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/mar...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.

[Davedoc7]

Having almost the same question, as I'm re-tubing one, for a guy. This is one awesome beast. There are a couple of thoughts on the biasing procedure. I expected to do the math, from the plate voltages, and then compare it to what Marshall, set it at the factory, as I understand it was -50volts on pin 5, of the 6L6. Then, if that was not desirable, a -40, says even better. I do have the current tube tool socket, that plugs into the fluke meter, and expect to use the 70% rule. .7 x 22wattsmax/plate voltage, of pin 3. Any recommended amplitude, of injection sine wave, and at what volume level, on the amp, to set the Balance, for each amp, within, one box, so to speak? As I've read, if I understand correctly, the balance, is to equal, the positive, and negative portions, of the sine wave. It has been modded by Voodoo Amps, with all Mercury Magnetics, transformers, and then some. Have a call into them, but holiday season, not sure when they'll get back. Rather be perfect, on this big boy, and ask plenty of questions. Some, speak of putting up near 1/3 vol. but still no mention, of amplitude of injected sine wave, if that is indeed, the proper procedure. I've scored the schematics, off the web, but no mention of the actual, proper procedures, with this one. Figured you guys, have crossed this bridge already, and very much, willing to listen, to all info. Thank you, and much respect. Dave

[The Dude]

As I recall, the balance adjustment is to adjust positive and negative halves of the waveform so that both clip at the same point. It's been a while since I worked on one of these, but my recollection is that I ran the amp into a dummy load, turned the amp up until one half of the wave form clipped and then adjusted so that +&- clipped at the same time. Of course this method requires a dummy load and a scope. I'm not sure the adjustment is that critical if you don't have the test equipment. If you don't, I would just leave the balance adjustment as is.

[Davedoc7]

Thank you, Dude. I've not touched anything yet, other than, resolder the board, and research, and am still going to take readings, etc. to learn anything, I can. I'm used to seeing the Hum balance, like Enzo, spoke of, on the Fenders, but this amp, and this customer, I want to go above, and beyond. I'm picky, that way, anyway. I was ready to scope my guitar signal level, and then feed that level into the input, with my sine wave generator, around 1khz, and see what I saw, on the scope, . Have a set of dummy loads, that I built, for amp testing, huge ceramic looking things,(looks like two sticks of dynamite, with wires, mounted on a 2x4, but been gun shy about using them. An 8 ohm load, is an 8 ohm load, isn't it? Never could find the kind of dummy loads I was looking for, so I just built these, but didn't want it building up any kind of weird problems, with this amp, of all things. Used to mainly use them rebuilding blown up car amplifiers, years ago. Any thoughts on that? These were adjustable resisters with a ceramic coating, with sliders, for making for adjustable ohms. About a foot long, each, and are held in place with welded hardware, I constructed, so they could set into place, and not conduct on anything. Otherwise, was just going to hook it to my 4-12 cab, (8ohm cab)and put up with the noise, during adjustment time. Good chance, I'll leave it alone. I don't like to fix something that doesn't need fixing. From the looks of some of the tricks inside, I've conquered a few things, that I don't let go, already. Just like to know absolute numbers, and so many times, I wish I'd done the EET program at Devry, instead of the technician. That being said, I do like to get in, get out, and get away from it, and get it up, on line, as fast as safely possible. Good ole professor's words from 1T. Thank you, again. Dave

[The Dude]

I don't know what your dummy loads are rated at, but you can check it with a meter to find resistance since it's a totally resistive load. They also must be able to handle 100W minimum. Let the amp warm up a good long time first so you don't have to re-check it often. It shouldn't take that long to make the adjustment. Sure you could use speakers for a load, but it's going to be loud! You could also probably run the amp at some lower power output and adjust the two halves of the waveform to be equal. Using this method, we're assuming the tube output is linear all the way up to clip, which may not be the case. It ought to be close enough. At any rate, it's a guitar amp- not some hi-fi piece, so I doubt any slight asymmetrical clip on either side would be noticeable. And, for pity sakes, if anybody is running a dual 100W guitar amp at clip, I don't want to be in the same room.