Champ cap

[Larry F]

I've had a Marsh Champ run 2-4 hours a day for two years without probs. Lately, I hear little pops, followed by a big volume drop. Doing the chopstick test, I see that the 22 nF cap between the volume and pin 1 of the pre-amp tube reacts with a little pop when I gently tap it. Moving the leads from the cap or the wires they are soldered to doesn't cause any sound, just the tapping directly on the cap itself. It is a Mallory. Is the cap bad, or the connection? This has been intermittent over the past few weeks. It started the day after I accidently left the amp on overnight.

[mojo2001]

I am not sure that a slightly microphonic cap is cause for great alarm and I don't think that cap is the root of your problem if the solder connections are good.

Of course, I'm just guessing since we're doing this on the internet...but I'd say try wiggling the tubes and see if a faulty tube socket connection could be at fault.

Are you sure all of your tubes are sound, particularly the rectifier?

If so, might be easier to wait until the problem surfaces, then see if you can chopstick and wiggle it back to normal.

Have a voltmeter? Check the B+ voltage when the fault condition is evident, then check the voltage at the plates of the 12AX7 and the cathode voltage of the 6V6 to make sure the tubes are drawing current.

Should not be a problem to leave new tube gear in otherwise good condition on overnight. Gets a bit hairier when dealing with older vintage gear where a dried-out paper insulated transformer can smoke, caps can go into dead short, etc.

[Larry F]

Yes, rectifier it was. Thanks.

[Larry F]

About the microphonic cap. Did it become microphonic, or was it always that way? What causes this? How does it affect the sound?

[mojo2001]

Hard to say, some caps are just like that. If not perfectly rolled there might be some give between layers. Even if perfectly rolled, there is some give in the plastic.

To some extent, perhaps, all parts are microphonic--but especially caps-- and many kinds of coaxial cables too.

Some amps, including vintage Ampegs, drizzle parts with glue or silicone to moderate this effect.

In any case, I wouldn't worry about it...might even add a little je ne sais quoi to the amp.