MING-DA 80W PP with 845

Picture 1. Ming Da, made about 1996, re-engineered 2011.

Ming-Da-reformed-front-on-bench-2.jpg

Picture 2. View under chassis after re-wiring amp.
MingDa-reformed-under-chassis-5.jpg
The original circuitry had many parts such as the diode arrays
hanging on "mid air wiring", a common bad practice in China
where use of good boards or terminal strips is avoided to
save expense.

Many circuit connections were done using 4mm brass spacers
with nuts and lugs, so there were many non-soldered connections
which would give trouble in future.

I gutted everything in both amps, but left in useful connector strips
with brass connectors which I soldered up so no connections
could come loose or suffer corrosion.

All resistors were replaced where more than 150Vdc was present
because a couple had gone open, and one had shorted resulting in
Vdc across 3 of 4 series electros being much too high. Chinese
resistors of 1990s were not reliable.
The Chinese communist party runs the industry in China and
sometimes includes fire crackers in its products, but you don't know
where they are, or when they will explode.
The Taiwanese metal film R I have used are much more reliable.
The 845 have B+ = +1,200Vdc. There is a CRC B+ filter using
117uF - 100r - 117uF.
The Vripple at OPT CT = 0.3Vac.
To make up each 117uF, 4 x 470uF/450V are used in series with 220k
across each. The 470uF were retained because they appear to be
reliable, and ripple current is very low.
I retained the use of Si rectifiers throughout. For +1,200Vdc,
I used a bridge using 12 x 1N5408 with diodes mounted on a board,
and all painted with varnish to stop effects of moisture.

I worked out what voltages were available from existing two PTs and the
properties of OPT.
Then I built a much better circuit of my own with all soldered connections
and boards placed to make all  connections and parts and service measurements
accessible. I removed all 4 existing biasing pots which ridiculously awkward
for anyone to adjust and get right. Two were for 300B, but they actually did nothing.

The board at top left is for active protection and 845 Ikdc balance indicator amp.

Fig 1. Reformed Ming-Da schematic.Ming-Da-schem-reformed-85W-845PP-mono-2011.GIF

Listeners here gathered around "The Mingies" when I had them reformed.
Music was what everyone expects from a good triode amp - just fabulous.
The OPT quality was poor by my standards. The turn ratios were all wrong.
Used with original labelled loads, the amps would soon cook the 845 to death
if high levels were used. Distortion, noise, and Rout were all way too high,
and bandwidth and stability bad.

The original OPTs had 3 output terminals meant for common, 4r and 8r.
If 4r speakers or 8r speakers were used where intended, the anode loading
RLa-a becomes way too low at about 8k0, when for best operation it should be
never lower than 19k0, where Ea is a high 1,200Vdc. So what went wrong in
Chinese minds during design? Quite a lot, it seems.

So I re-arranged the labels on OPT so 8r speakers are used across originally
labelled 4r portion of OPT sec winding, 4r speakers are used on winding portion
between 4r and 8r, and 16r speakers are used across whole OPT sec.
This means the RLa-a is doubled, and THD is more than halved,
Rout is halved, and tubes won't cook.

Graph 1.
845-RCA-curves-PP-loadlines-2014.GIF
Graph 1 shows 845 Ra curves with the PP load lines drawn for
Ea = 1,200V.
The minimum RLa-a which I would ever use for Ea = 1,200Vdc = 19k0.
You then get 72Watts of Class AB1 at anodes of 845, and if OPT
winding resistance losses = 7%, you get say 67Watts at secondary
terminals at clipping. The amount of initial class A1 Po depends on
the signal Ia change never being greater than +/- the idle current Idc.
In this case where Ia at idle = 40madc, the signal Ia for class A changes
up to 80mA and down to 0.0mA, ie, +/- 40 mA pk.
The Irms value = 40 x 0.707 = 28.3mArms.
This is the maximum class A Iac change in the RLa-a for the two tubes,
so Class A Po = RLa-a x Ia squared = 19,000 x 0.0283 x 0.0283 = 14.9Watts.

The straight line ADCB is the class B load for each 845, and has a value
of 0.25 x RLa-a = 4k75.
The straight line EQD is the class A load of each 845, and has a value
of 0.5 x RLa-a = 9k5.
The lower RLa-a becomes, the lower the class B RLa load ohms,
and higher the maximum Ia, and % of class A working becomes lower.
 
