TPR

My amp gives a squeal, sort of a harsh R2D2 sound when I turn it off, even though I'm turning it to standby for 20-30 seconds first. What might cause that? Here's a little more background in case it helps.

JCM 900 50W head with EL34s, bought last weekend. Barely any sound apparently because one tube was burned out. Put in all new tubes and cleaned up the jacks. Had bias and voltage measurement problems, but I think that turned out to be the meter. Played for about 15 minutes and volume seemed ok. Just as I was finishing, it started to cut out.

Today, I got a new meter. Plate voltage appeared to be 450 V. Biased the tubes and got 33 mA on one, about 39 mA on the other supposedly matched tube. Played through it again, but volume almost non-existent again. Tried both speaker jacks, and awful squeal happened again. What should I check?

Thanks.

Tim

Marshall Amps

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Jesstaa

Are the preamp tubes good?

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T-Bird

Hi.

Hi pitched squeals are almost every time a result of circuit oscillations. Something triggers them, in this case caps draining at different rates I'd guess.

In any case, a trip to a tech would most probably be the wisest choice (after a full set of fresh tubes of course) , he/she could determine whether the OT and especially the feedback circuitry is working as planned.

There's probably some voltage leakage somewhere (on either of the switches?), as only the heaters should have power when on standby. Only some faint hum should be present in the output signal on standby, whether switchin ON/OFF or when ON.

Regards
Sam

TPR

Nuts. OK, will check with a tech. All the tubes are new. Thanks.

TPR

OK, have made some headway by retensioning tube sockets. Now getting normal volume when playing. I still get a little of the squeal when I shut down, but that's not nearly as loud now. So, new questions.
- Should I get new tube sockets? When I jiggle the pins from underneath where soldered (after draining caps, of course), they move a bit.
- Should I go for the cap job? The big LCR cans and the other big one mounted on the board both show infinite ohms, as well as most of the smaller ones. A couple of the 100 V ones move the meter a tiny bit, but not much and no bubbles on the outside. However, the smaller ones all seem to be Samwha and I'm not sure how they stack up even when new. Any thoughts? Thanks.

Tim

TPR

Hey, I'd love to get some input on this problem. Here's the latest info. It turns out that the tubes I ordered aren't perfectly matched, even though I did order a matched set. Based on what I've read and heard about unmatched tubes, I have no problem with that. They're off by about 10%, so that shouldn't cause technical problems. It's just that it threw me off while diagnosing because I expected closer numbers in the bias.

However, it appears that the bias is moving quite a bit. 2 nights ago one tube read about 41 mA. Last night the same tube in the same socket with the same tester read about 36.5. Both readings were taken after the amp had been on for 10 minutes, and not in standby mode. That seems like a lot of drift in 24 hours. Also, when I go to standby, the meter goes to zero like you would expect. But when I turn the power switch off a minute later, the meter jumps up to about 5 or 6 mA and I get the little r2d2 sound. FWIW, if I turn the power on for a few minutes and then turn it off without ever coming out of standby, I don't get the r2d2 noise.

Almost forgot, I jiggled the tubes a bit with the bias probe on and it made no difference, so I think the connection in the socket is good.

So, in summation,

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T-Bird

Hi Tim.

To me it sounds like the filter caps drain themselves through the bias circuit. That's not unheard of, but does sometimes produce a faint sound that is annoying. Just like in Your case.

IMLE almost any older amp, regardless if it's SS or tube, will benefit from a cap job if it's not been used in daily basis. Leaving the caps drained for a long period of time and then powering up without a variac, will cause a partial internal shorting in the cap. That will result increased ripple and decreased capacity.

As for the drift in bias, that's perfectly normal IME within the first 20 hours or so of operation.

Regards
Sam

TPR

Ahhhhh, I see. OK, I'm getting the cap job. Thanks for the info, Sam.

Tim

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