B+ Drifts down after 2 hours

[Anchan]

Hello-
I built this amp about a year ago:
6V6 Amp

It started off as a thread here:
Dual Simple SE's in Push Pull w Differential Input

Basically, I am using 2 of Tubelab's Simple SE boards together to make a PP amp. No phase inverter needed, since my source is balanced.

Anyway, it sounds absolutely fantastic. I couldn't be happier with the amp. I've been running it with 6V6 tubes for a about a year now. I'm using the JJ 6V6 tubes which can take over 450V. Using a new PA060 transformer from Dynakitparts, my B+ is around 430+/- and it stays like that, even if it is running for 24 hours. And the amp, trannie and choke are not too hot. Great!!

So now finally the question. I want to change to EL34's since I am now buying some slightly less sensitive speakers, and want more power. These boards are designed around the fact that I can use different tubes provided I change the cathode resistor. When I run the amp w EL34's, I change the cathode resistor to 600-700 ohms, biasing somewhere around 40-45ma. That works and sounds great. And obviously it gets a lot hotter that w 6V6. But my B+ tends to drift down after about 2-3 hours. It will start off at 420, and then after a few hours it will drift down to 400, and then maybe even 385 after 4-5 hours. The trannie gets pretty hot, but being its the same one used in the ST-70, I am not too worried.

I speculate that as the transformer and choke get hot, the resistance in the windings climb, and therefore the voltage will drop. Is this right? And it is it safe? With the 6v6's, I've forgotten to turn off the amp overnight, and the amp was only pleasantly warm the next morning. With the El34's I'd be worried. Comments? Am I just being paranoid?

[LinuksGuru]

Yes, seems to be voltage drifts down because the resistance climbs as the result of temperature raise. I would suggest you to use low-flux toroidal transformer wound on M3/M4 to avoid this issue. It is not likely to be cheap, this is not commodity item which need to be custom ordered.

[Bob01605]

If the B+ went down and up and down in an irregular pattern I might suspect that your line voltage is not steady. As line voltage goes down (even a couple of volts) B+ will go down because the secondary voltage on the power transformer will go down. Same thing if the line voltage goes UP. Secondary voltage goes up.

But > If the B+ drifts just down as you say after a few hours then IMHO either ... > the quad cap is leaking and should be replaced OR your rectifier tube needs to be replaced.

Bob Latino

[Anchan]

Originally Posted by Bob01605

If the B+ went up and down in an irregular pattern I might suspect that your line voltage is not steady. As line voltage goes down (even a couple of volts) B+ will go down because the secondary voltage on the power transformer will go down. Same thing if the line voltage goes UP. Secondary voltage goes up.

But > If the B+ drifts just down as you say after a few hours then IMHO either ... > the quad cap is leaking and should be replaced OR your rectifier tube needs to be replaced.

Bob Latino

Well, it does go up and down in an irregular pattern- but I wasn't necessarily speaking to that. That's easy to see as a line voltage thing. But, what I am referring to is the average voltage declining over the course of hours- and only with the EL34's, not with the 6V6's. Its not a quad cap, but rather individual caps, and they are all new. But that doesn't mean they can't leak I suppose. It could be the rectifier tube I suppose, but I feel like I tried that. Unfortunately, I don't have a set of EL34's on me at the moment to do another round of testing, I sold them when I decided to go 6V6. But I wanted to see if this was a common problem, and if it could be fixed before I bought another bunch of tubes.

[Richard Ellis]

Your mains TX really shouldn't get hot to the touch...........warm yes , hot no. The windings are changing value as the temps climb & are giving you this reading...........Over time at this temp your insulation is degrading...My guess is you are right AT the max rating.........OK...but not for a truly long time(20+ yrs) lifespan. Upgrading to a toroid will get you a higher rating at or nearly the same size as you EI TX.
Have you calculated your full load on your mains TX?? Designing for a 1 1/2 times max load minimum.....or higher is the way to go. Designing for a 3X max load would be great, getting you a cool running mains.
Oh well.......you don't have any projects your running now? Do you? Put them on the back burner & get this taken care of..

__________________________________________________ __Rick........

[Anchan]

Originally Posted by Richard Ellis

Your mains TX really shouldn't get hot to the touch...........warm yes , hot no. The windings are changing value as the temps climb & are giving you this reading...........Over time at this temp your insulation is degrading...My guess is you are right AT the max rating.........OK...but not for a truly long time(20+ yrs) lifespan. Upgrading to a toroid will get you a higher rating at or nearly the same size as you EI TX.
Have you calculated your full load on your mains TX?? Designing for a 1 1/2 times max load minimum.....or higher is the way to go. Designing for a 3X max load would be great, getting you a cool running mains.
Oh well.......you don't have any projects your running now? Do you? Put them on the back burner & get this taken care of..

__________________________________________________ __Rick........

The transformer is the same one used in an ST-70, which would make me believe that it is beefy enough for 4 X EL34's, but maybe this is an issue with those was as well? I don't know. To your point, with the 6V6's + 12AT7's, on the B+ winding , I am drawing 4 X 20ma + 4 X 13 ma = ~130 ma. Again, with 6V6's the transformer is warm, but absolutely not hot, and the B+ is rock solid (minus the line fluctuations) even running for 24 hours.

With EL34's it's 4 X 40ma + 4 X 13ma = ~210ma. The PA060 transformer is rated at 360ma. So it appears to be right at 1.5 min that you recommended.

Maybe I stick with the 6V6's for a while, given that it's run flawlessly for a year. If I absolutely need the EL34's, maybe I'll consider a bigger trafo. A bit surprising that the PA060 is not up to the task, but maybe it is the problem. I have to think this over a bit. Maybe I'll get some more 34's, an IR temp gun, and do some further testing, and report back. Thanks for the feedback!

I wish there was octal tube with the same pinouts, in between the power of a 6v6 and an EL34, but I don't think there is.

[Rundmaus]

... what about running the PS section with a dummy load drawing the current the EL34 O/P stage needs?

Should be easier to identify the problem then...

Greetings,
Andreas

[Anchan]

Originally Posted by Rundmaus

... what about running the PS section with a dummy load drawing the current the EL34 O/P stage needs?

Should be easier to identify the problem then...

Greetings,
Andreas

Not a bad idea. I've got some homework to do, and will need to report back.
BTW I misstated my trafo's rating. Its 360-0-360V @ 300ma not 360ma as I stated.

[tubelab.com]

Put one meter, set on AC on the transformers HV secondary. Put a second, measuring DC on the B+ at the rectifier cathode. Let the amp play until the voltage drops and see if the percentage drop at the secondary is the same as the B+ drop.

I have seen transformers that lose a few volts when they get hot, but we are talking about 35 volts. That's a pretty big change in resistance. I would think it's a rectifier tube, or the first electrolytic in the power supply is losing capacitance as it gets hot. If you have a scope, check the ripple hot VS cold.

If the power transformer is too hot to touch, that's bad. If you can hold your hand on it for several seconds without using bad language, its OK.

[GloBug]

Good advice. That should fix him up.