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

[52 Bill]

Do you have a voltmeter? Have you tested the 35 volt power supplies to the preamp? Try monitoring the preamp power supplies when the signal drops out. Does the supply voltage drop with the signal?

[AndyDufresne]

That's good news. I originally bought it because our bass player needed a head and we couldn't afford a working one. I bought this one for $30 and was hoping that it was going to be a more obvious fix, like a bad solder joint or a big leaky cap.

I really like the sound I kind of wish it was the G300 with the reverb. It would make a good head to keep at the house. I really don't mind the troubleshooting though because I'm learning a lot. I love to fix things anyways.

Could you give me an explanation of how the diode could help? The diode would block current in one direction right? So you think that current might be flowing the wrong way somewhere? Are you thinking that the base lead on one of the transistors might need a diode in front of it?

[J M Fahey]

Don't hurry up.
I'm just thinking aloud, and, of course, I can imagine a couple things which can happen there, but can't suggest a medicine unless I'm certain about the sickness.
Example: short ago I went to see the Doctor for a numb pain in my left leg.
He examined me , then wrote a list of tests he wants done: various body fluid chemical analysis, XRays, ElectroMyography, renal and liver tests, the works.
I asked : "Dr, what may it be" he answered "bring the test results and we'll talk"
Repeat dialogue 3 times.
The 4th time he gave me a 600 page book covering about 2500 or 3000 illnesses: "it's *probably* there ... you want me to be more specific? ... there's the door ... go away and come back with those tests."

And so far my (presumed but not confirmed) diagnostic is "signal rectification shifting the bias point" ... but I need your results.

[AndyDufresne]

I do have a voltmeter. But I was wondering if you could help me a little with the schematic? Tell me if I have this correct.

So, there is a rail that runs throughout the preamp that is labeled +36 V. After this voltage is reduced by resistors to 14, 17 or 18 volts it goes to the collectors of all the transistors.

Voltage on the base of these transistors controls the flow of current through the transistors and out the emitters. The emitters are all connected to chassis ground.

This is where I get a little confused. I thought that the transistors acted like a gate of sorts. So, there is gain coming out of the emitter? Then shouldn't there be a load between the emitter and ground?

For Q1 and Q2 the emitter goes to -36 V, while Q3,4,5,6 all go to chassis ground. I'm not sure why.

[AndyDufresne]

The contour switch does not seem to affect the problem. Neither do any of the other switches. It appears that the problem is linked to the distortion. Initially, the problem did not surface. But after a few minutes of playing the problem showed up again.

I had not noticed before, but when the volume pumping occurs the sound is only distorted. When the volume problem is briefly not there, the sound is only clean. The distortion pot does not change the tone in either circumstance.

The volume pumping issue can be resolved by turning down the volume on the guitar, If the signal input is low enough then the problem won't occur.

I tried to test the +35v rail, but I got zero volts, so I obviously didn't get it right.

[J M Fahey]

Basically, bipolar transistors (Q3 to Q6) need their base around 650mV above the emitter voltage.
Some (biasing) resistors are used to provide that.
R21/23 , R30/31/33 , R34/35/39 , R45/43.
So far, so good.
Since the audio is either on a collector with around 15VDC or in a pot or jack with 0VDC (because it's referred to ground) you can't connect it direct to the base which in this case rests at 650mV DC above ground (because the emitter is at 0V) so you need an element which blocks DC but passes AC (audio), which is a capacitor: C10/26 plus a lot more in the other transistors, because they not only block DC but also make complex networks so they have special frequency response curves (active filters).
But what most forget or ignore is that *every* transistor base is part of a diode (base: anode ; emitter: cathode) and this, if fed with a *strong* audio signal will rectify it and charge said capacitor with its peak voltage , which will absolutely disrupt its biasing and make it distort, lose gain, whatever .
Ring a bell?
That's why I said that there are not bad parts there, simply it's an old design.
You need a scope to *see* what's happening on every collector when you play loud and then, *maybe*, I can suggest a cure ... or not.
Otherwise, you will hear what happens, at the end of a chain (through the speaker), but will not know which is the weak link, nor what's exactly happening.
And, of course, do not use the crappy "Distortion" nor the "boost" available there, it will onlñy make the problem worse.
My doubt is: *without* using them, does the problem still happen?
At what volume and tone settings does it appear?
More precisely: with Lo and Hi mid on 5 (on a 0/10 scale), bass and treble on 10 , Dist and Boost on 0 , at what volume setting does the problem appear?
Is the amp clean or already distorting/crunching?
Please answer these doubts.
Thanks.
PS: that's why GK switched to Op Amps, they do not have this problem.
PS2: you may build a new preamp, if you dare, and mount it inside that amp ... but the sound will be quite different.
Or use some Bass pedalboard or processor as a preamp and plug it straight into the Power Amp in.

