Vibrolux B+ drops by HALF at practice space

[reveriesOf]

Hi gang, first time poster. Ordinarily I can answer all my questions by searching around, pooling resources. Not today, so I beg a little indulgence.

So here's a strange thing: my 71 Virbolux sounds great at home, shitty at practice; very muddy, no headroom whatsoever. For good measure I recapped, blackfaced, matched the symmetrical components in the power section, put in some lowergain preamp tubes and a balanced 12au7 in the PI.

No avail. Fortunately I brought my biasrite to practice today and read 225V on my plates and 196ma bias current on each tube! At home it was biased at 43ma with B+ at 428V.

What would cause this? For what it's worth, my practice space is in the basement of a laundromat in an old building in SF. Does the problem lie therein possibly? Anything I can do here or in my amp to fight back?

Really, thanks everyone for any help yall can suggest, dying to realize this amps full potention within the mix!

[txstrat]

Welcome to the forum.
I suppose you have tried new tubes!?
When you measured the bias at practice, was the amp warmed up? Does the bias drift at home too when it's warmed up? I'm asking cause sometimes the components given values (resistors, caps or even tubes) drift after warming up, due to failure.
BTW the low headroom might be due to the 12AU7 in the PI since that tubes only has a amplification factor of 17, while a 12AT7 has 60 and a 12AX7 has 100.
I'd try to put in a different PI tube first. Provided the power tubes are good.

[52 Bill]

Have you tried reading the ac line voltage at the rehearsal space?

[MWJB]

If the wall AC voltage was low in the rehearsal space, the plate current would be proportionally low too...not saying that the practice space does not have a bearing but I don't see why plate current would rocket, while the voltage falls.

I suspect that either a loss of bias voltage is causing the immensely high idle current & pulling down the B+ (recheck the bias supply connections/tube socket tension), or the amp may be oscillating at a ultrasonic frequency & eating up current trying to amplify a frrequency that you can't hear. Whilst your idle current should be <40mA at idle you might well read signal currents of that magnitude. This scenario wouldn't explain why your amp ONLY does this in the rehearsal space, unless that's the only time you can push it hard/have it on long enough to make it happen?

[52 Bill]

I would think that even at a lowered plate voltage, if the tubes were really drawing 196 mA each, that they would be red plating to the point of meltdown.

[reveriesOf]

Hey fellas, much thanks for the tips.

txstrat- Definitely relevant q's: I tried two sets of tubes, the ruby tubes that I bought with it biased up neatly at home but the amp still sounded very poor at practice. So I was trying a new set of tubes at practice when I noticed this phenomenon. Switched back to the ruby tubes and they were doing the same thing. Lugged the amp home and my new TADs biased up just fine, plates returned to normal.

At my practice space I originally left the amp in standby for 5 minutes before checking the voltages, and turned it off immediately when I saw my bias current was so high! Switched in my cold Rubys and watched the currents as they warmed up, the plate voltage fell from 450 to 225V in about 20 seconds. I tried swapping out the rectifier tube with the same results. At home the voltages hover right around 428V, I left the amp sitting for 20 minutes to see what happened. As to the PI, I was trying to write my post before I fell asleep it's a 12Ay7, still low gain I know but I wanted to try it after reading an article that said lower gain PIs can lower output tube distortion. It really sounds sweet when the plates are right!


Bill- I will do so today! Also going to investigate what the plates are at in some other amps at the space.


MWJB- thanks for the insight into some of these scenarios. My first suspect was a faulty bias supply, I brought the amp home resigned to spending my night doing repairs. But this inconsistency is boggling me, I was hoping faulty AC line voltage might provide a neat explanation for everything.

FWIW the voltage drops almost immediately at the space, whereas at home it stayed normal for 20 minutes and then I played for thirty minutes on top of that and the amp sounds just as beautiful as god, leo, or uncle spot (who serviced it for the fellow i purchased it from) intended. I should definitely tension the sockets, just as a matter of course. Who knows, maybe I'll get it back to the studio and it'll still sound amazing???

[pdf64]

Was the 196mA at idle - no signal? On both 6L6s?
If so, maybe there's some sort of radio transmitter near your rehearsal room?
Whatever, you need to confirm that your bias supply is good - was a new bias supply cap included in your recap? If so, uprated from the original 50V spec?
I would replace every resistor between the bias transformer tap and the tube grid, including the 1k5s on the tube bases. And the coupling caps from the phase splitter,
Retension the 6L6 tube sockets.
Fit alternative, known good 6L6s and 5U4.
As mentioned above, I wouldn't put a 12AU7 in the phase splitter slot, they pull a lot more current than 12AX7 and 12AT7, and might not give sufficient gain/signal swing to drive the output tubes properly.
Having said that, I've never tried it though, so I'll put one in mine to see what it does.
Peter.

