This page updated April 2018.

This page for Power Transformer design and choke testing.

There are 3 pages about chokes :-

Fig1 PSU schematic for

Table 1. Fill factors for 4 common window sizes.

Table 2. Wire sizes, Grade 2 wire, rated 200C max, polyester-imide enamel.

Table 3. Core heat losses.

Graph 1. Power transformer temperature rise, degrees C for Watts heat loss per sq.inch.

Fig 2. 377VA PT for 2 x 90W channels.

Fig 3. PT for 362VA with GOSS E+I core.

Tables 4, 5, 6 for various Ea and Iadc conditions

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POWER TRANSFORMER design including a lot of calculations.

Schematic of amp power supply must be well understood to design 377VA power transformer.

Fig 1 Schematic includes PT with generous VA rating for 377VA input to make output 343W at idle,

and much more output during high Po class AB of up to 90W / channel.

The PSU suits 4 x KT120, KT90, KT88, 6550, and 4 x EL84 or ECC99 or 6CG7 or 12BH7 for two

LTP drive stages, with 2 x 6CG7 for preamp and input stages.

The PSU can supply all power needed for a good phono amp such as

preamp-rocket-phono-2005.html

Calculations are based on steady idle condition for filaments, input and driver tubes and bias,

but with higher Iac current from HT winding to allow for increase of Idc during class AB operation.

PT should not have T rise above +20C above ambient with NOSS core, or more than +15C with

GOSS core.

PT should remain silent and vibration free during all operation.

The production of B+ must NOT use tube rectifier diodes.

AC and DC Filament Heater Supplies.

NOTE. Allow for 4 x KT120 needing 2.1A each. Thus 4 x 6550, KT88, KT90, EL34, KT66 all OK,

4 x KT120 :- 4 x 6.3V x 2.1A = 53W.

4 x EL84 driver :- 4 x 6.3Vac x 0.8A = 21W,

2 x 6CG7 input :- 2 x 6.3Vac x 0.6A = 8W,

2 x 6CG7 preamp :- Idc heaters, include external preamp, 17Vdc x 5A = 85W.

Sub total all heaters = 167W :- 25.2Vac x 6.63A.

B+ ANODE VDC SUPPLY FOR ALL TUBES.

1. For 4 x KT120, Idle B+ power = +510Vdc x 50mA x 4 = 102W.

( At 2 x 90W AB audio Po, max B+ power = 300W, but is not continuous ).

2. Idc in R5+6, R9+10, each 68k, Idc = 520V / 136k = 3.8mAdc, dc power = 510V x 0.0038A = 2W.

3. 4 x EL84 driver stage, Idc is constant = 4 x 14mAdc, dc power = 510V x 4 x 0.014Adc = 29W.

This includes power in R to reduce B+ for LTP to +450Vdc.

4. 4 x 6CG7 input and preamp tubes, average 5 mAdc each, dc power = 520V x 0.02A = 11W.

5. 5 x 75V Zener diodes, max Idc = 14mAdc, dc power = 510V x 0.014A = 8W.

6. External phono amp, Idc for 3 triodes per channel, each 5mAdc, dc power = 510V x 0.03A = 16W.

7. B- cathode -Vdc bias supply, for 2 x 28mA CCS for 4 x EL84, and fixed bias R networks,

135Vdc x 038Adc = 5.4W.

Sub total 1 to 6 for B+ supplies = 168W.

Total of powers to Primary, and from Secondary windings :-

Primary 377W, 240V x 1.57A

Heaters 167W, 25.2V x 6.63A

HT, doubler diodes, 168W, 190V x 0.88A.

Bias 6W, 50V x 0.12A.

Total of all Sec power at idle = 343W.

( all Fe and Cu loss = 10% estimated for all Sec power = 34W, so Pri VA = 343+34 = 377VA )

For mains input = 240Vrms, Iac = 377VA / 240V = 1.57Amps.

Before the metric system made many things easier,

Afe in sq.in = sq.rt VA / 4.44, where 4.44 is a constant.

To find Afe in sq.mm, multiply both sides of equation x 645, because 1.0 sq.in = 645 sq.mm.

Then you get theoretical Afe = 145 x sq.rt VA.

For 377VA, Afe = 145 x sq.rt 377VA = 2,815sq.mm. Here is list of conditions for the ancient

formula to be valid :-

Core = wasteless NOSS E+I, Afe is square centre leg passing through winding with T = S,

145 is a constant for all size units in mm, and VA is input to primary = output from all secs + 10%,

Bac = 0.9Tesla, Frequency = 50Hz, fill factor, ff = 0.3, ie, total of Cu section area of all copper in

window = 0.3 x L x H, stacking factor Sf = 0.95, and current density = 2A/sq.mm.

The "simple equation" is dealing with about 7 different things.

These issues have been well known since 1920s and Sf 0.95 has not changed much at all.

Fill factor ff has improved because winding layers extend closer to Fe because of plastic moulded

bobbins and improved polyester-imide winding enamels, and better insulation than impregnated

woven cotton ( cambric ) made in India, or kraft paper or cardboard, lignum based, still made in

many places.

Consider a typical window size 76.2mm x 25.4mm, The area available for wire = 72mm x 20mm,

and if you have the bobbin full of layer wound turns of 1.0mm Cu dia / 1.093mm oa dia, with no

layers of insulation, the copper area =

0.786sq.mm x [ 72mm x 20mm / ( 1.093mm x 1.093mm ) = 947sq.mm.

Fill Factor = ACu for all turns / window area = 947 / 1,935sq.mm = 0.489.

But insulation layers the typical ff can now be 0.34, with average plastic moulded bobbins,

Grade 2 enamelled Cu wire, and Nomex 401 insulation.

The Bac can also be up to 1.0 for GOSS core because slight improvements have been made

since 1950 to non GO steels.

The PT designer must have a clear list of the normal Vac x Iac and output Watts wanted from

each secondary winding with some allowance for increased power output during use.

To this total he may add 10% for all winding losses and core losses and the total is the

Primary VA rating for the PT.

Using my design ideas, the PT will suit the idle condition and easily cope with +50% total

secondary power for class AB if a sine wave is used up to clipping on both channels.

For the PSU above, total sec power = 343W, allow 10% loss = 34W, thus input VA = 377VA

The designer will know what E+I lams he can source, or has in stock. If he knows Tmm, Lmm, Hmm,

he can easily calculate stack Smm for the wanted VA if he also knows Bac in Tesla, Frequency in Hz,

current density in A/sq.mm.

Assume the copper section area for all turns in the window area will vary with the thickness

of enamel insulation, and for 1.0mm Cu dia wire the Acu = 0.786sq.mm. The oa dia = 1.093mm.

Cu section area for 1 turn = 0.786sq.mm, and the square area needed to contain 1 turn

= 1.093 squared = 1.195sq.mm.

The factor for Acu contained in a given area of winding = 0.786 / 1.195 = 0.658.

For a window with area = 25.4mm x 76.2mm = 1,920sq.mm, the area available for wire allowing for

layered insulations = 15.82mm high x 72.2mm wide = 1,142sq.mm.

This area can contain 1.0mm Cu dia wire turns = 1.142sq.mm / 1.195sq.mm = 956turns and copper

section area total = 956t x 0.786sq.mm = 751sq.mm.

The maximum fill factor = total copper section area / window area = 751sqmm / 1,920sq.mm = 0.391.

But in practice, the factor is difficult to achieve because not all layers will be filled and wire sizes

and ratio of insulation thickness to wire dia varies and in practice the fill factor, ff = 0.34.

The moulded bobbins used for nearly all PT and OPT and Chokes used for hi-fi audio gear have

window size from L76.2mm x H25.4mm, to L38.1mm x T12.7mm and average bobbin wall thickness

plus clearance from Fe = 2.0mm.

Table 1.

