Maximum safe class AB1 Power,
= 0.125 x
( [ RLa-a / 4 ] + Ra ) squared
OPT-2A, RLa-a minimum
= 5,076 ohms, Ea = +500V,
Ra at Eg1 = 0V = 846 ohms,
Max class AB PO = 0.125 x 5,076 x 500 squared
( [ 5,076 / 4 ] + 846 ) squared
= 0.125 x 5,076 x 250,000 / (
1,269 + 846 ) = 35.45 Watts.
For more than one pair of output
tubes divide above by
number of pairs, for example, if there were 4 x 6550, RL = 16k6 / 2 = 8k3.
PO = 0.5 x Ia squared x RLa-a
where Ia is the Idle Ia dc for one tube.
OPT-2A, Pure class A1 PO max = 0.5
x 0.05 x 0.05 x 16,616 = 20.77 Watts.
Middle RLa-a =
Minimum RLa-a x square root ( RLa-a for Max class A1 / RLa-a safe minimum. ).
= 5k1 x sq root ( 16k6 / 5k1 ) = 5k1 x sq root 3.25 = 9k2.
It is coincidental that
Middle RLa-a for 6550 Beam Tetrode and
Triode have been found to be so close to each other.
62. Calculate PO for Middle RLa-a,
Max class AB PO = 0.125
9,200 x 500
( [ 9,200 / 4 ] + 846 ) squared
This result agrees with Fig 31 above.
63 can be summarized as listed :-
RLa-a Minimum AB1 = 5,076 ohms, PO
= 35W, Va-a = 421Vrms.
RLa-a Middle AB1= 9,200 ohms, PO = 29Watts, Va-a = 516Vrms.
RLa-a Max Class A1 = 16,600 ohms, PO = 21Watts, Va-a = 590Vrms.
The core size can be designed
according to the Middle RLaa PO and Va-a,
and for that the triode OPT is designed by following all the steps after
Step 14, but labelled 14T to 19T :-
Calculate minimum centre leg
cross sectional area,
Afe, for triode PP amp.
NOTE. I have left out references and
steps 14 to 19 above.
Confirm MIDDLE RLa-a and maximum power at clipping, 2 x 6550.
OPT-2A. From Steps above, Middle RLa-a min = 9,200 ohms,
Max PO = 29 Watts.
Afe = 300 x sq.rt ( audio power, Watts ), in sq.mm.
OPT-2A Theoretical Afe, thAfe =
300 x sq.rt 29
= 300 x 5.385 = 1,615 sq.mm
For a square core section,
dimension = Stack height, ie, T = S.
Theoretical T x Theoretical S = th
Therefore theoretical T dimension = square root th AFe = Th T, mm
OPT-2A, thT = sq.rt 1,615 = 40.2mm.
Choose suitable standard T size from list of available wasteless E&I lamination
core materials with assembled E&I plan sizes of :-
T sizes commonly available for
wasteless OPTs :-
20mm, 25mm, 32mm, 38mm, 44mm, 50mm, 62.5mm
The thT calculated =
40.2mm, which indicates the standard T size
38mm may possibly be best, because it is closest to 40.2mm.
But with triode there is
inefficient operation compared to UL or CFB operation.
So T should ALWAYS be larger than the theoretical T calculated.
Therefore T = 44mm should be
If a smaller T is tried, the
weight may be slightly less if the aspect ratio gives a
Stack height more than Tongue dimension. If it is found to be difficult to get low
winding losses with the slightly lower T size, the stack height may be increased to
reduce the number of primary and secondary turns so thicker wire with less
resistance may be used.
standard T size above thT gives lower copper winding losses,
higher weight, and choosing T below thT gives higher losses and lower weight.
Afe must be the same for either T = 44mm or 50mm so the LF response and
Fsat does not change with tongue size. HF peformance depends entirely upon
the interleaving geometry and insulations.
OPT-2A, choose core T = 44mm
Some constructors will
be using non wasteless pattern E&I lams,
or C cores which do not have the same relative dimensions as E&I
Wasteless Pattern cores.
The actual sizes of the T, S, H,
& L of the core to be used
patterns or C-cores have a much larger
for their effective T dimension so that larger wire sizes for less copper
loss may be employed or to give more room for more turns and insulation
layers. Regardless of the core pattern, the ratio of Afe size relative to
Bac max must be maintained.
thS = Afe / T,
then adjust to a larger height to suit nearest standard
plastic bobbin size if available, mm.
OPT-2A, S = 1,615 / 44 = 36.7mm. This may be increased to suit
a standard size bobbin allowing stack height of 38mm. A hand made
bobbin need not be used.