[reveriesOf]

Hi pdf, thanks for the response. The bias cap was replaced about a year ago by uncle spot, I will confirm that the replacement has a higher rating. Swapped powertubes and rectifier, and as stated everythings beautiful when I'm at home. I'm going to start with retensioning the sockets and investigating some of the other amps in the studio to see what's up. If it's still wack, and it's just my amp, I'll continue along replacing the resistors.

As to the PI tube, I've attached an article that led me to put a 12ay7 in there (I made a type last night, though the article suggests 12au7 also). Now that I'm home and everything's normalized on the amp, it sounds great w/ that tube. I haven't popped a 12at7 to compare though. Cheers

Was the 196mA at idle - no signal? On both 6L6s?
If so, maybe there's some sort of radio transmitter near your rehearsal room?
Whatever, you need to confirm that your bias supply is good - was a new bias supply cap included in your recap? If so, uprated from the original 50V spec?
I would replace every resistor between the bias transformer tap and the tube grid, including the 1k5s on the tube bases. And the coupling caps from the phase splitter,
Retension the 6L6 tube sockets.
Fit alternative, known good 6L6s and 5U4.
As mentioned above, I wouldn't put a 12AU7 in the phase splitter slot, they pull a lot more current than 12AX7 and 12AT7, and might not give sufficient gain/signal swing to drive the output tubes properly.
Having said that, I've never tried it though, so I'll put one in mine to see what it does.
Peter.

[reveriesOf]

well kind sirs, after a good cleaning and retensioning my amp is quite happy at the practice space. the voltage is still lower than at home, but only by 25V and the bias current stays in the right range. i'm so relieved the problem turned out to be so simple, it's difficult to know what to do when your results are so inconsistent. truly grateful to this forum, particularly for all the information i'm bound to cull from the archives. cheers, gents

[trem]

well kind sirs, after a good cleaning and retensioning my amp is quite happy at the practice space. the voltage is still lower than at home, but only by 25V and the bias current stays in the right range. i'm so relieved the problem turned out to be so simple, it's difficult to know what to do when your results are so inconsistent. truly grateful to this forum, particularly for all the information i'm bound to cull from the archives. cheers, gents

OK, I'm new at this, I'll take the bait. How can dirty, un-tensioned tube sockets cause sever plate current in one building, but have absolutely no ill affect on the amp in a different building?

[melvin]

I dunno, humidity? -did we ever get the AC wall voltage?

[MWJB]

"OK, I'm new at this, I'll take the bait. How can dirty, un-tensioned tube sockets cause sever plate current in one building, but have absolutely no ill affect on the amp in a different building?" Coincidence.

Melvin asked, "did we ever get the AC wall voltage?". Wall voltage may have been a secondary issue, but low wall voltage would not have caused plate current to rise, it would have dropped along with the voltage.

[melvin]

MWJB, I know, I read your previous post. I am just curious. And I am just wildly guessing, but I think bad tension/dirty sockets was just enough extra unwanted resistance to bring up the real problem which you also mentioned earlier: parasitic oscillations...maybe from a black-facing, perhaps without fixing the layout. this could even become apparent only in a noisy em environment, or really just when the amp is cranked at practice, couldn't it?

of course, it could also be a bad solder joint irritated by the metal band in the room next door with the svt stack backed right up against the shared wall. (the vibrolux doesn't stand a chance in this case.)

but the real lesson here is: ALWAYS clean and retension when servicing a tube amp...

[reveriesOf]

MWJB, I know, I read your previous post. I am just curious. And I am just wildly guessing, but I think bad tension/dirty sockets was just enough extra unwanted resistance to bring up the real problem which you also mentioned earlier: parasitic oscillations...maybe from a black-facing, perhaps without fixing the layout. this could even become apparent only in a noisy em environment, or really just when the amp is cranked at practice, couldn't it?

of course, it could also be a bad solder joint irritated by the metal band in the room next door with the svt stack backed right up against the shared wall. (the vibrolux doesn't stand a chance in this case.)

but the real lesson here is: ALWAYS clean and retension when servicing a tube amp...

That's a great point melvin, I didnt clean up the lead dress in the amp. I just assumed if you don't hear the oscillation it's not there, is this really a problem for some amps? My practice space does have pretty gnarly em issues, so maybe this is a real concern. Hah, thank god no metal bands next door, no shared walls--we unduhground!

My figuring was that my probe was not getting accurate readings in the dirty sockets, even though I reseated them many times and got the same consistent error in each socket... and then of course it worked the first time when I got home, and each time I changed tubes then. Could be coincidence but does seem like a long shot. Then there's the great improvement in sound... I'm gonna keep my bias probe around the space and just check periodically to see what my amp's doing. Could it have something to do with the laundromat right above our heads? Or the machinery that runs all that stuff being next door to us in the basement?