Now the reformed Ming-Da produced 70 Watts at 8r0 load so anodes
would make 75Watts, but there is a slight drop in B+ because of
increased Iadc across PSU resistance.
The reformed amp has RLa-a = 14k, and includes total P&S winding
resistances of maybe 1k0, so winding losses are just over 7%.
The RLa-a of 14k0 is a bit too low. But my schematic shows the 8r0
used across what was labelled for 4r0 originally.
If 4r0 speakers are used on these amps using original labelled
Com - 4r0 terminals, RLa-a becomes 7k5 including Rw of 1k0 so
winding losses = 13.3%. RLa-a = 7k5 gives B RLa of 1k87 and
Ia peak = 0.31A.
Pda due to signal current will be about 117Watts, and tubes will
over heat with continuous sine waves at high levels.
The B RLa is lower than the 845 Ra of about 2k2. Max Po for 7k5
at anodes = 88Watts, and at output terminals = 76Watts.
Class A Po = 5.2Watts, and while working in mainly class B the
DF is less than 1.
 
The point I make is that there's nothing to gain by using a lower
RLa-a below 19k0.
Using higher RLa-a will give more pure class A working, lower THD,
higher damping factor, lower OPT winding losses and wider bandwidth.
This always gives better sound.

With Ia = 40mAdc at idle, each 845 idles with Pda at a comfortable
48Watts.
Their rating is for 100W, but only a fool would idle any 845 at 100W,
and 70W is what I consider maximum safe level, and where only class
A is wanted. But sooner or later, someone will connect low Z speakers
to the wrong output terminals and this will overheat the already fairly
hot 845 at high levels.

If the reformed Ming-D are to work in pure class A only, their RLa-a
must be 51k! But you get a very nice 41 Watts.
But for pure class A with 4r0, the OPT should have a highest available
ratio = 51k : 4, ZR = 12,750:1, with TR = 112.9:1.

But the existing OPT has 13k : 4r for its highest ZR and turn ratio,
Com to 4r0.
If pure class A was wanted, the lowest speaker load usable = 15.6r.

So these Ming-Da cannot provide best class A performance unless
you have 16r0 speakers.

One might be tempted to increase the Ia for 845 to 58mAdc to have
idle Pda = 70Watts.
Then RLa-a is 33k0. Then the speaker load for pure class used
between 4r & 8r terminals needs to be 10r0.
So using more idle current hardly improves anything, and just
wastes electricity.

The amps sounded fabulous as I have made them.
Average levels for most listeners is less than 1Watt per channel,
with peaks perhaps reaching 30Watts. This amp will cope brilliantly.
The amp makes at least an initial 10W before AB action commences.
The crossover to class AB with all triode amps is usually completely
tolerable because the cut off behaviour is gradual and not many harsh
sounding harmonics are produced, so class AB with 845 sounds better
than say a pair of KT88 in ULAB1 mode.

And who would think 300B work well as drivers with only 6.6mAdc of
anode Idc?

The total RLa for each 300B = 39k, and with Va = 134Vrms,
the Ia change = 4.8mApk.
This is OK because it is safely less than 6.6mAdc of idle Ia.
Originally, the 300B stage had about 4mAdc in each 300B and it was
the first stage to clip, and THD was high. Biasing was all done incorrectly,
with two bias pots having little effect. The low Iadc in each 300B allow
them to not need Iadc to be balanced, and the common Rk R24, 4k7
regulates the Ia of both 300B very well, and with such low Iadc, the
300B run very cool and should last 50 years.
So I was able to delete the two bias pots Ming-Da used.


300B work well to make 134Vrms max at each 845 grid. 
It would have been better to use a choke with CT for balanced choke
feed to 300B anodes, and with series R between ends of choke winding
and anodes. This would allow about 20mAdc in 300B which would mean
less THD.
But as is shown, the 300B work with B+ between -110Vdc and +454Vdc,
and Ea = +254Vdc. The low Ra of 300B allows a max Ea swing of
+/- 230Vpk giving 165Vrms max!