OPT-2A Stack height = 38mm.
T = 44mm, H = 22mm, L = 66mm, S = 38mm.
OPT-2A. ThNp = sq.rt( 9,200 x 29 ) x 10,000 / 1,672 = 3,089 turns.
of the window area approximately = 0.28 x L x H. The constant of 0.28
works for most OPT.
Each turn of wire will occupy an area = overall dia squared.
Overall or oa dia is the dia including enamel insulation.
Therefore theoretical over all dia of P, thoaPdia, of wire including enamel
insulation = square root ( 0.28 x L x H / thNp ), mm.
OPT-2A, th oa dia P wire = sq.rt (
x 66 x 22 / 3,089 )
= sq.rt 0.132 = 0.362 mm
20T. Find nearest suitable overall dia wire size from
the wire size table. oaPdia, mm.
OPT-2A, For core window L = 66mm, Bww = 66 - 4 = 62 mm.
22T. Calculate no of
theoretical P turns per layer.
ThPtpl = 0.97 x Bww / oa dia from step 12.
NOTE. The constant 0.97 factor allows for imperfect layer filling.
Ignore fractions of a turn.
OPT-2A, thPtpl = 0.97 x 62 / 0.371 = 162 Primary turns per layer.
23T. Calculate theoretical number of primary layers.
Then round down or up to convenient even number of layers.
Theoretical N pL = ( Theoretical Np from step 18T ) / PtpL from step 22T,
then round up/down.
OPT-2A, thNpL = 3,089 / 162 = 19.067 layers; round UP to 20 layers
or down to 18 layers.
NOTE. Rounding down may reduce Np and raise Fs above wanted 14 Hz.
But the actual turns used will give low enough Fs, in this case 14.4Hz, less
than a 15% rise above design aim and OK. For those wanting to maintain
Fs = 14Hz, or have Fs marginally lower than 14 Hz, the Afe can be
increased by increasing S from say 38mm to 44mm or more and still be
able to use a standard size of pre-made moulded bobbin 44mm x 44mm,
and have Fs slightly lower.
The calculated number of primary layers should be an even number to avoid
a primary winding CT in the middle of a layer which is awkward to wind,
and because each 1/2 primary winding should have an equal number of
turns and a symetrical geometric layout either side of the CT.
24T. Calculate actual
Np = Number of P layers from Step 23 x thPtpl from Step 22.
25T. Calculate average turn length, TL.
TL = ( 3.14 x H ) + ( 2 x S ) + ( 2 x T ), mm.
where 3.14 is pye, or 22/7, and 2 are constants.
OPT-2A, TL = ( 3.14 x 22 ) +
( 2 x
38 ) + ( 2 x 44 ) = 233
26T. Calculate primary winding resistance, Rwp.
Rwp = 2.26 x ( Np x TL ) / ( 100,000 x Pdia x Pdia ), ohms.
where 2.26 is the resistance of 100 metres of 1.0mm dia wire and a constant,
and 100,000 is a constant, and P dia is the copper dia from the wire tables.
OPT-2A, PRwp = 2.26 x 2,916 x 233
/ ( 100,000 x 0.30 x 0.30 ) = 170
27T. Calculate pri winding loss % with MIDDLE RLa-a,
P loss % = 100% x Rwp / ( PRL + Rwp ), %.
OPT-2A, P loss = 100% x 170 / ( 9,200 + 170 ) = 1.81%.
If YES the design calculations must be checked and perhaps a larger core stack
or window size chosen.
If NO, proceed to Step 29.
OPT-2A, P winding loss is less than 3.0%.
The calculations so far
are based on using the MIDDLE RLa-a.
Under optimal normal operation, RLa-a will be higher or lower than the
MIDDLE RLa-a for class AB1. The winding losses for RLa-a of 5k0
are nearly double at about 3.3% and for pure class A of 16k6, losses will
be less at about 1%, and in all cases losses are low enough.
It is better to have low winding
losses so that the primary windings are
unlikely to overheat if a tube malfunctions and draws excessive Idc during a
"bias failure event". Such occurences were a main reason why so many
OPTs of the past failed so easily after being designed by accountants
rather than engineers who know "shit happens" :-).
29T. Choose the interleaving pattern.
Inspect tables 2, 3, 4, 5 ABOVE for the power from the transformer.
Choosing an interlaving pattern
may entirely bamboozle many readers
or designers who have not much experience with winding audio frequency
transformers for wide bandwidth between about 14Hz and at least 70kHz.
point in the design process for PP triode OPTs,
I will now abandon you all and leave you to proceed
through all steps to a final design.
Back to PP OPT Calc Page 4.