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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Dynaco MKIV 7199 driver circuit

Can anyone who is familiar with the 7199 driver circuit, (used in Dynaco MKIV and ST-70 amplifiers), tell me what the purpose of the .05uf capacitor, (connected between pin 3 and pin 6 of the 7199 driver tube), is? I have been using a pair of cloned MKIV's, for several years, and have enjoyed the sound immensely. Just recently, the left monoblock amplifier sounded dull and became lower in amplitude. Using a signal generator, and a scope, I can see that the output of the left amplifier drops off above about 4KHZ rapidly. Resembling a low pass filters response on the oscilloscope. I tried all the usual tips. swapping tubes, checking solder joints, etc.
Through trial and error, I have traced the problem to the .05uf capacitor shown on the schematic ,that is connected between pin 3,(grid 2 of the 7199 pentode section), and the pin 3 cathode. The capacitors value measures fine with a capacitance meter, but removing it completely from the circuit restores the frequency response to normal. No roll off, clean waveform, etc. I am baffled why this is happening. Can someone tell me what the purpose of this capacitor is? Feedback stabilization?
Could I simply remove this capacitor from both amps from the circuit without ill effect?
I only looked at the output on the scope, with the .05uf cap unsoldered from the board. I did not feel confident in listening to music with that part removed.
Any help appreciated.
Thanks in advance.
 

Koonw

Member
2013-04-09 9:37 pm
I quoted from http://www.valvewizard.co.uk/pentode.html:

"The screen bypass capacitor can be chosen according to:
Cg2 = 1 / (2 pi f Rg2)
If we want to pass all audio frequencies without any loss of gain then we might set f to 10Hz or therabouts: <
Cg2 = 1 / (2 pi × 10 × 390000) = 40.8nF
A more common standard value is 47nF."

Since Cg2 and Rg2 form the timing constant together with screen impedance of g2, so you also check the value of Rg2 in ST70 which 1.5M has drifted or not. If you remove it you'll lost some gain but could be no harm. If Rg2 and and Cg2 is normal, then the tube 7199 could be abnormal.
 
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Found this I wrote down in 2011:

Sound Practices Issue 10, p35. Matt Karma says that most ST-70 & Mark IV
need 7199 replaced as there is Too Much Heater Cathode Leakage of 100-135V, were it should be 75V above ground.

Put a 390K/1 Watt resistor between the B+ & filament center tap + 82K resistor between the center tap + ground.

Add a 1 uF cap parallel to 82 K R (1 uF cap will reduce ripple at full power)
 
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... most ST-70 & Mark IV need 7199 replaced as there is Too Much Heater Cathode Leakage of 100-135V, were it should be 75V above ground.

This assertion is likely false. Not that it matters much anymore, because the RCA 7199 is essentially extinct, but the RCA datasheet lists a heater negative wrt cathode spec of 200VDC.

A more significant issue with similar tubes in similar circuits is poor control of pentode plate voltage. It's important for this voltage to be within 25-30% of B+ or performance will suffer, and there's precious little margin in some amplifier designs. This is one place where tube rolling can yield real differences. My personal bias is that it's more productive to make the screen dropping resistance variable instead. This solution is particularly useful when subbing the 7199 with more readily available types.
 

rayma

Member
2011-04-29 8:37 pm
it's more productive to make the screen dropping resistance variable instead.
This solution is particularly useful when subbing the 7199 with more readily available types.

Exactly, and instead of the 1.5M screen resistor in the ST70, use 500k in series with a 1M cermet pot.
Adjust the pot for the best phase splitter performance, which is at about 120VDC at the cathode of the triode section.