[teemuk]

That's why I said that there are not bad parts there, simply it's an old design.

J M, I think you are missing some pretty important clues in this:

Initially, the problem did not surface. But after a few minutes of playing the problem showed up again.

I had not noticed before, but when the volume pumping occurs the sound is only distorted. When the volume problem is briefly not there, the sound is only clean. The distortion pot does not change the tone in either circumstance.

- The problem seems to appear only occasionally, sometimes the amp sounds fine, then it drifts into misbehaving again
- The tone becomes distorted when the volume begins to pump
- It is not something related to the built-in distortion feature
- It is triggered by high amplitude input signals

To me all this sounds exactly like a part going bad. I find it hard to believe they would have let a design pass if it did this in "stock form". With "part" I mean everything from plain components (caps, resistors, semiconductors, etc.) to parts like solder joints; it could be an intermittency issue. If the volume pumping keeps appearing frequently then locating the stages where it originates shouldn't take long with proper tools.

We can discuss days and days about this but only thing that gets your amp troubleshot and fixed is getting into the action: pop up the hood and start finding out what causes the issue. Don't have a scope? Built an audio probe, loan one, rig up one of those "soundcard scopes", anything. You NEED tools to troubleshoot and fix the amp.

[J M Fahey]

That's the point I'm trying to stress from day 1.
My "no bad parts" recommendation is meant to try to stop our friend Andy from butchering that PCB by changing parts at random, with no clue.
As you also agree, this is one case where a scope is needed.
That's why they sit on troubleshooting benches, not only because they look cool
About one year ago my trusty old Kikusui scope died ; of course on a Saturday afternoon and me, as always, with a lot of backñogged work pending.
I downloaded a PCScope, the kind that uses the PC card as input.
Worked acceptably well, only audio frequencies of course , no problem with that.
In fact I wrote something about it, including two attenuators , a 5 step one and a fixed one, because they only accept a few hundred mV signals . I guess it was in SSGuitar.
The *BIG* problem in this case is that software PC Scopes do not read DC.
And here I want to clip the probe to different collectors, from Q3 to Q6, play loud, and not only watch the distorting waveform but the DC level dancing wildly.
I'm sure it happens at least in one of them and is the cause of the perceived sound problem.
And once I find that, I'll clip that probe to that same transistor base and also watch for rectification and bias shifting.
And only then, decide on what to do, *after* seeing and confirming the problem.
Shotgunning an amp (changing parts at random hoping to find the bad one) often causes *new* problems.
Yes, maybe some part drifted value or some cap became lossy , making the preamp more sensitive to higher level signals.
It's possible.
But, which one?
In a nutshell, this problem deserves being scoped.

EDIT: download
Winscope
As they insist, you *need* an attenuator or you will kill your Soundcard input.
USE THE SOUNDCARD LINE INPUT, NOT THE MIC INPUT
The full attenuator I suggest is:

or you can use a fixed one, a simplified version:
.022uF>220k>output to mini plug>2K2>Ground.
Don't forget the protection diodes across the output leads. You can use 1N4002 or 1N4148.
They will clip any signal above 700mV peak (500mV RMS), that´s why my attenuator is calculated to supply signals around 200mV to the PCCard Line input.
Within its limitations, it works and has saved me more than once.
You expect to find some stage (transistor collector) where you see signal distort in a funky way.
Unfortunately this PCScope will not show DC drifting in response to the audio signal, which is *the* telltale symptom.
Oh well. Better than nothing.

[kscience]

The real issue may be the cheesy connectors used- with 18 awg single conductor wire. The weight of the wire when vibrated will move the connector; the connections to the pc board can be fractured. Also the connectors can be oxidized; either way this can cause issues you speak of.

That said- you really should replace the electrolytic caps in the old beast, as they dry out over 40 years. Especially if left unused for any length of time.

[AndyDufresne]

Hello, I finally have access to an oscilloscope and some free time. I know it's been a few months but I was hoping someone was still up to help me out. How should I test the transistors? What should I be looking for? Thanks.