Tube amp troubleshooting - what would you do?

[tony_clifton]

Hi,

I know this question is not bass amp related, but I don't really know another place with a lot of tube amp knowledge so I figured I could give it a try...
If not, feel free to delete this topic - after all this is a bass forum.


Anyway, my dad has a faulty Fender Deluxe Reverb Reissue (22 watt).
The problem is - we can't hand it over to a tech because the problem doesn't occur very often.

Sometimes the amp kicks into an extremely loud boom, regardless of what volume you are running it. The volume knob seems to have some minor effect on it - can't tell for sure you can adjust the volume of that noise (because it's that loud - it might just distort without hearing it), but if you turn it down completely it goes away. It doesn't happen a lot - maybe once in two months, hence why I think we can't give it to a tech. Even if he hears what it's like, he can't reproduce it to fix it.

I've changed powertubes but it didn't help. I still have to try new preamp tubes but a replacement set is expensive (about 7 tubes in there and here in Belgium tubes and things aren't as cheap as in the US, think 130 dollar) and I doubt it's tube related. It just doesn't look that way.

Anyway, any opinions/suggestions?
Would appreciate it!

 

[BassmanPaul]

Possible crack in the element of the volume control at the ground end or a poorly soldered wire again on the ground end. When the ground disconnects the volume control stops working properly and the signal maxes out.

Paul

 

[tony_clifton]

Thanks so much! I'll try to replace it or ask a tech to do it if it's a little tricky.
(I know about the dangerous parts like high voltage caps etc.)

Hope this gonna work, if not thanks anyway!

 

[fingertap]

Intermittent problems are often the most difficult to diagnose.

I am guessing this is a mechanical issue. Specifically, an unwanted oscillation arises and persists from time to time.

Perhaps one of the other tubes has become slightly microphonic.

At symptom's next appearance, does a tender but firm tap, knuckle rap, or slap on the top or side near where the circuitry is affect it in any way?

Perhaps this is best attempted just after you get it to stop, then without turning off the amp, raise the volume and tap, etc. Does the symptom recur as a result of the tap, etc.?

(I am assuming that you can raise the volume after it "goes away" by the method you reported above and it stays 'gone away' without turning the amp off.)

If so, this might indicate a microphonic tube, however, it could also be that something else has become microphonic.

i.e. a wire or a chassis screw(s) that has worked loose. You should be able to replace the tubes. Without knowledge and experience, anything further is better left to a qualified technician.

If you find that you can affect the symptom in this manner you should be able to take it to a technician with directions on how to recreate the symptom so that it could be diagnosed and repaired.

It may be cheaper to simply replace the other tubes.

Whatever you do, aside from replacing the tubes, do not fiddle around inside the amp because dangerous voltages and currents are present,
sometimes even after being shut off for quite some time.

(Present for days, weeks, or even much longer in the case of some tube-bearing circuitry.)



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