The output 845 have fixed bias and Iadc at only 40mA which is simply
balanced by a VR1 10k, 3W wire wound. Should anyone build the amp
shown, they could change R33 to 12k0, and put a VR2 10k linear wire
wound pot in series to 0V so the total Iadc for both 845 can be adjusted
to equal 80mAdc. I have found this to not always be necessary.
If one 845 Ia goes up to say +60mAdc, while the other is at 40mAdc,
the VR1 should be able to reduce the higher Ia and increase the lower
Ia so they are equal at about 50mA each. This is quite acceptable.
If Ia of one or both 845 go higher than about 100mAdc, then the
protection circuit will turn off the amp.

Chinese Tangin 845 were tried, and all 845 in both amps arced over
with Ea at over 1,100Vdc. I built a breadboard test circuit.
These 845 arced over internally with B+ only +800Vdc.
The owner didn't get a refund, and was most disappointed.
The internal appearance of these crap tubes showed flimsy construction
 compared to the Shuguang B, which I think are best, IMHO.

The amp is fitted with  2 yellow Leds to indicate bias balance, one blue
LED for 'on' = OK, and one red Led for "NOT OK" when the amp is turned
off if the 845 conduct too much Iadc.
The 845 idle Iadc = 40mAdc, so idle Pda = 47Watts, so they always run cool.
The Iadc is easily balanced by turning the head
small black metal knob at
front of amp
which adjusts VR1 pot. This means the owner never has
to remove these very heavy amps from their hi-fi stand, then turn the amp
upside down on carpet and remove a cover, then fiddle around adjusting
pots and probe around circuitry with volt-meter leads with lethal Vdc
present.
Its now 2014, and its been 3 years since I reformed the Ming-Da and the
owner has had no problems.

Should anyone try to build a new PP class AB amp with 845, they may
find it a big challenge.
Here is a sample of what I might do if I made a new amp with PP 845 :-

Fig 2.
80W-PP-AB1-amp-6SN7+EL34+845-august-2014.gif
Fig 2 shows a similar schematic to Ming-Da, with 4 tube stages but
with different OPT, 19k : 2r, 4r, 8r, and EL34 triode driver stage with
Choke Feed and CCS for V2 LTP.
The B+ rail voltages and general schematic has been rationalized to
benefit music, not benefit anyone's profit margin.
No doubt nobody will try to build this amp, it is too difficult.

Fixed bias is used with adjustments for both Idc balance and the
overall level of Idc for both tubes.
A protection circuit is wise, because there are some 845 being
produced now (2014) which may not last very long.
The use of two EL34 in triode, each with idle Ia = 20mAdc,
makes an excellent balanced amp driver stage with maximum possible
Va-a = 360Vrms, and THD < 1%.
The choke L1 is wound using about 5,000 turns 0.2mmCu dia wire on
25T GOSS E&I core with stack 32mm, and it has a CT, and no air gap.
L1 functions acts like the primary inductance of an OPT to give very
high reactance between 20Hz and 20kHz. The resistance loading of
EL34 by choke is negligible between 20Hz and 20kHz.
R28 & R29 prevent the choke shunt C and its shunt inductance from
being directly connected to EL34 anodes at extreme F outside the
F band so the EL34 gain remains at least 7 even outside the AF band
and phase shift by C&L shunting is avoided. The dominant load on each
EL34 anode are R32 & R32, 33k. This RLa of 33k = 25 x Ra of EL34
so that voltage gain approaches . Peak Ia swing at 134Vrms = 5.6mApk,
which is much less than Ia = 20mAdc, and you can expect THD < 0.3%,
mainly 3H, and declining to say 0.03% at 13.4Vrms at typical listening
levels.

The pair of EL34 require 33Vrms grid to grid for amp clipping.
The 6SN7 input stage and following 6SN7 LTP both have gain of about
16, so the total gain = 256. So therefore about 0.13Vrms is needed at
V1 Vg-k for clipping. The global NFB voltage applied to V1 cathode
= 0.5Vrms, so input for clipping is about 0.66Vrms.

To get more class A with lower class AB, a 4ohm speaker may be
plugged in between 0V and 2r0 labelled outlet.
This doubles the Class AB RLa-a from 19k0 to 38k0.
With RLa-a = 38k0, the initial class A increases to 30Watts,
with about 47Watts maximum class AB.