L mm |
H mm |
Window area |
Max bobbin content height 0.8 x H |
Insulation layers = 0.1 x H + bobbin base 2mm |
All wire ht including enamel insulation |
Bobbin wind width = L - ( 2 x 2mm) |
Area occupied by all wire |
Copper section Area Cu per LxH |

76.2 |
25.4 |
1,935 sq.mm |
20.32mm |
2.5mm + 2.0mm |
15.82mm |
72.2mm |
1,142sq.mm |
0.388 |

66.6 |
22.2 |
1,478 sq.mm |
17.76mm |
2.2mm + 2.0mm |
13.56mm |
62.6mm |
849sq.mm |
0.378 |

57.1 |
19.05 |
1,088 sq.mm |
15.24mm |
1.9mm + 2.0mm |
11.4mm |
53.1mm |
605sq.mm |
0.366 |

47.6 |
15.8 |
756 sq.mm |
12.64mm |
1.6mm + 2.0mm |
9.1mm |
43.6mm |
396sq.mm |
0.345 |

for all window sizes.

Notice that the height of all wire is about 0.61 x window H.

The current density for Pri and all Secs should be same, but Primary input VA is 10% higher

than total Sec output Watts.

With 377VA at primary input, 343W from all secondaries, total in + out power = 377+343 = 720W,

and copper section area for all Pri turns = [ Input VA / total Sec W ] x 0.34 x window area

= [ 377VA / 720W ] x 0.34 x L x H = 0.524 x 0.34 x L x H.

Now Np = Vac x 226,000 / ( T x S x Bac x F ). (( Bac in Tesla, F in Hz, and 226,000 is a constant ))

Cu area for 1 Primary turn = Acu for all P turns / Np = 0.178 x L x H / Np

= 0.178 x L x H / ( Vac x 226,000 / S x T x B x F )

= 0.178 x T x S x L x H x B x F / ( Vac x 226,000 )

= 0.787 x T x S x L x H / ( Vac x 1,000,000 )

The area of 1 primary turn x wanted Iac density Id = Primary input Iac.

Primary VA = Vac input x Id x Acu for 1 turn.

Therefore VA = ( Vac x Id x 0.787 x T x S x L x H x B x F / ( Vac x 1,000,000 )

The Vac is cancelled,

Therefore

For any chosen value if T, S L or H, ff = 0.34, dimensions in mm.

Now for wasteless core and where S = T,

VA = 0.78 x 1.5 x T x 0.5 x T x T squared x B x F x Id / 1,000,000,

VA = 0.585 x T to 4th x B x F x Id / 1,000,000,

T to 4th = VA / ( 0.585 x B x F x Id / 1,000,000 )

= 1,000,000 x VA / ( 0.585 x B x F x Id )

Afe = T squared = sq.rt [ 1,000,000 x VA / ( 0.585 x B x F x Id ) ]

Afe = 1,000 x sq.rt VA / ( 0.765 x sq.rt ( B x F x Id )

If B = 1.0Tesla, F = 50Hz, Id = 2A/sq.mm, ff = 0.34,

Then Afe = 130 x sq.rt VA for wasteless core.

This suits those clever ppl who can work out the optimum wire sizes and winding positions

without much reliance on any formula.

The old "safe" formula was Afe = 145 x sq.rt VA, with fill factor 0.3 and 0.9Tesla.

The better NOSS grade allows 1.0 Tesla and fill factor 0.34, so that Afe can be slightly smaller.

From (2),

From this, it is possible to use any chosen T and S and get VA for wanted B and F and Id.

From (2),

Examples :-377VA transformer, B = 1.0 Tesla, F = 50Hz, Id = 2A/sq.mm, T = 51mm, wasteless.

With wasteless, L = 1.5 x T and H = 0.5 x T.

(4) S = 377 x 1,000,000 / ( 0.78 x 51mm x 76mm x 25mm x 1.0 Tesla x 50Hz x 2A ) = 49.87mm

49.87mm is less than T51mm, so we would use S51mm which suits a standard bobbin with

T51mm x S51mm.

What is VA rating with T51mm x S51mm, 50Hz, 1.0Tesla, 2A/sq.mm?

(4) VA = 0.78 x 76 x 25 x 51 x 51 x 1.0 x 50 x 2.0 / 1,000,000 = 385VA.

Afe = 51 squared = 2,601sqmm, Np for 240V = 417t.

What is VA rating with T51mm, S62mm, 50Hz, 1.0Tesla, 2A/sq.mm?

(2) VA = 0.78 x 76 x 25 x 51 x 62 x 1.0 x 50 x 2.0 / 1,000,000 = 468VA

Afe = 51 x 62 = 3,162 sq.mm, Np = 343t.

What is S for chosen T = 44mm, VA = 377, 1.0Tesla, 50Hz, 2A/sq.mm?

(2) S = 377 x 1,000,000 / ( 0.78 x 44 x 66 x 22 x 1.0 x 50 x 2.0 ) = 75.6mm.

T44mm x S75mm is a standard bobbin which could be used.

Afe = 3,300sq.mm, Np = 328t.

What is S for chosen T = 38mm, VA = 377, 1.0Tesla, 50Hz, 2A/sq.mm?

(2) S = 377 x 1,000,000 / ( 0.78 x 38 x 57 x 19 x 1.0 x 50 x 2.0 ) = 117mm.

There may not be any standard plastic bobbin with S over 117, so make one.

Afe = 4,446sq.mm, Np = 244t

FOR NOSS, Bac = 1.0T and Id = 2.0A/sq.mm for F = 50Hz. Use of 60Hz is possible because

Bac reduces to 0.833T with less core heating.

FOR GOSS / CRGO, Bac may be 1.1Tesla, Id = 2.2A/sq.mm, F = 50Hz.

What is S for chosen T = 51mm, 1.1Tesla, 50Hz, 2.2A/sq.mm?

(4) S = 377 x 1,000,000 / ( 0.78 x 76 x 25 x 51 x 1.1 x 50 x 2.2 ) = 41mm

I am not aware of any usable bobbins for T51mm x S41mm. But if bobbin is made with S41mm,

Afe = 2,244sq.mm, Np = 440t.

The maximum dimension is large, shape is somewhat odd, so most ppl would prefer T44 :-

(4) S = 377 x 1,000,000 / ( 0.78 x 44 x 66 x 22 x 1.1 x 50 x 2.2 ) = 62mm.

Afe = 2,728sq.mm, Np = 362t.

If T = 38mm, for GOSS, S will be 97mm, and a 38mm x 102mm bobbin is available.

Afe = 3,876sq.mm, Np = 254t.

Note. For the 377VA PT, for max class AB, B+ power may increase +200W for B+ sec to make total sec

power increases from 343W to 543W, and winding losses increase but core loss is constant, so

loss is about 54W so input = 597VA. Primary Iac density slightly increases and HT winding Id may

increase to over 3A.sq.mm, but the condition is temporary, and well tolerated. If PT is in a guitar amp

used by a heavy metal enthusiast who likes all controls on guitar and on guitar amp amp turned up full,

the output tubes become grossly over driven and grid current charges up coupling caps so -Vdc bias

increases and output tubes work in class C, and class C is quite efficient so working PT VA does not

increase exponentially.

For a NOSS wasteless E+I core, there are a couple of core size choices.

What is VA with T51mm x S51mm for B 1.0Tesla, F 50Hz, 2A/sq.mm ?

VA = 0.78 x L x H x S x T x B x F x Id / 1,000,000.................................(2)

For T51mm x S51mm, VA = 0.78 x 51 x 51 x 76 x 25 x 1.0 x 50 x 2.0 / 1,000,000 = 385VA.

But I prefer the alternative core with T51mm and higher stack of 62mm, and VA = 468, which

will run cooler with NOSS and with working VA = 377.

Core has T51mm x S62mm, for 468VA rating, but for working 377VA at idle.

Afe = 51mm x 62mm = 3,162sq.mm.

Wire ACu = VA rating / ( mains Vac x Id ) = 468 / ( 240V x 2A/sq.mm )

0.975 sq.mm.

theoretical Cu wire dia = sq.rt ( 28 x Acu / 22 ) = sq.rt ( 28 x .975 / 22 ) = 1.114mm.

From wire size table, try Cu dia 1.12mm, for 1.217mm oa dia including enamel.

Calculate primary turns per layer = bobbin winding width / oa dia.

Bobbin may have cheeks each 1.8mm thick, plus clearance off Fe core so

Bww = 76mm - 4mm = 72mm.

Pri tpl = 72mm / 1.217mm = 59.16turns. Allow for easy fitting of turns, try = 58tpl.

