Ampeg V4BH

[tubby]

I bought an old V4BH, which tested fine at medium volume. I put in new power tubes, cause the old ones were almost dead. The first time i cranked it, the volume went down after 5mins or so, then it went dead. I put it on stand-by for 5 mins, then powered it up again: the same results. Obviously something doesn't take the heat too well. I replaced all the components which had drifted more than 10% from spec values. Didn't help.

Any ideas are much appreciated....

[Rob Mercure]

Tubby,

I hope you retrained all the parts you removed? Tight off put in all of the old tubes and see what happens. Then replace the output tubes and see what happens. If I read you post correctly the amp worked until you did the above. So now try replacing the tubes one by one and see if you've got a working set.

If no first off remember that amplifiers were often specified with parts having a +/- 20% tolerance - that is, parts could vary by 20 percent - 10% was considered "tight" and often what makes a vintage piece of gear sound so are the parts that have drifted in with each other - restoring it to stock would ruin a great tone.

OK, now I'm going to recommend dooing what you're avoiding - go though each part of the circuit you altered and ensure that you haven't miswired a part of have a cold solder joint or an accidental bridge to ground or another part, etc. We've all done this more times than you can imagine. If you can't face the thought and have a buddy see if s/he will do it for you and offer to do the sam for her/him when they wind up f'ing up.

Rob

[Gtr_tech]

Those amps really put the boots to the power tubes voltage wise so a bias adjustment is *very* important after replacement of output tubes. What did you use for replacements? They must be of good quality or that amp will just eat 'em up....so you may have had one fail already....'specially without a bias check. Screen resistors OK? Be a good idea to replace the 470s with 1k 2w types just to be nice to the screens. The JJ 6L6s will hang in those just fine, just don't bias too hot.

[tubby]

Hmmmm....

Rob and Gtr-Tech, thanks for your help.

Yes, I kept all the old parts (in fact, just one resistor and one cap were replaced). For power tube replacements i used JJ7027As which should hold up for a while. I made sure the problem is not in the power amp section by using the power amp in with another preamp. I guess the prob was there all along, even with the old power tubes. I just didn't had the chance to crank it yet.

So, the amp still shuts down after 15mins 3/4 vol. playing. As i mentioned above, the power amp still works in that condition, so it's somewhere in the pre section. After going to standby and back the amp works again for 10mins or so. Maybe it's a resistor that opens the circuit when it gets too hot? Faulty pre tube? Any ideas?

[telemoxy]

Rob and Gtr-Tech, thanks for your help.

Yes, I kept all the old parts (in fact, just one resistor and one cap were replaced). For power tube replacements i used JJ7027As which should hold up for a while. I made sure the problem is not in the power amp section by using the power amp in with another preamp. I guess the prob was there all along, even with the old power tubes. I just didn't had the chance to crank it yet.

So, the amp still shuts down after 15mins 3/4 vol. playing. As i mentioned above, the power amp still works in that condition, so it's somewhere in the pre section. After going to standby and back the amp works again for 10mins or so. Maybe it's a resistor that opens the circuit when it gets too hot? Faulty pre tube? Any ideas?

Did you check the switch? If you have converted to a 3 prong plug you could try using the polarity switch temporarily in place of the standby switch and see if you still have the problem.

[David Schwab]

Bad caps? Or maybe a crack in a tube socket, or something that changes with heat?

[tubby]

Problem solved (finally)....

Thanks again for all the input!

Today I got the preamp tubes I ordered, and swapped those in the amp one by one. Turns out it was the second 12DW7 from the input. I never would have thought a prob like this could be caused by one of the pretubes...