Marshall 9200 adjustments

[Kurtsa2]

Hello all,

I decided finally change the power tubes of my Marshall 9200 dual mono block power amplifier. One question came up when I opened the amp. There is two trimmers (for each channel) inside the amp: one for bias adjustment and another for somekind of balance adjustment for the output stage (RV2 above the V2 in the schematic below). (The second trimmer seems to be varying the gain of the signal path going to inverted pair of the output tubes.) Does anyone have a suggestion how to tune the latter trimmer? First I was thinking to tune it by ear, but there must be better ways (that's too easy :) ). One possibility is to tune it with sine-generator and oskilloscope (preferrably with low frequency spectrum analyzer for the dynamic range), but that would need me to take the amp to my workplace during a long nice holiday (I have a oskilloscope here at home but no signal generator). Does anyone have a service manual for this?

The schematic can be found here:
http://www.drtube.com/schematics/marshall/9100-60-02.pdf

_
Kurtsa

[Mars Amp Repair]

I have no first hand experience with this amp, but the balance pot does seem to be adjusting the gain of one phase of the drive signals to the power tubes as you ascertained. I agree an oscilloscope would be necessary to set for equal clipping point on neg & positive sinewave at full output under load, or possibly just equal output at about half full volume.

You wouldn't necessarily need a generator for that....you could just use a music audio source like the radio. Then set the horiz time on the scope for a slower scan rate to compress the signal display...just makes it easier to determine the plus max & negative max from a complex waveform.

Interesting. I haven't seen a balance pot like that since the early days of solid state hifi amps. glen

[Enzo]

Set the balance pot by ear. No signal. Listen carefully to the speaker. Turn that control until hum is minimized. That is the balance point. Then adjust the bias to suit, and after that go back and recheck the balance. It was their way of balancing the tubes instead of expecting you to have matched sets. 20 years ago, matched sets were not NEARLY as common as they are now. it is nothing high tech, nothing cosmic. In fact, many amps over the years have had "hum balance" controls like this. In addition to the ones on the heater 6v.

There are no "service manuals" for these amps. Schematics are all there is.

[Mars Amp Repair]

Enzo,
This balance control is on the P.I. plate side of the coupling capacitor...as far as I can tell it would not affect the 'bias balance', just the 'audio drive balance'...it's more like a gain balance adjustment, unlike the Fender 'bias balance' arrangement.

I'm not sure you would hear any difference adjusting that pot with no signal... glen

[Enzo]

Sorry, must have a different circuit n mind. I should have gotten out the print instead of trying to remember everything.