74 JMP with volume control issues

vmazz

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74 JMP with volume control issues. I’ve got a 1974 1959JMP that I just re-capped it had a few things done to the bias circuit but I repaired them. I fired it up and it sounds amazing but the volume controls are acting strange. When I plug into the bright channel theThe gain jumps when I turn the volume up to about one or two as if it’s on full and when I adjust the normal channel it works in reverse ( so turning up the normal vol cuts the volume on bright channel) so somehow the normal channel is affecting the Bright channel even when not jumpered. I was assuming a mixer resistor or a coupling cap (although the amp doesn’t display any other symptoms of bad coupler) but I haven’t been able to track down anything out of the ordinary yet. Anyone have any ideas on what could cause this symptom?
 

Matthews Guitars

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This sounds familiar.

Try disconnecting the Volume 1 bright cap and see if the volume still jumps between 1 and 2.

If it does, the volume pot is suspect.


Did this amp display any of the described issues BEFORE you started working on it?

When someone brings me an amp to work on, I note what the owner reports to be the problem, and then I run the amp through every playing test I can think of BEFORE I start addressing the reported issue. Because sometimes, there's more than one problem and they're related but the customer only noticed the one, because he never messed with another control that affects the outcome.

Post a good clear photo or two of the preamp section of the board. So it can be cross checked by other eyes that might notice something different.
 

neikeel

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Sounds daft but is the polarity of the v1 cap the right way round?
How about a picture?
 

vmazz

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Great advice. I did not test prior. I often do that. This amp had not been played in many years. Was toured hard had been modded in the bias circuit and had a replaced coupling cap. So I didn’t feel comfortable firing it up to start. Owner said it needed to be gone through thoroughly and didn’t give a specific issue. I will get some better pics later.
 

Pete Farrington

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When I plug into the bright channel theThe gain jumps when I turn the volume up to about one or two as if it’s on full and when I adjust the normal channel it works in reverse ( so turning up the normal vol cuts the volume on bright channel)
I suspect oscillation.
But then I always do.
Because vintage valve guitar amp designs tend towards a low to nonexistent margin of stability.
Try 100pF between pins 6 & 8 of V1.
 

neikeel

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Looking at the bottom of the board some of those pads look very full of solder - have the channels been linked internally?
 

Matthews Guitars

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Yeah, there's too much solder on those pads and clearly it's had some work done.

Be cautious when cleaning up the bottom of the board. My own '73 Superlead had even more crud on the bottom side
of the board than yours did, and when I went to clean it I discovered that several traces were corroded to the point that
I made sure to give them a thick fresh coat of solder to reinforce them, and one trace crumbled and fell off so I had to
bridge it with buss wire.

Micro rant: Even in the early 70s Marshall COULD have made the ST1 board much better, with solder mask, heavier traces, and plated through holes at every location. I wish they had. But no, they had to save a few pence...
 

StingRay85

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I suspect oscillation.
But then I always do.
Because vintage valve guitar amp designs tend towards a low to nonexistent margin of stability.
Try 100pF between pins 6 & 8 of V1.

Hi Pete, would you mind explaining why a high pass filter between plate and cathode on the first triode could solve the issue? I still have a London City amp that terribly noisy since I recapped it, and you also expected oscillation. Maybe I could also try this (still got something else to fix first)

Cheers
 

Pete Farrington

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The stray parasitic capacitances in the amp chassis link everything to everything else, to a greater or lesser degree. That creates numerous feedback loops, negative and positive. Even though the impedances in the amp are high, the tiny nature of these capacitances means that those feedback loops only really come into play at high frequencies, above the audio range for the most part.
So that’s the scenario we’re dealing with in a valve guitar amp chassis.
The positive feedback loops are what we’re mostly bothered about, as they’re what cause major operational issue of oscillation; negative loops will just mainly tend to roll off very high frequency range, beneficial really in keeping oscillation at bay (though too much and the audio response may become muffled).
With a typical Marshall 4 holer, there will be a significant positive loop between the bright channel input jacks and the V2 outputs to the tone stack. The vol control bright cap, treble peaking cap on the channel mixer resistors, and cathode bypass on V2’s common cathode stage (if fitted) will exacerbate this. A useful method to prevent that positive loop becoming strong enough to cause trouble is to roll off very high frequencies after the input stage. The cap I mentioned will do that, as in conjunction with the output impedance of the input stage (about 40k), it creates a low pass filter of about 40kHz. Well above audio, but often it’s enough to bring things jnto a happy place with negligible effect on the audio response.
 

vmazz

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Traced the issue down to a shorted coupling cap on normal channel side .022 mustard. Shame. But interesting that the symptoms where as such. Thanks all for the advice and suggestions.
 

Matthews Guitars

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Replacement mustard caps can be found. Ebay and other places like Valvestorm....
 

StingRay85

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The stray parasitic capacitances in the amp chassis link everything to everything else, to a greater or lesser degree. That creates numerous feedback loops, negative and positive. Even though the impedances in the amp are high, the tiny nature of these capacitances means that those feedback loops only really come into play at high frequencies, above the audio range for the most part.
So that’s the scenario we’re dealing with in a valve guitar amp chassis.
The positive feedback loops are what we’re mostly bothered about, as they’re what cause major operational issue of oscillation; negative loops will just mainly tend to roll off very high frequency range, beneficial really in keeping oscillation at bay (though too much and the audio response may become muffled).
With a typical Marshall 4 holer, there will be a significant positive loop between the bright channel input jacks and the V2 outputs to the tone stack. The vol control bright cap, treble peaking cap on the channel mixer resistors, and cathode bypass on V2’s common cathode stage (if fitted) will exacerbate this. A useful method to prevent that positive loop becoming strong enough to cause trouble is to roll off very high frequencies after the input stage. The cap I mentioned will do that, as in conjunction with the output impedance of the input stage (about 40k), it creates a low pass filter of about 40kHz. Well above audio, but often it’s enough to bring things jnto a happy place with negligible effect on the audio response.

Would you consider doing this preventively?
 


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