Univox tube amp oscillates/feedbacks

[mmcfarlane]

I recently purchased a used, broken Univox 1051 tube head. The schematic is here: http://www.univox.org/pics/schematics/u1050b.gif This is my first tube amp and I am not very knowledgable. Its a 3 12AX7 and 4 6L6GC design.

I spent ~US100 for the amp head, and just got it back from the shop with another $120 to rewind the output transformer, change out 2 of the 3 12AX7 preamp tubes, and replace a burnt out resistor (I don't know which one it was). The guys who did the repair were from a general purpose electronics shop in Bahrain (the best I could do). They tested it by hooking up a raw speaker and feeding a cd player through it and smilled and said - 'see it works'.

Well, needless to say I brought it home (to Saudi Arabia where I am currently stationed), plugged in my bass and found some more serious problems

1) Intermittant crackles
2) A horrendous feedback loop if I play anything from a c# to f. The sounds keeps going even if I unplug my bass. At low volumes the feedback is brief, at higher volumes I have to shut off the amp to stop the feedback.

The current 6L6GC's are from Radio Shack, probably from the 1970s. I have 2 spares from the same supplier and same vintage. Replacing and shifting around the tubes has no effect on the feedback. I also replaced all of the 12AX7 's with new tubes I had laying around from other amps.

Any ideas what might be wrong?

Should I just write this off as a bad $250 investment and not throw any more money at it? I don't know if I can find a qalified amp technician in this neck of the woods, although there is a Marshall repair shop in Riyadh, I don;t know if they'd accept this amp fo repairs and I have friends with horror stories of repairs they have done before.

I don't think this will be a great bass amp when it is fixed anyway, even at low volumes it didn't sound as good as my Eden head...

I may be able to borrow a quad set of 6L6GC's from a friend to do some testing.

Spending Foolishly,

Mark

[mmcfarlane]

a brief update

I did a little swapping around of the 6 6L6GC's that I had and I've been able to significantly reduce but not eliminate the feedbac. My thrid open D string is the worst offender.

Also, if I turn the treble knob all of the way to the left I get a very loud hiss in the last few degrees of rotation, i.e. if the knob is set anywhere between 0-1 on a scale to ten. Tjis hiss just 'appears' all of a sudden as I turn down the knob, it s not a gradual build up.

[mmcfarlane]

Well, I borrowed a pair of 6L6GC's and was able to reduce the feedback to a simple 'bloom' when I play the D string. The D is probably 6db louder than a B or F. This was with two known 'good' tubes and 2 supposedly new 1970's era from Radio Shack.

Could it stillbe the tubes that are causing this bloom, or should I look into something else?

Thanks,

Mark

[tubeswell]

Could it stillbe the tubes that are causing this bloom, or should I look into something else?

Thanks,

Mark

Get a quad of new output tubes and get some good new (NOS) pre-amp tubes (not just ones you've had lying around in other amps). Usually its bad pre-amp tubes that make the amp howl and shriek no matter what (and its usually the first pre-amp tube in the signal path that makes it howl the worst, with the next ones having progressively less of an impact). Bad output tubes will usually make the amp crackle/hiss, but having said that could be other possibilities I guess, but the first one to eliminate is microphonic tubes.

[mmcfarlane]

Get a quad of new output tubes and get some good new (NOS) pre-amp tubes (not just ones you've had lying around in other amps). ...

Thanks Tubeswell.

Two of preamp tubes were ones I pulled out of a brand new Fender Blues Jr amp, where they seemed to worked fine. They are Fender branded GTs. The other one (in position 1 - left side of the amp, first of 7 tubes) was a brand new Mesa tube from a shop in the States. I'll try a few other 12AX7's I have around that work but I don't think in this particular case it's the preamp tubes.

Is it possible the blooming is caused by non-linearity in the output transformer due to an improper rewind job, or is 'double volume a few notes away' too much effect for the transformer to be the cause?

I'll go ahead and buy a quad set when I'm visiting the States in a few more weeks.