No of Primary layers = th Np / th tpl = 342t / 58tpl = 5.89 layers.

Increase layers to next whole number = 6.0 layers.

Height of Pri wire = 6 x 1.217mm = 7.302mm.

Most PTs have following allowed maximum heights for contents :-

Height of primary winding wire including enamel coating......0.32 x H,

Height of all secondary winding wire incl enamel coating.....0.30 x H,

Height of all layer insulations................................................0.11 x H,

Height of bobbin base plus clearance for bulge...................0.27 x H,

For H = 25.4mm, Pri wire = 0.32 x 25.4mm = 8.128mm.

The chosen Pri wire will fit under allowed Primary wire height.

Note. For all PTs you might wind, the bobbin has a certain base thickness, there are various

thicknesses of insulation and there must be clearance between wound turns and the iron when

E+I are inserted to a wound bobbin. Wires do not lay flat, and after many layers the winding

has bulged and height of bobbin contents plus bobbin base thickness may be found to be close

to the window height H. In general, calculated bobbin contents should not exceed height = 0.8 x H.

Note The VA input to Primary is always between 5% and 15% more than total power from Secs,

so the area occupied by primary turns should be more than all secondary turns to keep the

current density constant.

Height of all secs = 0.3 x 25.4mm = 7.62mm, and heights of Secondary Heater, HT, Bias may

be divided in proportion to the power from each winding.

Total Secondary power = 343W.

For 167W 25.2V heaters, height of wires = ( 167W / 343W ) x 7.62mm = 3.71mm.

For 168W 190V HT, height of wires = ( 168W / 343W ) x 7.62mm = 3.73mm.

The 8W 50V Bias theoretical winding height = ( 8W / 343W ) x 7.62mm = 0.177mm, which is

a Silly Result, because the bias winding should be rated for the wire size used which will be larger

than needed and say 0.569mm oa dia for 0.4A, or 20W, so Bias winding wire height is theoretically

0.44mm, but it is really 0.569mm, and 1 layer will not occupy the bobbin winding width.

Bias winding may fitted where HT winding does not fill a complete layer.

Adjusted Np = 6.0 x 58tpl = 348 turns. Np must be an even number to get 2 equal primary

windings to match international mains Vac between 240V in Australia to 110Vac in USA, 100V

in Japan, and 230V in UK.

There will be two windings each 3 layers, and each winding has taps for 120V, 110V, 100V.

Height of primary wires = 6 x 1.217mm = 7.302mm = OK

Check Iac density = VA / ( mains Vrms x Cu wire section area )

1.12mm dia Cu has ACu = 0.986sq.mm. At 2A/sq.mm, Ia = 1.972A and with 240Vac, OK for 473VA input.

For working 377VA, Primary Iac = 377 / 240V = 1.57A. Primary load input = Vac / Iac = 240V / 1.57A = 153r.

Average Turn Length TL = ( H x 22 / 7 ) + 2 x ( T + S ) = ( 25 x 22 / 7 ) + 2 x ( 62 + 51 ) = 304mm.

RwP = Np x TL / ( 44,000 x Cu dia squared ) where 44,000 is a constant.

RwP = 348t x 304mm / ( 44,000 x 1.12 squared ) = 1.92r.

Primary loss for 377W = 100% x 1.92r / ( 153r + 1.92r ) = 1.22%.

Wire heat = RwP x Iac squared = 1.92r x 1.57 squared = 4.7W.

In other words, Pri loss = 1.22% of 377W = 4.7W.

NOTE. Australian mains is nominally 240V, but could be up to 250Vac or down to 235Vdc.

FILAMENT HEATER WINDINGS.

Heater winding for 25.2V x 6.63A must have ACu = 3.31sq.mm or larger for 2A/sq.mm.

Wire dia for given Acu = sq.rt [ Acu x 28 / 22 ] = sq.rt [ 3.31 x 28 / 22 ] - 2.05mm Cu dia.

Wire table says 2.0mm Cu dia for 2.12mm oa dia is close enough.

For 25.2V the turns needed are Vac x tpv = 25.2V x 1.450 = 36.54. But to get 25.2V with load

connected, 38t should be used.

38t x 2.12 oa dia = 80.56mm which cannot fit on one layer 72mm wide.

Therefore use two layers of wire, but each one can use wire Acu > 0.5 x 3.31sq.mm,

say 1.66 sq.mm minimum.

Cu dia for 1.66sq.mm = 1.45mm. Wire table shows 1.50mm Cu dia at 1.608mm oa dia and

38t easily fits on across 72mm width with a spare 10mm of width. The empty 10mm should be

filled with 1.61mm of insulation to keep the following windings flat for full width.

Height of 2 wire layers = 2 x 1.608mm = 3.216mm which is less than allowable height above

= 3.65mm. 1.5mm Cu dia has ACu = 1.77sq.mm and Iac can be 1.77 x 2A/sq.mm = 3.4A.

TWO windings give 6.8A = OK.

RwH for both heater windings in parallel = 38 x 304 / ( 44,000 x 1.5 squared x 2 ) = 0.058r.

Power heat in RwH with max idle current = 6.63A x 6.63A x 0.058r = 2.55W.

Heater RL = 25.2V / 6.63A = 3.8r, RwH loss = 1.5% = OK.

The process of finding the best heater winding arrangement does NOT always follow formulas,

but requires human intuition which has yet to be achieved by application of any AI design program app.

Most computers are so dumb they cannot even read any schematic, let alone any winding diagram.

HT winding for 168W, 190Vac for 0.884A. Turns = Vac x tpv = 190V x 1.450 = 276t.

HT is for doubler rectifier to make +513Vdc at 327mAdc

Acu = 0.884A / 2A / sq.mm = 0.442 sq.mm, Cu dia = 0.75mm.

From wire table, oa dia = 0.832mm. Tpl = 85t.

HT wire layers = 276t / 85tpl = 3.24, so height of all wire = 4 x 0.832mm = 3.328mm, less than

allowable 3.67mm.

3 layers each 85t = 255t, with 4th layer = 21t.

RwHT = 276t x 304mm / ( 44,000 x 0.75 x 0.75 ) = 3.39r

Load on HT winding = 190V / 0.884A = 215r. Loss % = 1.55%

Heat loss = 2.65W.

Bias winding = 50Vac = 73t. Use 73t x 0.5mm Cu dia, 0.569mm oa dia.

Current at 2A/sq.mm = 390mA, much higher than needed.

Negligible heat loss for this winding.

Sub-total of bobbin content height, wire + insulation so far :-

Primary = 6 x 1.217mm..........................................................7.302mm

0.05mm insulation between P layers = 4 x 0.05mm...............0.25mm

0.10mm insulation between P layers = 1 x 0.05mm...............0.10mm

1.0mm insulation between mains Pri to Htr 1.........................1.0mm

Heater 1 winding....................................................................1.608mm.

0.05mm insulation between Htr 1 to Htr 2..............................0.05mm

Heater 2 winding....................................................................1.608mm.

0.25mm insulation between Htr 2 to HT winding....................

Sub total to here...................................................................

HT winding 4 x 0.832mm......................................................3.328mm

0.20 mm insulation between 3rd on HT layer to B-

B- bias winding 1 x 0.569mm = no height increase.

0.05mm insulation between HT layers = 3 x 0.05mm.............0.15mm

0.25mm cover insulation over all contents..............................

Provisional Total sub-total 1..................................................

Insulation height = 2.18mm, could be 0.614mm more to reach allowed 2.794mm.

Wire height = 13.844mm, and could be 1.904mm more to reach allowed 15.748mm.

Clearance off Fe + bobbin base thickness should exceed 0.27 x 25.4mm = 6.858mm.

The HT winding generates theoretical heat = 2.65W. But with high C values, the charging current

to C for perhaps only 1/5 of each 50Hz half wave and average charge current is 5 times the Idc

current and heat in Rw = average charge Iac squared x Rw / 5, which gives more heat than if the

load is a pure resistance.

If possible, RwHT should be 1/2 the value calculated above. And during class AB operation at high

Po levels, Idc and charge peak currents to C increase, and so does the winding heat.

Wire height so far for Pri and heaters = 7.302mm + 3.216mm = 10.518mm.

Thus available height for HT = 15.74mm - 10.518mm = 5.22mm.

