Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ACHIEVEIT on February 02, 2022, 10:15:36 am
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Hello guys,
I recently got a Peavey Ultra 60 tube head. Nothing has been modded/tampered with as far as I can tell (opened and examined the boards). Even has 3 out of 4 original Peavey-branded preamp tubes. V1 (JJ ECC83) and the power tubes (Tung-Sol 6L6GC STR) must be newer.
The problem that I have is that there is a strange sound whenever a note is abruptly muted. There is a distorted trail that's always there. It is particularly prominent in the clean channel but it is also there in the Crunch and Ultra channels (it's just less apparent because of the distortion). I have made a short clip to demonstrate it. Of course when playing normally it is not that prominent but for clarity's sake I insisted on the abrupt notes on the clip.
I intend to change the filter capacitors as they are now 30 years old (they are original and the PCB dates back to 1991). I didn't spend too much time troubleshooting but here's the things I've tried already without any difference:
- Swapped all preamp tubes with known-good ones (didn't have a spare 6L6 set to test)
- Visually examined the PCB/components and tested very crudely for any short on all electrolytic and most coupling capacitors
- Relays are energized correctly when I apply external voltage
- Bypassed CR1 & CR2 (that act as a 'noise-gate' of sorts)
Could the filter caps be the culprit or should I be looking for something else?
Thanks for your input!
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I'd verify bias first.
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I guess this can be ruled out: Listening to just the preamp from either the FX Loop send or the Preamp Out exhibits the same behavior, while connecting the guitar to just the FX Loop Return (bypassing the preamp effectively) shows no such characteristics (or to a much smaller degree if I have to be picky). Assuming that it's present in both scenarios (although to a smaller degree in the power amp section), would it make sense to assume it's something that's shared by both -> power supply rail -> filter caps?
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If it's in the preamp, you're going to have a lot harder of a time figuring it out. Id assume that if it's in the "preamp out" signal that it is indeed in the preamp. If you can find out settings that make it worse or better you may be able to nail down which gain stage it is. A lot of things can go wrong in an amp that complex and old and PCB's and SS circuitry gets extra complicated.. but, if youre patient and willing to nail it down, you should be able to find the bad component. Start measuring resistor values against the the schematic and look for ones way out of spec.. 10% or more. Then look at the electrolytic capacitors in the preamp sections. I can test capacitance with my MS8268 to see if ones way out of line, maybe you have a way to do that as well?