The Ra-a at 40mA = 2k7 + 2k7 = 5k4 and if RLa-a = 38k, then
damping factor without any GNFB = 38k0 / 5k4 = 7.0, and this
could be the highest DF possible for any power triode.
Rout at 2r0 ohm outlet = 5k4 / 9500 = 0.57r. Winding resistance
may raise this to say 0.6r, and with 4r0 load the DF = 4 / 0.6 = 6.7,
quite acceptable.

GNFB need not be used. Then V1 could be eliminated.
About 2Vrms is needed at V2a grid for clipping while V2b grid is grounded.

For slightly better PP pure class A operation, the Ea for 845 may be
reduced from +1,200Vdc to +1,050V as shown in my page for single
ended 845 at
http://www.turneraudio.com.au/monobloc845se55.html

The parallel pair of SE 845 produce 55Watts of pure class A with OPT
primary RLa = 6k0.
Each 845 has RLa = 12k0, and with the same pair used in a PP amp
you can expect the same 55Watts with OPT RLa-a = 24k0.
There is a slight amount of grid current draw above 50Watts,
so 55Watts will actually be slightly Class A2.

Fig 3.
55W-PP-A1-amp-6DJ8+EL84+845-august-2014.GIF
Fig 3 shows a simpler amp than Fig 2 or the Fig 1 reformed Ming Da.
Fig 3 has operating conditions for pure class A :-
Ea = 1,050Vdc, Ia = 72mAdc, Eg1 = -155Vdc, idle Pda each 845 =
75.6Watts.
For Po 55Watts, Va-a = 1,148Vrms, and Vg-g = 218Vrms, ie,
109Vrms at each 845 grid.

The use of EL84 in triode for the V2 &V3 balanced amp driver gives
very similar operation to EL34 with nearly same Ea and Ia conditions,
only the EL84 gives more than twice the gain of EL34.
This means the gain required by an input stage need only be about 26,
and use of 12dB GNFB will give Vin sensitivity of 1.8Vrms for clipping.
The best V1 tube I can think of is 6DJ8, which has to only produce
6Vrms to drive EL84. The V1a & V1b are in an LTP so its a balanced
amp with very low THD.
I show Input to V1a grid and GNFB to V2b grid.
You will find the arrangement functions excellently and gives good sound.
For operation without GNFB, just disconnect R6 & C3 from top of R4.
and 47r then grounds V1b grid.
The input for clipping with no GNFB = 0.46Vrms.

V1 6DJ8 could be replaced with 6CG7 without any changes to the
Fig schematic.
Because there is a cathode CCS the Iadc for V1a & V1b will remain
constant and anode Ea will also remain constant and the 6CG7 will
work perfectly with no change to pin out or to any R values, although
6DJ8 require less heater current.
But 6CG7 gain will be about 16, and this means V1 Vg-g = 0.74Vrms
instead of 0.45Vrms needed by 6DJ8 for clipping.
The drop in gain means the amount of applied GNFB is reduced from
12dB to say 9dB, but that's enough, and because its a class A triode
amp the sound will be fabulous. The input with GNFB will be about
2.1Vrms with 6CG7.
Some people have told me that 6CG7 sound better than 6DJ8,
and to avoid controversy, I always agree.

I should talk about 845 !

In 2008, the SE55 amps I made used KR Audio 845, but I found much
cheaper Shuguang type B were just as good sounding and far cheaper.

There are some very interesting comments about anode and cathode
materials at
http://www.emissionlabs.com/html/products/Warning-Chinese-Fake-mesh.htm
The use of carbon anodes is discussed, and from what I read, only high
temperature bright emitter thoriated tungsten filaments should be used
with carbon anodes, because of carbon evaporation within the tube.
This carbon settles on inside of glass tube.
Higher efficiency barium oxide cathodes like those in a 5U4 or a
300B have been tried in several versions of 845 including KR Audio, and
their dull red color averages 777 degrees C. See page 2, RDH4.
Temperatures of +/- 10% make tubes unreliable, with shorter life.
Such low temperature oxide coated cathodes are prone to positive
ion bombardment where electrons are knocked off a gas molecule
to leave it positively charged and the the heavy gas molecule rushes
towards negative cathode at high enough speed to damage oxide
coating.
Ea should be kept below +900V to avoid ion bombardment.
The higher the Ea, the higher is the positive ion speed.

