Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dpm309 on July 10, 2017, 11:30:31 am
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I have my bandmates Crate Vintage Club 20 on my bench that has a bad squealing problem. This amp is about 15 years old and has never been opened up or had tubes replaced. Replaced all of the tubes (3- 12AX7WA and 2 EL 84) with no difference. Cleaned the jacks, pots and sockets with no luck. Does not appear to be any cold joints or any other loose connections. I am now thinking it could be either the filter caps or OT. Is there anything else I should be looking for?
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was a new speaker put in before you got it? could it be as simple as swapping speaker connections? otherwise measure all the volts, look for anomalies, pull tubes 1 at a time n see where the squeal stops.
guessing it's squealing without any input, can you hear music through the squeal?
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I doubt that inverting the speaker connection can result in squealing, and the amp seems without NFB
(http://i.imgur.com/dy9rFvh.png)
Franco
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The speaker is the original. The squealing occurs with or without a guitar plugged in and am getting music with the guitar plugged. Pulling any of the pre amp tubes one at a time stops the squealing but am getting no sound through the speaker. Seems that the gain (volume) control affects the squealing. With the gain on 1, and the level up to 10, no squealing until I turn up the gain.
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Sometimes it hard to see a cracked solder joint on a PC board. My guess would be a loose or cracked solder joint. Look around the PI tube, second 12AX7, grids and plate solder joints, maybe re-flow all the solder joints around each 12AX7, use a small tip and low heat (the traces can lift easily with too much heat). Do that to both 12Ax7s. A maligning glass help spot bad solder joints.
Doesn't sound like a filter cap and there are no wires from the tubes in that amp so lead dress wouldn't seem like the cause. If the re-flow doesn't work, maybe the OT went...? Someone else might chime in.
BTW, I have an original OT for that amp, it's 16 ohm. You pay for shipping and it's yours.
al
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Thanks Dude. I will go over the underside of the board again. I have a magnifying lamp and a good Weller soldering station. I am also going to look at the gain/volume pot while I am at it. If all else fails, I will take you up on your offer for the OT.
Dan
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AHA, that did the trick. Re-flowed the solder joints on all 3 preamp tubes as well as a couple of other suspicious looking joints and cleaned the volume/gain pot with tuner cleaner and the squeal is gone!
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Three 12AX7's...? Guess your VC20 is a one with reverb and a twelve.
Glad you got got it running, a bad joint can sometimes look no different than a good one.
al
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Yes it is a the Vintage 20 with Reverb. Not a bad little sounding amp and built like a tank. You have to pull the speaker baffle off first before you can get to the tubes and chassis. My bandmate will be happy to get it back.
Thanks