Try increasing HT winding wire dia to 0.90mm Cu dia = 0.99mm oa dia.

Tpl = 72mm / 0.99mm = 72t. No layers = 272t / 72 = 3.77 layers.

The wire occupies height = 4 x 0.99mm = 3.96mm which should be OK.

Use 4 full layers of 0.9mm Cu dia wire for total HT turns = 4 x 72 = 288t, giving 198Vac,

but it will sag under load a bit.

RwHT = 288t x 304mm / ( 44,000 x 0.90mm x 0.90mm ) = 2.46r.

Allow HT heat loss at average Iac at say 1.3A = 1.3 squared x 2.46 = 4.2W.

Revise heights.

Sub total to after 0.38mm insulation after Heater 2 winding......12.298mm.

HT winding 4 x 0.99mm............................................................3.96mm

3 x 0.05mm insulation between HT layers................................0.15mm

0.25mm insulation between HT winding and bias winding.......0.25mm

Bias winding = 73t x 0.569mm, part of 1 layer..........................0.569mm

0.25 cover insulation over bias winding....................................

Provisional sub total 2.............................................................

Clearance off Fe plus 2.0mm bobbin base = 25.4mm - 17.477mm = 7.923mm, exceeds 6.858mm = OK.

For CRGO cores, see http://www.aksteel.com/sites/default/files/2018-01/litecarlite201304_0.pdf

For NOSS cores, see http://www.aksteel.com/sites/default/files/2018-01/dimaxm-132013pdf_1.pdf

Use of M4, M5, M6 should be OK for any PT, OPT or choke and give 1/2 the core loss of

M13 NOSS material. For the example PT here, these are the losses :-

CRGO, GOSS M4, M5, M6 | 1.7T |
1.5T |
1.2T |
1.1T |
1.0T |
0.9T |
0.8T |

Heat Loss
per Kg |
2.2W |
1.0W |
0.6W |
0.51W |
0.42W |
0.37W |
0.27W |

NOSS M13 |
1.7T |
1.5T |
1.2T |
1.1T |
1.0T |
0.9T |
0.8T |

Heat Loss
per Kg |
3.3W |
2.7W |
1.4W |
1.2W |
1.0W |
0.8W |
0.7W |

The weight of the core must be calculated to find heat loss of the core.

Volume of a wasteless E+I core = 6 x T squared x S. To make things simpler, use centimetres,

cm for T and S.

Density of NOSS or GOSS = 7.6grams / cc.

For 377VA PT, T = 5.1cm and S = 6.2cm, Volume = 6 x 5.1 x 5.1 x 6.2 = 968cc, weight = 7.35Kg.

Core heat at 1.0T for NOSS = 7.35Kg x 1.0W/Kg = 7.35W.

The same cores with GOSS / CRGO at 1.0T have 0.42W/Kg for heat = 3.1W which is a minor factor

in core heating.

In theory, use of higher Bac at say 1.2T with GOSS gives 0.6W/Kg = 4.4W heat, but Np can be reduced

by factor = 0.625, so a larger wire sizes can be used with much lower Rw so higher VA rating.

But the PT may be noisy, and vibrate a bit.

In hi-fi amps I always found Bac higher than 1.0T leads to vibration and noise.

Even with toroidal or C-cores which usually are GOSS, the noise and vibration are a problem and it is

always necessary to pot the transformer with 2 pack potting mix, or casting resin with sand.

The problem with potted PTs is that its like wrapping the PT in a blanket, and heat cannot get

out of the transformer so easily. The result is that copper and iron runs hotter and the outside of pot

can run slightly hotter.

Where you do find usable NOSS, it may not like running at 1.7Tesla.

AK steel seems to say its possible without complete core saturation. I found about 1.3T was max

for old Lycore 150 NOSS. I suspect 150 meant 1.5W / Kg but Bac was never specified.

The total winding heat for 377VA PT :-

240V Primary = 4.7W.

Filament heater = 2.65W,

HT = 4.41W.

Bias = negligible

Total Cu heat = 11.76W = 3.11% of 377VA, < 5%, = OK.

Total loss for Fe and Cu = 7.35W + 11.76W = 19.11W = 5.07% of operating 377VA.

On page 238, RDH4, there is Fig 5.18B giving Temp rise versus W / square inch of

external core area.

On page 237, RDH4 formula to calculate surface area of power transformer with wasteless E+I :-

Surface Area = T x [ ( 7.71 x T ) + ( 11 x S ) ].

Unfortunately, this formula for wasteless E+I seems incorrect.

The dimensions of a wasteless core PT with lams laying flat on chassis are :-

Plan area, top and underside = 2 x ( 2.5 x T ) x ( 3.0 x T ) = 15 x T squared.

The 4 vertical sides area = S x ( 2.5T + 3T + 2.5T + 3T ) = S x 11 x T.

The surface area omits the slight increase due to bell end covers or projecting windings.

The RDH4 formula forgot to include for heat radiation from underside of PT.

The core radiates most heat, and convection with air flow gives some cooling.

The formula should assume the PT hangs in air of a room, is not potted, and without bell-end

covers, and is unaffected by how it is fixed to a chassis or by what tubes are located nearby,

or the temperature of the under-chassis volume. Usually, the PT runs hotter when on a chassis

of amp than in free air because of hot tubes nearby.

I don't know exactly how RCA staff composed the graph, but there is no such graph on Internet

because the latest generation is Bone Lazy.

For 377VA PT with T 5.1 cm and S = 6.2 cm, SA = 5.1 x [ ( 15 x 5.1 ) + ( 11 x 6.2 ) ] = 738sq.cm

= 114 sq.in. For total Fe + Cu heat = 19.11W, heat / sq.in = 19.11W / 114sq.in = 5 / 105 = 0.167.

From Graph 1 the T rise = +23C. If ambient T = 25C, PT temp = 48C.

What if core was smaller size with T51mm and S51mm, and GOSS was used,

but with same wire, Bac, and Id ?

Weight = 6.05Kg, and core loss = 0.42W/Kg x 6.05 = 2.54W.

Copper loss is less because turn length is 22mm shorter = 282mm.

Copper loss = 11.35W x 282mm / 304mm = 10.5W.

Total Fe + Cu loss = 10.5 + 2.54 = 13W.

With surface area is less = 676sq.cm = 105sq.in and W/sq.in = 10.5W / 105sq.in = 0.1W / sq.in.

T rise = +15C, and if ambient = 25C, PT = 40C, which feels much cooler than 48C, and you

will find the results are pleasing.

I have wound a number of PTs with old NOSS made after 1960, and sometimes used Bac = 0.8T

where core loss may be assumed to be = 0.7W/Kg.

One example is the PT in amp at

This OPT has T38mm x S62mm and core weight = 4.1Kg, and surface area = 475sq.cm = 74sq.in.

VA at idle = 154W.

The VA rating for core is 160VA according to above formulas Core volume = 537cc, weight = 4.1Kg.

Fe Core loss = 4.1Kg x 0.7W/Kg = 2.9W.

Cu loss estimated at 5% of load of VA = 7.7W.

Total heat = 10.6W = 0.143W / sq.in, and T rise should be +18C so that T = 43C on a day of 25C.

I can leave my hand on the transformer without pain, and the RDH4 graph appears to work about right.

The measurements I made indicate PT T at about 43C.

It seems only RDH4 has presented the world with a simple graph that relates T rise above ambient to

the number of watts of heat and the surface area of the transformer, for most PTs used in old radios

and old or new amplifiers.

Just what comprises the core loss? it is present with or without secondary loads connected.

The primary current measured for PT primary consists of current flow in the primary inductance,

and harmonic currents due to eddy currents and hysteresis of iron creating a non linear load to primary

input Vac. As the Vac peaks swing above Bac = 0.6Tesla, the current in primary increases non

linearly as the core moves towards saturation. If Vac pushes Bac to saturation and above, the magnetic

field ceases oppose the current flow so the load where Vac exceeds the mains becomes the winding

resistance RwP and current becomes extremely high. If you have a primary meant for 120Vac mains,

and it is used where a 240Vac mains exist, the mains fuse should blow instantly.