I don't believe all I read at websites.
Emission Labs say its impossible to get a good electrical connection
from wire leads to carbon anode, which is brittle, so a tensioned rivet
is needed, but that this can cause small cracks from movement during
shipping.
But then carbon plates have been around a very long time and I've never
read anything bad about 845 and many other carbon anode tubes in
OLD BOOKS written before 1960.
Emission Labs rave on about their metal MESH anodes being better
than "punched metal" used by Chinese and cite the woven mesh
does not give rise
to microphony compared to Chinese tubes. Well, I ask then why are
nearly all DH tubes microphonic no matter what the anode material?
The anode in fact does not vibrate very much like a bell to any great
extent. But the cathode wire plus grid wires are flimsy and flexible in
most DH tubes and do move move when the tube vibrated.
If you tap a 300B with a finger, you hear a ding-ding from the tube
itself because of cathode and grid wires move to change cathode
to grid distance which modulates the electron stream so you get
ding-ding at the speaker. If there is 12dB GNFB, this noise by tapping
tube is reduced -12dB.
But sound from speakers usually is not enough to cause significant
microphonic effects greater than other phenomena such as THD, IMD,
and emission noise.

So, here I am in 2014, and I might bravely say the Shuguang type B 845
is still the best you might buy.
I do not accept that each time someone launches a new 845 on a fickle
market it means that 845 is better than anything produced since 1930.
A recent maker ELROG in Germany has started making 845, $1,000
a pair, and my advice is to stay with Shuguang B for a couple of years
to see if there's a flaw in ELROG reliability.
Usually many boutique tube makers will never admit to their tube failures.

The OPT with 24k0 anode winding must be extremely carefully wound for
low shunt C and with very good insulation.
Sec outputs should be for for 3r0, 6r0, and 12r0, so TR = 89:1, 62.5:1, 44.5:1.
The OPT must have total winding losses in class A1 of less than 5% so
that if class AB  is used for a lower minimum RLa-a of 15k, winding
losses do not exceed 5%.

How did grandfather use 845 in 1935? I think he was 25 years old.
He may have used a pair of 2A3 to drive a 1:2 step up IST between
2A3 and 845, so 2A3 would have produced 109Va-a, or 54.5Vrms
at each anode, and 13.6Vrms at each 2A3 grid.
The input stage might have been a single smaller triode, 6J5, with IST
with 1: step up so its anode Va = 13.5Vrms, and its input would be
about 0.9Vrms.
With so many transformers of questionable quality, perhaps he got
bandwidth of 80Hz to 7kHz if he was lucky.
The use of GNFB would have been impossible because of phase shift.
But within 12 years a Mr D.T.N Williamson taught the world how to
make better OPTs and how to get rid of ISTs and how to get noise
and distortion down to acceptable levels using moderate amounts of
GNFB.

Be my guest if you want ONLY real DH triodes in your amp,
and without GNFB.
Expect huge problems with hums and microphony and difficulties with
biasing etc. Expect high THD and IMD, and remember, in 1935, the
best OPTs and ISTs were quite primitively designed and E&I
laminations generated as much THD or more than did the tubes.
The use of GOSS cores, and the more modern driver tubes including
pentodes strapped in triode, plus better filtering of B+ supplies
all do less awful things to music than grandfather's efforts.

Rear end of Ming-Da amp.
Ming-Da-reformed-rear-on-bench-3.jpg
Notice the TWO PTs side by side.
If Ming-Da really wanted to make a good amp, they would have
used just ONE good big PT.
Notice the Chinese writing on labels at rear of amp.
Notice the left Chinese label which has "220V" written.
But we have 255Vac some days. The working voltages I show on
my reformed are with about 250Vac mains. I don't what would
happen if mains was 220V, I guess all Vdc rails would reduce by 1
5%, so instead of 1,200Vdc B+ it would be +1,056Vdc.
I found heater voltages slightly high, and with 220V maybe they'd
be slightly low. I biased the 845 for only 40mAdc to avoid
overheating, especially with the original OPT strappings which
has the RLa-a much too low.

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