What's causing my old-school Galien-Krueger GMT 300B to drop volume when i pick hard?

[AndyDufresne]

Hello everyone, I have a Gallien-Krueger GMT 300B from the '70s. The problem is that with lower volume and gentle picking the amp sounds great. However, when you strum hard and increase the input signal the amp kind of fades and the volume drops in proportionately with the strumming.

When I run the "effects send" to the "effects return" I get the same problem. If i put a signal into the "effects return" the amp works fine, so I'm assuming that it's a problem in the preamp. When I run the "effects send" to another amp it gives me the same dropping volume effect.

Thanks.

[Enzo]

Without me looking it up, does that one have a compressor? And can it be turned off?

[AndyDufresne]

It has switches for "Contour" "Current Lim." "Bright" and "Inp. Atten.", but there is nothing labeled compression.

[J M Fahey]

The "Current Lim." might be a limiter or Compressor.
And yes, old FET compressors (the kind used way back then) usually had such problems.
That's why most manufacturers stopped using them.
SWR continued for many years, had the exact same problem, and most bass players preferred to switch them off and cope with the buzzy clipping.

[AndyDufresne]

I'm not familiar with how a FET compressor would work. Is it just a transistor that it will only allow a set amount of signal through?

The switch has 10, 5, and 2 as settings. To me it looks like the "10" setting is off, because the terminals on the switch have nothing connected to them on the "10" side. Would these numbers represent amperes? So it is limiting the current to either 10, 5 or 2 amps?

Any advice on how I could test the circuit? Any advice on how I could disable the circuit? I've found it hard to locate any schematics.

[J M Fahey]

Email them stating Model and Serial Number and politely ask for the schematic.
They usually send it.
Post it here, so we all talk about the same.
Then we continue.

[teemuk]

There is no built-in compressor/limiter in those amps.

The current limit feature is an user adjustable VI limiter for the power transistors and therefore naturally in the power amp section.

There are lot's of switches and jacks in the signal path (e.g. "input attenuator", "boost/distortion") that could have corroded along the years.

I'm more inclined to believe it's a failing component like a capacitor, JFET or a generic bipolar transistor.

I have a faint memory I posted some links to archive of old GMT/G&K schematics into the schematic requests section. There should be three ungodly large .pdf files titled "Old Products" from which you'll also find schematics for 300B model. As said, the files are simply way too large to attach here. Search for the link.

[AndyDufresne]

I cleaned the all the pots, switches and jacks with electronic cleaner.

Here are the schematics that GK sent me.

300B Bass Preamp 60002.pdf300B-300G Early Power Amp 60000.pdf

[J M Fahey]

Yes, no compressors there.
And the Curr Lim *is* a variable current limiter.
Not clear what it was used for.
It allows signal clipping in the power stage at less than maximum power, not too interesting in an SS amp.
Oh well.

[AndyDufresne]

Any advice on which components or circuits I should check? Any theories on what could cause this?

[J M Fahey]

Yes.
Rather than check "for bad components" , of which there may be none, we must make a functional test.
It's a 6 gain stage preamp.
Some stages provide raw gain, some provide frequency sensitive gain, meaning equalization, but all have the same core structure: a single stage, common emitter transistor (Bipolar or FET), with approppriate biasing resistors and a coupling capacitor in series with the input.
All of them are capable of rectifying the audio gignal and blocking, or at least losing gain .
It will be difficult to happen on the first two, Q1 and Q2, because they have 220K resistors in series with their gates (and Q1 is DC coupled) plus they have a very "strong" biasing system which is not easy to disturb.
But from Q3 on, it may happen.
I suggest you scope Q3/4/5/6 collectors and play your bass, at stage levels.
I'm sure one of them (or more) will show an important DC voltage shift in sync with your playing.
You might also see signal clip, assymetrically, and with a varying clipping point or duty cycle.
Such unpredictable behaviour made me switch to Op Amps already in the early 70's, when most anybody else was still going discrete.
Of course, do not use either the Distortion nor the Boost, which will make the problem worse.
I also think that your bass signal must be hotter than what GK imagined way back then.

PS: what about the Contour switch?
On/Off, does it affect the volume "pumping"?

[AndyDufresne]

Unfortunately, I don't have a scope. Is it feasible that just going ahead and replacing the Q3/4/5/6 transistors will give me a good chance at solving the problem?

Are the coupling capacitors just there to block DC?

I'm guessing that if one of the biasing resistors was bad then the transistor would just be constantly open or shorted and wouldn't cause this kind of effect. Is that correct?

Also, the schematics note that these transistors should be MPSA18, but on my unit they are TIS97. The TIS97 seem harder to locate than the MPSA18.

[J M Fahey]

No.
As I said, I don't think there are "bad" parts there, specially the transistors.
Don't change parts needlessly.
You will solve nothing, and probably cause real problems.
If you don't have a scope, sorry, this is one of the cases where it's needed.
*Even* if you have one, this basically confirms what you hear, but knowing where and how much it happens may lead to some suggestion ... which may work or not.
This amp is not broken, is "as-is", an old design.
You didn't answer what effect, if any, had the controls on the problem.
If possible, post some gut picture, would love to see the internal chassis space distribution, whether the preamp is on a separate PCB, etc.
The coupling capaitors are there to block dc, correct.
No biasing bad, or the amp would sound funky all the time, not just on signal peaks.
And I still suspect a bass signal hotter than what was common in the early 70's, plus today's bass players pop, tap, slap, do all kinds of percussive and peaky stuff on bass, while in that age most (as in 99%) just played smooth bass lines.
Try to find which control settings trigger that problem.
Good luck.

[AndyDufresne]

Here are some pics. I'm seeing if I can get a hold of a scope.

[J M Fahey]

Hi Andy, thanks a lot.
INCREDIBLY well built amp.
Parts of the best quality.
I had asked for pictures to see the internal space, "modern" GK are ultra-compact and do not allow for Mods, other than altering some part value.
Yours has a lot of space and, more important, separate boards so you *may* build, say, a modern GK preamp or whatever you want and stick it there .
Only, I don't think it's worth it.
I see a killer amp now, with a minor inconvenience.
Maybe by scoping we find what happens where and maybe, by adding a diode or something we can compensate for it , but otherwise I wouldn't touch it.
Remember Led Zeppelin'd Bass player, after using his wall of Acoustic 361 from '69 to '77 (their "Golden Age") , then switched to GK600B (same as yours but higher power) for 3 more years.
Not a bad endorsement !!
John Paul Jones » Gallien-Krueger GMT 600B AMplifier