MING-DA 80W PP
Picture 1. Ming Da, made about 1996, re-engineered 2011.
Picture 2. View under chassis after re-wiring amp.
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
Many circuit connections were done using 4mm brass spacers
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
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
3 of 4 series electros being much too high. Chinese
resistors of 1990s were not
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
- 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
ripple current is very low.
I retained the use of Si rectifiers throughout. For +1,200Vdc,
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
properties of OPT.
Then I built a much better circuit of my own with all soldered
and boards placed to make all connections and parts and service
accessible. I removed all 4 existing biasing pots which
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.
Listeners here gathered around "The Mingies" when I had them
Music was what everyone expects from a good triode amp - just
The OPT quality was poor by my standards. The turn ratios were all
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
and bandwidth and stability bad.
The original OPTs had 3 output terminals meant for common, 4r and
If 4r speakers or 8r speakers were used where intended, the anode
RLa-a becomes way too low at about 8k0, when for best operation it
never lower than 19k0, where Ea is a high 1,200Vdc. So what went
Chinese minds during design? Quite a lot, it seems.
So I re-arranged the labels on OPT so 8r speakers are used across
labelled 4r portion of OPT sec winding, 4r speakers are used on
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 shows 845 Ra curves with the PP load lines drawn for
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
up to 80mA and down to 0.0mA, ie, +/- 40 mA pk.
The Irms value = 40 x 0.707 =
This is the maximum class A Iac change in the RLa-a for the two
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
of 0.25 x RLa-a = 4k75.
The straight line EQD is the class A load of each 845, and has a
of 0.5 x RLa-a = 9k5.
The lower RLa-a becomes, the lower the class B RLa load ohms,
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
The reformed amp has RLa-a = 14k, and includes total P&S
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
If 4r0 speakers are used on these amps using original labelled
- 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
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
With Ia = 40mAdc at idle, each 845 idles with Pda at a comfortable
Their rating is for 100W, but only a fool would idle any 845 at
and 70W is what I consider maximum safe level, and where only class
wanted. But sooner or later, someone will connect low Z speakers
wrong output terminals and this will overheat the already fairly
at high levels.
If the reformed Ming-D are to work in pure class A only, their
must be 51k! But you get a very nice 41 Watts.
But for pure class A with 4r0, the OPT should have a highest
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
Com to 4r0.
If pure class A was wanted, the lowest speaker load usable =
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
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
The amp makes at least an initial 10W before AB action commences.
The crossover to class AB with all triode amps is usually
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
The total RLa for each 300B = 39k, and with Va = 134Vrms,
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
feed to 300B anodes, and with series R between ends of choke winding
anodes. This would allow about 20mAdc in 300B which would mean
But as is shown, the 300B work with B+ between -110Vdc and
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
balanced by a VR1 10k, 3W wire wound. Should anyone build the amp
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
the VR1 should be able to reduce the higher Ia and increase the
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
The owner didn't get a refund, and was most disappointed.
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
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
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
Its now 2014, and its been 3 years since I reformed the Ming-Da
owner has had no problems.
Should anyone try to build a new PP class AB amp with 845, they
find it a big challenge.
Here is a sample of what I might do if I made a new amp with PP
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
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,
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.
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
R28 & R29 prevent the choke shunt C and its shunt inductance
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
µ. 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
The pair of EL34 require 33Vrms grid to grid for amp clipping.
The 6SN7 input stage and following 6SN7 LTP both have gain of
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
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,
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,
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
reduced from +1,200Vdc to +1,050V as shown in my page for single
The parallel pair of SE 845 produce 55Watts of pure class A with
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,
55Watts will actually be slightly Class A2.
Fig 3 shows a simpler amp than Fig 2 or the Fig 1 reformed Ming
Fig 3 has operating conditions for pure class A :-
Ea = 1,050Vdc, Ia = 72mAdc, Eg1 = -155Vdc, idle Pda each 845 =
For Po 55Watts, Va-a = 1,148Vrms, and Vg-g = 218Vrms, ie,
at each 845 grid.
The use of EL84 in triode for the V2 &V3 balanced amp driver
very similar operation to EL34 with nearly same Ea and Ia conditions,
EL84 gives more than twice the gain of EL34.
This means the gain required by an input stage need only be about
and use of 12dB GNFB will give Vin sensitivity of 1.8Vrms for
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
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
The input for clipping with no GNFB = 0.46Vrms.
V1 6DJ8 could be replaced with 6CG7 without any changes to the
Because there is a cathode CCS the Iadc for V1a & V1b will
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
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
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
The use of carbon anodes is discussed, and from what I read, only
temperature bright emitter thoriated tungsten filaments should be
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
leave it positively charged and the the heavy gas molecule rushes
towards negative cathode at high enough speed to damage oxide
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
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
But then carbon plates have been around a very long time and I've
read anything bad about 845 and many other carbon anode tubes in
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
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
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
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
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
is still the best you might buy.
I do not accept that each time someone launches a new 845 on a
market it means that 845 is better than anything produced since
A recent maker ELROG in Germany has started making 845, $1,000
pair, and my advice is to stay with Shuguang B for a couple of years
if there's a flaw in ELROG reliability.
Usually many boutique tube makers will never admit to their tube
The OPT with 24k0 anode winding must be extremely carefully wound
low shunt C and with very good insulation.
Sec outputs should be for for 3r0, 6r0, and 12r0, so TR = 89:1,
The OPT must have total winding losses in class A1 of less than 5%
that if class AB is used for a lower minimum RLa-a of 15k,
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
each anode, and 13.6Vrms at each 2A3 grid.
The input stage might have been a single smaller triode, 6J5, with
with 1: step up so its anode Va = 13.5Vrms, and its input would be
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
distortion down to acceptable levels using moderate amounts of
Be my guest if you want ONLY real DH triodes in your amp,
Expect huge problems with hums and microphony and difficulties
biasing etc. Expect high THD and IMD, and remember, in 1935, the
best OPTs and
ISTs were quite primitively designed and E&I
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
less awful things to music than grandfather's efforts.
Rear end of Ming-Da amp.
Notice the TWO PTs side by side.
If Ming-Da really wanted to make a good amp, they would have
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
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
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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