Amp problems. Maybe you guys can help.

[black mariah]

I have this Sunn Concert Lead that I love to death, but last year it decided to start spewing this really horrid high-pitched squealing sound on top of the regular sound. Everything functions perfectly. Gain and volume knobs work fine, EQ knobs work, bright switches work, but there's this squeal above it. I tried poking all the components while it was running to see what happened, and found that if I flex the first channel PC board, the pitch changes. WTF?!??? It's a solid state amp, built in '76 IIRC. Any recommendations based on this info? If it's an involved repair I'll leave it up to the pros, but something simple I don't mind doing.

[sir_modulus]

seems pretty serious (mind you, don't poke live components (possibilty of shock and static discharge will damage either you or the component)) IMO, get some pro help.

[Lonestarjohnny]

Sir Mod is correct, if you have no knowledge of High Voltage tube amps you should seek Qualified help, your  problem could be caused by several different reason's, sound's like your amp has developed to much Gain, maybe only a broke wire or a part that has changed value from old age or overheating, your amp even if it's turned off can be storeing very high voltage in the B+ Capacitor's so be very careful if you open it up, if your not very Experienced at this I would for shure take it to a amp repair shop,
JD

[csj]

Yeah, there are lots of possible causes. Since it's not a puttering sound, or chirpy; and there's no cause for the tell tale microphonic howl I'd guess it's a bypass or decoupling cap which has opened causing signal currents which need to be separated to crash into each other. Since my intuition is very poorly developed I'd need a schematic to see how the preamp layout is bypassed and decoupled. You would normally troubleshoot this by bridging the suspected caps with a known good cap and see if it stops. If you do this be sure to discharge the test cap because it'll load up with that part of the circuit's voltage peaks.

When you push on your PCB and the oscillation changes I think you are just physically changing the signal waveform characteristics... in effect serving as a human variable tuner. Mr. Vex could answer this better than I. He's a theremin/oscillator expert.

Also take a magnifying glass and check your lead dress for cracks etc.

As a side note...
I had a version of that amp back in '77. It had a 6 X 10" cab which came with it. I've still got pictures somewhere around here when I used it for a High School talent show. I still remember the 2 songs we played.
Crazy on You by Heart and Ridin' the Storm Out by REO.

Good luck with it and like what has already been said...play it safely.

[phillip]

Here's a schematic of the Concert Lead...pretty intense!

http://www.dangpow.com/~sunn/schematics/SunnConcertLeadOld.GIF

Hope this helps a little
Phillip

[R.G.]

The first step in any tube amp diagnosis is the Tube Amp Debugging Page at GEO (http://www.geofex.com).

Most of the ways tube amps can go wrong are neatly encapsulated there.

Run through that first.

[Mike Burgundy]

hullo all - is this a tube amp?

We'r e talking SUNN here. I think this is an all-tranny amp, maybe with some output MOS if it's an expensive one.
Diagnosis still largely goes along RG's lines though.
If you have some kind of squeal (What kind? 50Hz? 60Hz? real high - 1-15kHzh? What?)
we want to identify the specific bit of circuity that causes it. You might want to buld a signal-tracer.
(look around on Aron's site, it's there)

[R.G.]

Sorry - I missed that "solid state".

If it's a solid state amp from '76, it's almost thirty years old. Thomas Vox amps of that age or older frequently squeal from dryout and wearout of the electrolytics in the circuits not giving low enough impedance. Could well be that for this one.

There are a few things to check. First, make sure all the wires are making immaculate contact, especially connectorized, non soldered connections. Loosen and retighten all the controls and jacks on the chassis, as well as any screw connection of ground to chassis, and the screw connections to the main filter caps, if they are not soldered. You're trying to restore good, solid, low resistance grounds since ground impedance is a great way to cause oscillations.

Flexing one circuit board is fairly diagnostic - that stuff is involved in it somehow. If tightening and cleaning up connections doesn't help, replace every electrolytic cap on the suspect PCB, preferably the entire amp. Even if this does not fix the problem, it's not wasted money or effort. If one of them hasn't failed yet, they're well due for it.

Tighten connections, clean up ground, and let us know what happens. We'll take it from there.