Amplifier oscillates - advice sought

[Rob11966]

Dear All,

My newly completed Marshall 18w amplifier is up and running. It's pretty good but I am still troubleshooting one small problem....

The amplifier oscillates on the TMB (treble, mid, bass) channel but only when the treble is turned right up to level 9 or 10. The oscillation begins suddenly and is high frequency. All is fine apart from this. No other settings cause any problems. Before I start pulling parts out, I wanted to run some ideas up the flagpole. The circuit is a variation of the fairly well known Richie TMB.

Here are my proposed solutions. With reference to the attached schematic (the problem pot has the arrow), I plan to first replace the 300pf cap (marked 'A') with a 270pf cap. If this does not work, I will replace the 47K resistor (marked 'B') with a 56K resistor. If neither of these options work, I will try both solutions in combination. Failing all of the above, will place a 10-20K resistor at position 'C' in series between the 300pf cap and the pot, as the amp only oscillates when the treble wiper is approaching zero resistance.

What do you guys think? Reasonable approach or should I try something else.

Thanks in advance,

Rob

[bigwill]

Are you sure it's not just microphony? Try swapping valves around

[Mooly]

Hi Rob,
First better say I'm no expert on valve gear, but a couple of thoughts anyway.

I would say you need to determine where the problem is... is it the stage before the pot going unstable, or the stage around V3.

Could it be a "wiring" issue to the pots... stray capacitance etc

My thought would be to try a highish value resistor in series with the grid/s (both) of V3 ???
Also, does the setting of VR7 not have any effect... you imply it doesn't matter where any of the pots are... which seems a bit odd.

[bigwill]

Does the oscillation continue with the amp turned down but the treble turned up?

[Rob11966]

Originally Posted by bigwill

Does the oscillation continue with the amp turned down but the treble turned up?

Hi Bigwill and Mooly,

Just tested it - as you both suggest, it is in fact not entirely isolated as I first thought. The oscillation does not occur when the TMB master volume is turned down. In fact, the higher the TMB master volume, the lower the treble has to be turned up to cause oscillation. I missed this first time around due to complaints from the wife about noise - children sleeping, neighbors complaining etc etc. Copped it just then as it whistled away .

Pretty certain it's not just microphony, swapped all tubes around, no change - but will put it on the scope sometime over the WE just to be sure.

Rob

[Mooly]

A 'scopes the best way for sure.

Are you sure the wiring is OK... no pickup from other stages or the tranny.

Sometimes it helps to think back to what causes oscillation in the first place... what the conditions are... when the signal feedback (whether stray or by design) becomes positive or "in phase" and when there is sufficient gain in the circuit to overcome the inherent losses of the "network" causing that feedback.

[SemperFi]

The tone stack is not at fault, no reason to 'fix' it.

Although this is not a high-gain amp, the TMB channel does have anough gain to where problems easily occur if:
-Wire dress is not perfect
-No grid stoppers directly at sockets
-unessesary lead length
-all kinds...
From your schematic I see no grid stoppers! They are a must. Preferably carbon composition, try 10k-68k on 12AX7, and 100-1k on output tubes.

I've had the same problem many times, high frequency oscillation as the treble is upped. Usually wiredress and higher value grid stoppers fix it.

Sometimes a small cap, say 1nF needs to go from plate to cathode (or gnd) to get it stable. Some tweek their amps just that way, even the famous Dumble I've been told.

[Globulator]

There is a 470k grid stopper after the volume control but thats the only one..

I suspect it is stray capacitance between wires or the input going too close to the output transformer or speaker.

Get it feeding back and push the wires about with a wooden stick to see what changes it?

[SemperFi]

Are u sure that's not just the resistor to the tail of the phase splitter, as shown on the schematic?

[Globulator]

Originally Posted by SemperFi

Are u sure that's not just the resistor to the tail of the phase splitter, as shown on the schematic?

On the top-left of the schematic, into the triode that drives the cathode follower