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Hey folks. I'm working on an amp for a friend and I'm hoping to get some guidance. I am not a professional amp tech, but I do know the basics. The amp is all tube with 5 12ax7s and 4 EL34s or 6l6 power tubes. The amp developed a loud static noise so the owner replaced all of the valves with no change. The noise is present with or without a guitar connected to the input. I am pretty sure the problem is in the pre-amp section because plugging into the effects loop from another amp eliminated the noise and the amp works as it should. Plugging anything into the send or return removes the noise. Removing V3 or V5 eliminates the noise, with diminished output of course. I opened the amp up and it's spotless inside. Clean as could be. I used contact cleaner on all tube sockets, and send and return jacks. I re-soldered all pre-amp tube sockets with no change. I have been in contact with Carvin and the tech suggested replacing all of the pre-amp tube sockets. Any ideas? Thanks in advance! P.S. I am pretty sure the attached schematic is for the head version of the amp which is the same as the combo.
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Even with the loud static noise, does the amp pass guitar signal as it should?
If a tube socket is misbehaving, you should be able to influence the staticky noise by wiggling the tubes while the amp is on. If you can not make any difference in the sound by wiggling tubes, it's not a socket.
Use a wooden chop stick to probe every point, every connection, every component while amp is running so see if you can find a localized issue. -
I wouldn't replace all of the tube sockets. Maybe a single one if I proved it to be required, but to do all of them is invasive and may not be the problem anyhow.
So, you've already narrowed it down to the preamp and everything beyond the return is fine - that means V3a and V5A/B are good. What about if you turn all the controls to zero - does it still make the noise?Comment
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Does it happen on both channels? In that case the problem is somewhere between relay channel switching circuit and send/return jacks.Originally posted by guitardog View Post
Plugging into SEND or RETURN breaks the preamp to power amp path in both cases, but try patching them with a short lead, just to eliminate the possibility that the switching contacts are badly corroded and contact cleaner couldn't deal with it.
Static noise in my experience comes from a loose (ground) connection so a bit of reflowing around that area in case patching doesn't work should do the trick.
You could try grounding the signal path in that area with some alligator clips, but please make sure you're not shorting out any high voltages to ground.Comment
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Yes, I can play through the amp normally. The noise is present at all times once the amp is off standby, both channels. I wiggled all of the tubes with no change. I have checked with a chop stick but I will do it again.Comment
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Yes, the noise is there with all controls to zero. I also disconnected the reverb tank to eliminate it as the cause. No change.Comment
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The static is present on both channels, with or with out a guitar connected and with all controls at zero. I did patch through the effects loop and the noise persists. If I only plug a cable into one or the other, the noise is eliminated. I have checked all grounds, tightened all connections etc. There is no corrosion that I can see. The amp literally looks brand new inside and out.Last edited by guitardog; 06-04-2018, 10:06 PM.Comment
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That leaves only V1B (since it's shared by both channels) and the area between relay and SEND jack as suspects.Originally posted by guitardog View Post
Try grounding signal paths starting from the relay up to SEND jack.
Also, try grounding C46&R1 junction in case input jack is acting up. Do the same with C2/R4 junction.
Noise disappearing will give you an idea where something's wrong.
PLEASE, avoid shorting high voltage to ground, you can make sure there is none by measuring it before shorting to ground.Comment
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Turning all controls to zero eliminates V1b as well. The problem lies after both volume controls but before the FX return socket. As the problem is with both channels I think it lies after the relay. It wouldn't be impossible for the relay contacts to be playing up, though I think this is unlikely.Comment
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I tried all that you suggested to no avail. I thought I noticed a change in the static when tapping on C20 with a chopstick, but it I think it was just random noise. It seems like the longer the amp is on, the louder it gets, and it starts popping and hissing.Comment
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I think you are right. No just to figure out whats generating the noise. Maybe a leaky capacitor? Bad diode? I poked and prodded every component in the thing, re-soldered anything that looked questionable, and no change.Comment
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Regardless, short SIGNAL path at different points.Originally posted by guitardog View Post
After the noise stops, your faulty component/joint is located back towards the input.Comment
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That points towards a thermal fault. My favourite go-to for these is a source of heat (hairdryer will do) and a can of freeze spray. My gut feeling is to start with Q5 and then the associated components. With the amp running, warm up the board in that area until the noise worsens and then target the freeze spray on individual components and see which responds to chilling. You may need to heat/freeze a few times to pinpoint the fault.Originally posted by guitardog View PostComment
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Be careful with freeze around tubes. They will shatter. A short piece of 1/2 inch heat shrink or something similar is helpful. I’ve used a small plastic funnel. You never said if you deoxidized and jumpered the effects loop and/or pre out - main amp in jacks? AND how you did it. I recommend a plug and some Craig.Originally posted by guitardog View PostComment
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