I recently tested a large PT for 240Vac input, with wasteless NOSS E+I core, T38mm, S75mm,

L57mm, H19mm. Transformer SA = 530sq.cm = 82sq.in, Volume 650cc, weight 4.94Kg.

No secondary loads were connected.

I applied a switch-variable 50Hz mains Vac from 14Vrms to 240Vrms.

The mains input Vac which measured 240Vrms had flat topped waves with about 5% 3H+5H etc,

due to hundreds of other local ppl using electrical appliances with rectified mains inputs. My switched

Vac source is like a variac, and had a 1:1 isolation PT before it so Vac could be reference safely to my

local Earth connection.

I used a 10 : 1 Vac divider to use to view mains Vac on on channel of CRO, and was able to view

primary current in other channel using 1r0 x 10W in series with neutral line to Primary.

There was very little THD in Iac wave up to 50Vac input, indicating the primary was acting like a

pure inductance. The Lp reactance rose from 1k1 to 2k7 at 50Vac because permeability, µ, increased

from say 0.06T at 14Vac to 0.21T at 50Vac.

The high THD of mains input Vac did not appear in Iac waves because 3H at 150Hz is much reduced

by higher L reactance at higher F. There may be other reasons I am not aware of.

But above 50Vac, Iac wave peaks became flattened and by 100V 3H = 20% approx.

At 150Vac, the flats began sloping upward and 3H increased, and there was probable other 5H, etc.

At 180V there are peaks at end of flats, and as a result of phase shift of 3H relative to the 50Hz.

At 240Vac, each 1/2 Iac+ wave goes up to 1/2 max, then levels, then up to a peak at max before steep

drop to the same inverted shape for the Iac -1/2 wave.

The Iac wave was measured with DMM rms meter, and the primary input impedance =

1k1 at 14Vac, 3k0 at 70Vac, 3k5 at 160Vac, then reducing to 2k7 at 240Vac. I have no idea of the

max Bac, but if I assume it is 1.0Tesla, then max Zin is at 160Vac where Bac may be 0.65Tesla.

Primary Lp at 160Vac is probably about 12H, quite a good high figure.

At 240V, Iac = 86mAac, so VA with no load = 20VA, and it is possible core heating = 10W.

The other 10VA is due to Iac in primary inductance which causes no heating.

This sample transformer has a VA rating of about 252VA, Primary input load = 228r, RwP = 3r2,

Pri loss = 1.4%.

I left the PT on wood bench for 4 hours with no load, and recorded ambient T and core T,

and T rise = +11C. The RDH4 graph begins with rise of +15C at 0.1W / sq.in. +11C indicates I had

0.07W / sq.inch, so with SA = 82sq.in the core heating power = 0.07 x 82 = 5.74W.

This was a very good result with a NOSS core because many other old PTs I have tested will get too hot

even with no load connected.

1N5408 have 3A rating which may mean 3A continuous. Data says forward voltage = 1.0V which is

true for highish currents > 0.5A. So on R = 1.0 / 3A = 0.33r, and diode heat = I squared x R =

3A x 3A x 0.33r = 3.0W. They will feel hot if you touch one. If the current = 20A, the forward voltage

is still 1.0V, so R = 1V / 20A = 0.05r and heat = 20A x 20A x 0.05r = 20W, and the 20A could not

last for longer than about 0.14 seconds or else the diode would explode with heat at PN junction going

high enough to melt and vaporize the hard plastic diode capsule.

The data says diode will survive a short pulse of 200A. OK, but only if pulse lasts 15uS, and not more

often than every 10mS. Peak inverse V = 1,000V for 1N5408. It means that if you have a HT winding

with CT at 0V for 320Vac - 0V - 320Vac, the Vdc = +450V. The winding swings +/ 450V peak so

max PIV = 900V. With a bridge rectifier winding of 320Vac, two diodes conduct during charging and two

are in series with 450Vpk across each. Therefore I like to use 2 x 1,000V diodes in series where I replace

tube diodes with Si diodes to make +450Vdc.

In a doubler with 200Vac max at one winding, max piv = 560Vpk so one diode is OK but in some amps

I have used 6A rated diodes with 1000V piv. Where iac is high, I have sometimes used 2 parallel diodes

with 1r2 in series with each diode to force each to have fairly equal peak Iac. P600M are OK

Do not use 1N4007 for a power amp. These have 1A rating for continuous current, and peak short term

current rating of 30A. These diodes soon overheat and are destroyed if you had 3A continuous because

their size is small and you get a high T rise, and if there is a short circuit you want a fuse to blow, not

the diode, and not the transformer winding.

In any B+ supply with Si diodes, peak Iac during charging can be surprisingly high. Diode peak current

is limited by the sum of diode R + HT Rw + primary RwP + mains supply resistance at the wall plate

which includes house wiring to the heavy supply wires in the street.

If the Idc flow = 332mAdc, and mains Vac is a sine wave, and C = 470uF, and there was no series R

the Iac peak charge could be 5A. But I have made SS amps with +/-70Vdc and used 2 x 100,000uF

and 35A rated bridge diodes with 50V-0V-50V winding and at 700W output the dc power = 1,000W

and Idc = 7A at each +/- rail and peak charge may be 35A. It has lasted the test of time.

The mains often has flat topped Vac waves where the flat is about 1/5 of each 1/2 wave, so charge current

remains fairly constant and not too high but when most ppl are asleep, wave is more sinusoidal so peak

charge Iac is higher, and PSU must be built with that in mind.

Using some added R between HT winding and diodes before high C values can much reduce peak

charge currents, but it makes the B+ output resistance higher. But that is going to always be low enough

with a small R series R. The B+ will not be as high with any series R but in case of 377VA PT the HT Vac

is rated at 198Vac to give +525Vdc with some loading, but the series R can drop this to +510Vdc without

lowering max Po much, but it may reduce peak charge Iac by -50% or more, so the HT winding runs cooler,

and it does not matter if R has 10W of heat. 10r0 should halve the peak charge current to each 470uF in

Fig 1 PSU for doubler diodes. 10r0 should be 30W rated at least.

The HT winding should have 5 taps at 20t apart which allow other tubes to be used.

The windings will bulge as turns are wound on and will never lay flat where wire is inside the core,

so the clearance of 4.7mm is needed. It may be possible for bobbin content to be 2mm higher but

the wound bobbin would need to have slow setting epoxy varnish applied while winding and the

wound bobbin then cramped up with wood blocks and a G-cramp while still held on lathe chuck.

It is a hugely messy job with toxic fumes. The wound bobbin is left cramped for 2 days before

removal from the lathe.

The windings must be drawn up carefully for any winding trades-person to easily follow.

If this person is YOU, you need to wind a few chokes first with neat layer winding before winding

a complex PT like this one.

It is extremely easy to become totally confused when winding tube amp PT or OPT because there

are so many connections and taps.

Fig 2.

Fig 2 shows a good design for 377VA PT to power your favourite amp that could make

2 x 90W or 1 x 180W.

It is heavy. The two OPT will be heavy, and then there is a choke, and the chassis must

be steel and be strong enough so it will not bend if the amp is dropped off a bench, or badly

handled by conveyor belts and careless ppl during transport.

( But a thief will find it difficult to steal, and a few have been caught trying to run away with

the amp, which they usually drop :-)

For all iron cores :-

dimensions in cm.

Wasteless E+I Volume = [ ( 3T x 2.5T ) - ( 2 x { 1.5T x 0.5T } ) ] x stack S

dimensions in cm,

Weight = core volume in cc x density of transformer steel density in gm / cc.

Density of NOSS or GOSS = 7.6 grams / cc.

Bac = 1.0Tesla, F = 50Hz, Id = 2A / sq.mm.

1. T = 5.1cm, S = 6.2cm, VA rating = 468. Core volume = 6 x 5.1cm squared x 6.2cm = 967cc.

Weight = 7.35Kg, SA = 114sq.in ( = 738sq.cm. ) For working 377VA, total losses = 20W,

W / sq.in = 0.175.

2. T = 4.4cm, S = 7.5cm, VA rating = 374.

Core volume = 871cc. Weight = 6.62Kg, SA = 101sq.in ( = 653sq.cm. )

For working 377VA, total losses 25W, W / sq.in = 0.247.

