New (but used) amp problem, can I really have a bunch of bad tubes or is it the amp?

[my_shadow22]

I just picked up a really cool Gibson GA-20rvt Minuteman. It is in great shape, sounds good cranked, and was pretty cheap. I played it for a while at the shop and it started to make some funky noises. Turned out a tube was microphonic so I switched a few around and no more problems. I put it through all the paces, and bought it.

So I get home, plug in, and there's that noise again. I tap the tube and it's making all sorts of noise. Actually when I touch the faceplate to turn a knob, you can hear it through the speaker. I tried changing the tubes again and every one I put in is microphonic. Some aren't as bad as the other, but still. Anybody have any idea what is wrong? Is it time for a check up?

Thanks!

[nickA/T]

Hi,
I don't know if this will help, but it is worth a try. I bought a DR Z Maz Sr. and every knob I touched sounded microphonic. I changed all of the tubes, both power and pre/amp with no avail. I called and talked to Z and he thought it was a tube or a ground. By the way, he is a great guy and will do all he can to help his customers. Well to make a long story short, I tightened the four screws in the top of the cabinet that hold the chassis in the cabinet... and now it is the best amp I have ever had. I have a feeling the guy I purchased it from was having the same problem and couldn't get the problem resolved so he unloaded it. Let me know if this helps. It sounds like the exact problem i was having.

Eddie

[Blue Strat]

Originally posted by my_shadow22
I just picked up a really cool Gibson GA-20rvt Minuteman. It is in great shape, sounds good cranked, and was pretty cheap. I played it for a while at the shop and it started to make some funky noises. Turned out a tube was microphonic so I switched a few around and no more problems. I put it through all the paces, and bought it.

So I get home, plug in, and there's that noise again. I tap the tube and it's making all sorts of noise. Actually when I touch the faceplate to turn a knob, you can hear it through the speaker. I tried changing the tubes again and every one I put in is microphonic. Some aren't as bad as the other, but still. Anybody have any idea what is wrong? Is it time for a check up?

Thanks!

The first step would be to replace all tubes with KNOWN good tubes. If this doesn't help (and even if it does) the amp is way overdue to be checked out by a qualified tube amp tech.

[Sonar]

Wiring can be funny business. Sometimes the run or location of certain wires to a preamp tube socket can give symptoms that SEEM like it might be the tube. Take a wooden chopstick and move the individual wires leading to that suspect tube socket as sometimes a simple rerouting of a single wire will surprise you by clearing up multiple -- seemingly unrelated -- problems.

With that in mind, I remember another Gibson GA20RVT as one of the worst wired amps I've ever seen which lead me to recommend the above. The GA20RVT has LOTS of thin gauge wire traveling all over the place, Much longer runs than needed and it seems to take every opportunity to cross the circuit board until it looks kinda like Gulliver being tied down by Lilliputians. Mine appeared to have been wired by a elementary school students wearing mittens. :D

You should consider rewiring the amp or at least have the excess trimmed up and and cleaned out. It WILL help.

A few other thoughts on that particular amp:

1. Try NOT to use the "three-way rotary" power switch. If it isn't already broken, it will be soon. It does duty as power switch/polarity switch. You would be better off FIRST getting a 3-prong grounded power cord installed and the just unplug the amp when not in use.

2. Don't believe the schematic for this amp in Gerald Weber's book. One of the preamp tubes listed there is incorrect. See if your unit has a tube chart inside and copy it down outside the amp somewhere. (I mean, if you bring it anywhere for service, mention that the tube chart is correct and the popular schematic is WRONG).

3. If you do have the footswitch, treat it with respect. Lose it and you won't get another because of the BIZARRE plug/jack configuration it uses.

4. Treat the handle with respect also. They seem a bit flexable, but they tend to break in the middle. Just stress of aging on old-school molded plastic parts -- it will just snap clean and fast like taffy kept in the freezer. Also this handle is next to impossible to replace. I'd recommend NOT carrying it by the handle (sorta like old Echoplexes that have handles break if you look at them wrong). Just pick the amp up from the bottom.

HTH ;)