3. T = 3.8cm, S = 11.7mm, VA rating = 375.

core volume = 1,013cc. Weight = 7.70Kg, SA = 109sq.in ( = 706sq.cm. )

For working 377VA, total losses 25W, W / sq.in = 0.23.

surface area so may run hottest of 3 options because W / SA factor is highest.

Use of GOSS core would reduce heat considerably without any other changes.

What if non wasteless E+I was used? It is a silly question because you cannot buy small

quantities of cores with say T = 32mm and windows = L76mm x H25mm. You could buy it

in 1955, and it was more expensive than wasteless because cutting the shape produced

waste, and waste costs money.

16mm for T dimension of 32mm, with windows 76mm x 25mm.

The turns can be the same as for wasteless E+I with T51mm x S62mm wasteless = 348t.

But Afe must be same as T51mm x S62mm = 3,162sq.mm.

Therefore C-core S = 3,162sq.mm / 32mm = 99mm, say 100mm, so that perhaps 4 x C-cores

are used, one pair stacked on top of other pair each with strip width 50mm, a standard size.

C-core volume = No C-cores x strip width x build up x average length of wound strip

4 x 5.0cm x 1.6cm x [ ( 2 x { 7.6 +2.5 } ) + ( 1.6 x 22/7 ) = 4 x 5.0cm x 1.6cm x 25.2cm = 807cc.

Weight = 6.13Kg.

To determine the temperature of PT.

Surface Area SA for 4 x C-cores for double O, each with build up 16mm for T32mm,

window L76mm x H25mm, and total strip width giving S = 100mm =

( 2 x oa plan area ) + ( distance around external curved shape x total strip width )

= ( 2 x 11.4cm x 10.7cm x 0.95 for curvature ) + ( 41.6cm x 10.0cm )

= 648sq.cm = 100sq.in. ( The C-core weight and surface area is almost identical to

use of wasteless T4.4cm x S7.55cm. TL is nearly same. Np is same, but C-cores

have bigger window than T44mm E+I, so copper losses should be lower because

wire size is larger.

C-cores are always GOSS, and will run cooler than NOSS but be 3 times the price

of NOSS E+I. So C-cores do not give any DIYer or small volume manufacturer a free lunch.

So far, for E+I, Cu losses are twice the Fe losses even with NOSS core. With GOSS,

T rise for 377VA PT should be carefully worked out with Bac = 1.1Tesla and Id = 2.2A / sq.mm

and with less turn length and less surface area and lower weight.

But GOSS E+I can cost much more than NOSS.

--------------------------------------------------------------------------------------------------------

Many DIYers may be tempted to use core material taken from old PT made 50+ years ago.

I did this often during my 20 years career being an audio tech and "amp worker" and I found

some awful iron with high loss W / Kg and low permeability was just fine to use in dc filter

chokes where permeability needed was often below 700.

There was a lot of junk NOSS core material that is not as good as anything made by

AK steel and if you use it at 1.0Tesla for a PT, it runs so hot after 4 hours in warm weather

that you might be able to fry eggs on it. Its more than 50C. Hot tubes nearby help it to be

hotter. Buying small amounts of C-cores of GOSS E+I has become difficult. Nobody wants

to sell say 40Kg for your amp project. They like orders for 40 tonne or 4,000 tonne.

This shows the difference between poor 1950 and 1960 E+I core material and 1990 C-cores for

three small PT with same Afe.

The no load VA input for 1950 and 1960 PT is high, and a high % of that VA heats the core

But the 1990 C-cores have far lower no load input VA and make less heat.

The 1950 and 1960 PT had wasteless E+I E+I cores with T25mm x S30mm, and appeared to

have VA rating of about 20W. With Pri input = 240Vrms, the Pri load

=

Both the 1950 and 1960 PTs have XL 3k3 at 240Vm so that

The Pri RL is not much less than the XL so core losses would be considerable. The

magnetizing Iac = 240V / 3,300r = 72mA. VA = 17.3W. I don't know how much of that power

heats the core, and how much is just Iac flow through primary Lp, no heat is generated in Lp,

although some heat in windings is due to Rw x Iac squared.

The 2,880r load Iac = 240V / 83mA Because inductance is reactive, the total Iac input is not just

the sum currents, but maybe 112mA, and total input VA = 27W.

Now the 1990 PT with double C-cores with Afe = T22mm x S25mm and window 51mm x 16mm

has XL at 240V at about 23k, and THD in Iac is only 10% so the magnetizing current is very

much less than for the E+I, so the core cannot heat up much. The window is a much larger

size than 12.7mm x 38.1 for T25mm E+I, so the wire size can be larger and so nearly all the

heat in PT is in the winding and not the core, so the VA rating is probably 30W, and Pri load

could be 1,920r.

Obviously, the C-core tranny is superior to the E+I.

---------------------------------------------------------------------------------------------------------

I should mention toroid cores. Buying toroid cores may be easy, but winding them requires a

toroid winding machine which is difficult to make yourself.

They can be fine for PTs but I found they need to have Bac < 1.0T to get noise and vibration low.

Many commercial toroid core PTs which you can buy were designed for 220Vac or 110Vac so

when used in Oz with 240V the Bac might be 1.3Tesla+ . All the toroidal PT I bought from Jaycar

and Harbuch were far too noisy to use in a hi-fi amp because you can hear them humming

metres away.

Toroidals might be OK for a factory where noise and vibration does not matter.

I found they got hot, and produced high external 100Hz stray magnetic fields so they needed

to be potted. None had varnished windings and all relied on wire tension pulling on polyester

tape used for insulation between Pri and Sec. So the micro-movement of wires is allowed to

occur. On many toroidal PT, wires are messily layered, and so I never used any toroid cored

PT in any amp except one. This had 800VA rated core, and ran at 1.2T, and was noisy.

I increased the primary and secondary turns by 30% to stop the noise by getting Bac < 0.8T.

I added the extra turns using a hand shuttle made with a 20mm wooden dowel about 1 turn

long, with V-cuts each end to hold 30t of wire on shuttle, then unwound onto toroid core by

passing shuttle through the centre hole 30 times for 30t turns, and each turn was pulled tight,

and kept tight for the next turn. It was a painful exercise which took two days.

The best toroid core PT use woven polyester tape for insulation so that when soaked in liquid

varnish it can easily penetrate the fill the voids to expel the air,wetting every surface before

heating to 125C for 4 hours. The the best toroid PTs are potted, and then you have a very

good PT. Core loss in W/Kg is so low for strip wound GOSS that it may be ignored. When

potted, the pressure on windings in pot is low. And you pay through the nose for such PTs

if ever you can find them. Plitron in Canada may have something like this.

I cannot recommend toroid PT or OPT and certainly not for DIYers. Try watching Google

Videos on toroid coil winding, one is https://www.youtube.com/watch?v=v-Upv7Mo7YU

BEWARE the sites claiming to offer toroid transformer calculations. NONE I found were

any good, or were so riddled with mistakes and bad grammar they were useless.

The turns for primary must obey the same equation as for any E+I or C-core, ie,

Np = Vac x 226,000 / ( Afe x F x Bac )

And for any hi-fi amp, the Bac should not exceed 0.9T for 50Hz.

--------------------------------------------------------------------------------------------

These are defined at https://en.wikipedia.org/wiki/Amorphous_metal_transformer

I have never tested any amorphous core PT or OPT sample but from what I read it is said

they saturate at Bac less than 1/2 that of GOSS. They are becoming popular in the heavy

power industry where multi kW or MW are involved. The core heat loss is low, so although

the Afe has to be twice that for GOSS, less power is wasted keeping the transformer cool.

For the PT detailed on this page the Afe would have to be doubled to allow the winding

shown to be used. Amorphous cores are very brittle, and dropping a transformer could

ruin it. The cost is prohibitive, and DIYers should ignore these cores.

Nickel-iron alloys are very expensive, and difficult to source and use is not justified for any

power transformer. In OPT, I see little benefit, but in SE OPT there may be less slightly THD

so this enables all sorts of silly claims to be made to boost sales of amps with Ni-Fe alloy

cores or those with a mix of Fe-Si plus Ni laminations. I base my design on wasteless GOSS

E+I laminations or C-cores, or NOSS if you are poverty stricken, and you don't mind wasted

heat and high weight. Where there is a +20C temperature rise above ambient for a generously

designed PT with NOSS, the same sized GOSS core with same winding and load may give

+10C after 4 hours. The T rise for GOSS or NOSS with low Bac will be mainly due to copper

losses being more than core losses.

------------------------------------------------------------------------------------------------

voltage at 50Hz, you should wind a test coil on a bobbin suitable for the sample of iron you have.

I made a test coil 429t x 0.8mm Cu wire dia to fill the bobbin window to suit T25mm x 32mm stack.

The wire was random wound. The E+I lams filled the bobbin and were fully intermeshed.

I used switched levels 50Hz, 14.5Vrms, 29Vrms, 44Vrms to 87Vrms.

Maximum XL was 4,700r at 44Vrms, and THD = 10%.

L = 4,700r / ( 6.28 x 50Hz ) = 15H.

Graph 3.

The core of T25mm x S32mm could be rated for 25W PT.

In this case, if Bac is allowed to be 1.0Tesla at 76Vrms x 50Hz, and if this winding was for

a PT primary, then the Pri load = 76V squared / 25W = 230r with N = 429t.

The XL = 1k2, magnetising Iac = 64mArms, load current = 334mArms.

Now if Vac input = 240Vrms, Bac = 1.0T, N will have to be 1,355t, and load = 2,282r,

and XL may be 10k and magnetizing Iac = 24mA, and load Iac = 105mA. The total

magnetising VA = 5.76, and I don't know what amount of this heats the core.

But for GOSS, the core loss / Kg at 1.0T is about 0.5W per Kg, and this core weighs

0.912Kg, so expect heat = 0.45W, and I doubt the core would get hot.

Now

are constants, N is the No turns in coil in

µ is permeability, ML is magnetic path length in mm.

For this example, at 50Hz, µ = 15.3H x 138mm x 1,000 / ( 1.26 x 0.429 squared x 25mm x 32mm )

= 11,381.

The core sample must be GOSS, because anything other than GOSS will give less L and

lower µ < 3,500, and that always means the XL at 1.0T would be much lower and core

heating is higher.

You can also work out magnetic field strength Bac in Tesla because you have know Vac,

Afe, N, F.

226,000 is a constant.

This case, Bac = 76Vrms x 226,000 / ( 25mm x 32mm x 429 x 50Hz ) = 1.0T

If I wanted to see if the core was suitable for an OPT, I would have to use a 14Hz test

signal and draw the graphs to find where Fsat would occur.

When I used a power amp to apply up to 40Vrms at 14Hz to the coil, I found :-

The graphs here show that the highest XL, and thus highest µ and L are where Bac is

between 0.4Tesla and 0.7Tesla at the top of the arched graph.

The µ of core and XL and the THD that is generated by core is mainly important for OPT.

For PT, which operate at only 50Hz or 60Hz, the noise, the core heating and efficiency

is the biggest concern. There is nothing wrong with using NOSS E+I if you have the

Bac max at below 0.8T. The magnetizing currents are much smaller than trying to

have Bac at 1.2T, for best GOSS. The lower Bac is not efficient, because the

you need more turns per volt and the wire takes up room so the core for a hi-fi amp PT

will be larger than a PT with same VA rating and used in a factory.

These become available when an old E+I transformer has been retired, or it has fused

and those replacing the transformer find it cheaper to buy a new one from China than

re-wind it.

So, providing old laminations are not rusty after being left in a pile in the rain for 20 years,

You may re-use their cores. DO NOT RE-USE THE WIRE.

During my 18 years as "amp worker", aka audio tech, I acquired a pile of old fused

transformers which I dismantled for re-use of the cores. Some were very well varnished

and it was difficult to separate Es from Is.

The best way to prepare old iron for re-use is

1. Remove all bolts, bell ends, and yokes. Leave the wire on.

2. Use an old oil drum with holes in the bottom to allow air flow and stack fire wood

around the pile of transformers.

I have a fireplace in my lounge, and and I built small fires around the gathered cores.

3. Cremate the cores, and give good wishes to the transformer spirits ascending.

4. Don't breathe the smoke, it is a little toxic because a small amount of vaporised

un-burnt varnish and plastic is in the smoke. My neighbours are not close, and they

kept their windows shut tight on freezing July late nights.

5. The transformers only need to appear dull red, there is no need for more than 900C.

6. Let the fire die down. Do not quell the fire with water.

7. Next day rake out the cool transformers and remove loose ash and dust, and

take them to the workshop where you should have a nice solid wood bench and a

large vice.

8. The roasted trannies will appear kind of partly loose, and you may find the E+I

just fall apart and out from the coil of wire. The wire goes to the bin for scrap to pay

for the beer at the Christmas party.

9. Clean up the mess you make and you will find the lams are ready for re-use.

The oxide coating insulates each lam from the next. But where lams are rusty,

discard them.

10. You should find the magnetic properties are unchanged or even improved by

the slow heat which in effect is annealing.

Testing second hand iron is essential. The magnetic properties of iron from old

transformers does not change over time for GOSS or NOSS.

However MOST old E+I lams you find will be NOSS, and you need to find out the

properties, ie, max µ before re-use. Most µ max is under 3,500,with quite a lot only

2,500, and with low µ the Bac max possible is under 1.2T, so to be sure you don't

end up making a hot running PT, never have Bac higher than 0.8T.

Winding transformers successfully is a skill that must be learnt and practised, like

shearing sheep, laying bricks, sewing clothing, and a winding lathe is needed along

with a great deal of patience and knowledge to avoid the transformer having a short

life, or to find yourself dealing with a tangle of wires and your mind is confused.

Before winding any PT or OPT you should learn to wind a nice layer wound DC

filter choke.

I suggest you read my page at

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This is usually more easily available than T51mm or T44mm material.

Bac = 1.1Tesla, Iac density = 2.2A/sq.mm, 50Hz, 240V mains.

S = Pri VA 377 x 1,000,000 / ( 0.78 x 38mm x 57mm x 19mm x 1.1T x 50Hz x 2.2A/sq.mm )

= 97mm, so use 100mm because this is a standard bobbin size with 38mm x 100mm core hole.

From this the TL = 340mm, SA = 643sq.cm = 99.7sq.in.

With GOSS, expect efficiency = 95%. Total Fe + Cu heat loss = 5% x 377VA = 19W.

SA = 642sq.mm = 99sq.in, W / SA = 19W / 99sq.ins = 0.191.

T rise from

If room temp = 20C, PT temp after 2 hours = 46C.

Np = 240V x 226,000 / ( 38mm x 100mm x 1.1Tesla x 50Hz ) = 260t.

For total Pri input = 377W and mains = 240Vac, input = 1.57A.

Core window = L57mm x H19mm = 1,083sq.mm

From Table 1 for T38 x L57mm x H19mm, wire occupies area = 605sq.mm, and total

Area Cu section, all wires, can be 0.34 x 1083sq.mm = 368sq.mm.

Primary uses 0.52 x 368sq.mm = 191sq.mm, and for 260t, Acu per turn = 0.735sq.mm.

The wire size might be 1.0mm Cu dia for Acu = 0.786sq.mm, Id = 1.57A / 0.786sqmm

= 1.99A/sq.mm, less than 2.2A/sq.mm = OK.

1.0mm Cu dia wire has oa dia = 1.093mm

Bobbin wind width = 53mm, tpl = 53 / 1.093 = 48t. No layers = 260 / 48 = 5.41.

Use 5.0 layers of 48tpl = 240t. Divide into two windings for 120V + 120V for international

mains Vac, using 2.5 layers each.

Turns Per Volt = Vac input / Np = 240V / 240t = 1.000tpv.

Corrected Bac = 1.16Tesla = OK.

Height of primary winding wire including enamel coating..........0.32 x H = 6.08mm.

Height of all secondary windings including enamel coating......0.30 x H = 5.70mm.

Height of all layer insulations.....................................................0.11 x H = 2.09mm.

Height of bobbin base plus clearance for wire bulge.................0.27 x H = 5.13mm.

Height of Pri wire including enamel = 5 x 1.093mm = 5.46mm, < 6.08mm, probably OK so far.

Average tun length TL = 340mm.

Primary RwP = 240t x 340mm / ( 44,000 x 1.00 squared ) = 1.85r.

RwP heat at 362VA input = 1.85r x 1.57A squared = 4.56W, loss = 1.21% for 377VA input, = OK.

25.2Vac Heater winding x 6.6Amps. 166W. 1.0tpv. No turns = 26, for 13V-0-13V with CT,

at no load.

Vac will sag to 12.6V - 0V - 12.6V with load.

Possible winding in one layer has 26t with oa dia = 53mm / 26 = 2.038mm.

Wire table shows 2.018mm oa dia for 1.90mm Cu dia with Acu = 2.83sqmm.

With 6.6A, Id = 2.33A / sq.mm and close enough to wanted 2.2A / sq.mm.

RwH = 26 x 340 ( 44,000 x 1.9mm squared ) = 0.056r.

RwH heat at 6.6A = 0.056r x 6.6A squared = 2.42W = 1.49% of 166W heater power. = OK

190Vac HT winding x 0.89A. 168W, ( nominal, might be more for class AB )

1.0tpv, try 190t. For 2.2A/sq.mm, Acu = 0.89 / 2.2 = 0.40sq.mm, wire size = 0.71mm Cu dia.

Wire table shows 0.71mm Cu dia with 0.79mm oa dia for 66tpl.

For 190t, need 2.878 layers so use 3.0 layers = 199t = 190V.

Height of wire = 3 x 0.79mm = 2.37mm.

Wire for 50Vac bias winding could be up to nominal 0.5mm Cu dia = 0.569mm oa dia.

At 1.0tpv, 52t needed, and may occupy 30mm of bobbin winding width.

But wire size could be 0.355mm Cu dia, 0.393mm oa dia for two layers 48t x 19mm wide

and for 48V-0-48V to make -123Vdc bias, depending on layer room used for HT.

Winding wire heights :-

Primary = 5.46mm,

Heater = 2.018mm,

HT =2.37mm,

Bias = 0.569mm.

Total = 10.4mm, so far, may be adjusted.

The max allowed total wire height can be 11.78mm.

Therefore there is 1.38mm of spare height which should be filled.

The HT winding wire should be increased because class AB operation increases Iac, and low Rw

is wanted because of high Iac pk in diodes to make B+.

Try 0.90mm Cu dia, 0.99mm oa dia = 53tp. Turns for 190V = 190t, so 3.58 layers are needed,

with 3 layers at 53t, 1 layer at 31t.

The 31t occupies 31mm of bobbin width, There is 22mm of bobbin width which can be filled with

2 layers each 19mm wide with 0.355mm Cu dia wire having oa dia = 0.39mm.

Height of HT winding including bias winding = 3 x 0.99mm = 3.96mm.

Revised winding wire heights :-

Primary = 5.46mm,

Heater = 2.018mm,

HT + bias = 3.96mm,

Revised Total = 11.438mm.

The max allowed total wire height can be 11.78mm. Is OK so far.

Nomex 401 Insulation heights :-

Within 5 primary layers, 4 x 0.05mm.......................0.2mm

Pri to Heater 1 x 1.0mm..........................................1.0mm

Heater to HT 1 x 0.38mm........................................0.38mm

Within 4 HT layers, 3 x 0.05mm..............................0.15mm

Cover insulation over all windings 1 x 0.25mm.......0.25mm

Total insulation height............................................1.98mm.

This is 2.09mm less than max height allowed for all layer insulation = OK.

Total revised heights for 377VA PT :-

Primary 5 x 1.093mm.................................................................5.46mm

0.05mm insul x 4........................................................................0.20mm

1.0mm x 1 insul..........................................................................1.00mm

Heater 1 x 2.018mm...................................................................2.018mm

0.038mm insul x 1......................................................................0.38mm

HT 4 x 0.99mm...........................................................................3.96mm

0.05mm insul x 3........................................................................0.15mm

(( Bias = 2 x 0.393mm, +0.05mm insul, within last layer of HT, so no height increase. ))

0.25mm insul x 1 cover over all windings...................................

Total height of all bobbin content..............................................

Clearance including bobbin base = 19mm - 13.206mm = 5.74mm = more than 5.13mm in above list = OK.

Therefore designed windings will fit into window OK.

HT Rw = 190t x 340mm ( 44,000 x 0.9mm squared ) = 1.81r.

If nominal idle Iac = 0.89A, heat loss = 1.81r x 0.89A squared = 1.43W, but high peak charge currents

in diodes may cause losses = 3.2W.

Bias winding heat = negligible.

Total Rw heat losses = Pri 4.56W + Heater 2.42W + HT 3.2W = 10.18W.

Core weight = 883cc x 7.6gm / cc = 6.72kg.

See Table 2, GOSS loss / Kg = 0.6W/Kg. Core loss = 6.72Kg x 0.6W / Kg = 4.03W.

Total losses = 14.21W.

Transformer SA, surface area in square inches = 99.7sq.in.

Heat W / sq.inch = 14.21W / 99.7sq.in = 0.1425W / sq.in.

See Graph 1, T rise = +20C.

If ambient T = 25C, PT temp = 45C = OK.

Draw the PT bobbin details.

Fig 3.

This PT needs to have a GOSS or CRGO core, such as M6 E+I wasteless lams.

A NOSS core would run too hot.

Here are 3 useful tables for Ea and Iadc idle conditions :-

Ea = Vdc anode to cathode, KT120. B+ at OPT CT = Ea for fixed bias, approx |
360 360 |
400 400 2 |
440 440 489 |
480 480 536 |
520 520 583 |

Idle Pda+g2, each KT120, W |
40 |
40 |
40 |
40 |
40 |

Idle Pda+g2, 4 x KT120, W |
160 |
160 |
160 |
160 |
160 |

Idle Ia+g2, ea KT120, mAdc |
110 |
100 |
90 |
80 |
70 |

Idle Ia+g2, 4 x KT120, mAdc |
440 |
400 |
360 |
320 |
280 |

Vac for HT winding, Vrms approx |
140 |
155 |
170 |
185 |
200 |

Pdc from PSU at max Idc, W | 160 |
160 |
160 |
160 |
160 |

Idle Iac for HT winding, Arms |
1.24 |
1.13 |
1.02 |
0.90 |
0.80 |

Class A Po max, One channel, W | 34 |
35 |
36 |
37 |
37 |

B+ at OPT CT = Ek + Ea for cathode bias approx Ea = Vdc from abode to cathode. |
395 |
442 |
489 |
536 |
583 |

Idle Pda+g2, each KT120, W |
40 |
40 |
40 |
40 |
40 |

Idle Pda+g2, 4 x KT120, W |
160 |
160 |
160 |
160 |
160 |

Idle Idc, ea KT120, mAdc | 110 |
100 |
90 |
80 |
70 |

Idle Idc, 4 x KT120, mAdc | 440 |
400 |
360 |
320 |
280 |

Vac for HT winding, Vrms approx | 152 |
170 |
188 |
206 |
224 |

Pdc from PSU at max Idc, W | 175 |
176 |
176 |
174 |
165 |

Idle Iac for HT winding, Arms |
1.24 |
1.13 |
1.02 |
0.90 |
0.80 |

Class A Po max, One channel, W |
34 |
35 |
36 |
37 |
38 |

B+ at OPT CT = Ea, fixed bias only | 360 |
400 |
440 |
480 |
520 |

Idle Pda+g2, each KT120, W | 28 |
28 |
28 |
28 |
28 |

Idle
Pda+g2, 4 x KT120, W |
112 |
112 |
112 |
112 |
112 |

Idle Idc, each KT120, mAdc | 77 |
70 |
63 |
58 |
53 |

Max Ia+g2, AB1, each KT120, mAdc |
150 |
150 |
140 |
130 |
120 |

Max Ia+g2, 4 x KT120, mAdc | 610 |
600 |
570 |
540 |
520 |

Vac for HT winding, Vrms approx | 140 |
155 |
170 |
185 |
200 |

Pdc from PSU at max Idc, W |
220 |
240 |
250 |
260 |
270 |

Max Iac HT winding, at max Idc Arms | 1.7 |
1.7 |
1.6 |
1.5 |
1.5 |

Class AB Po max, 2 Channels, W | 90 |
100 |
110 |
130 |
150 |

Index Page