Traynor YBA-2 eats EL84 for lunch B+ too high?

[jhelm_waterw]

Hey all... just joined and am very confused

I have an old ~25 watt EL84 Traynor (YBA-2) I picked up years ago.
It is the simplest amp ever. One channel -> 2 x EL84 power section.
I have the schematic too.


The problem is... It EATS output tubes (glow red after 5 minutes).
I thought I had a bad phase splitter coupling cap. (replaced them with orange drops)
Thought it was bad bias circuit. The schematic say's -15.5 fixed bias.
I installed variable bias, turned it up to -21V !!! & still is screaming hot.
How is that possible?

Then I realized (DOH!!!) ... this thing is idling at B+ of 410V.
Even the schematic says 395V
I thought maybe it was supposed to really use 7189s but the schematic say's 6BQ5.
WTF??? EL84s have no business being run that high.

I am ready to zener drop the power supply by 30v but someone help me
understand how/why this amp EVER worked.
With standard line fluctuations... who would run EL84's that close to 400v ???

Is this common with this amp?

Thanks,
Jeremy

[Euthymia]

Measure the actual voltages you're getting at pins 2,3, 7 and 9.

Also, do you have it on a load, either speaker or dummy, when you are doing your testing?

410V is not unreasonably high B+ for an older amp being run from a modern wall socket.

As for how a tube can be cooking, it can be a matter of the grid not being negative enough (which you have tried to remedy), and it can also be that there is an oscillation somewhere in the amp, one that may be outside the range of hearing or outside the range of what the output transformer and/or speaker can reproduce. Other things can make it happen, too.

Do you have another pair of EL84's you can swap in? Does the redplating happen with the preamp tubes pulled?

[Gtr_tech]

You need to get your plate dissapation down to what the tubes will handle. Plate voltage really ain't an issue here....yer pulling too much current. New production tubes are all over the map for bias requirement and 40 yr old published specs are meaningless when the tubes it ws designed around are no longer available. You may try a set of outputs that require less bias to get them to a safe operating current.

[Dan T.]

So C9 and C10 are definitively ok?
Did you actually measure -21V at pin 2 of the EL84 tubes?
I've seen Traynors with EL84s dimed into attenuators with no problems(405v on plate)
I set my Studiomate to idle the El84 tubes @ 15-20ma at 405vdc so the plate voltage is not the problem.

[jhelm_waterw]

Thanks for all the replies.
#1) Euthymia, good point. I actually also thought about an HF messing with my bias. So I put my scope C9 and C10 but saw nothing. I even PULLED C9 and C10 to completely isolate the power circuit from the pre circuit.
#2) Dan T, besides "knowing" c9 and c10 were good, I actually removed them to further troubleshoot (see #1) the bias problem. If I could get these tubes down to 20ma, I wouldn't mind but I'm looking at 40ma ! (way way too hot)
#3) Gtr_tech, Yep. I'm just pulling too much current but I just cannot imagine how on earth a bias of -15.5 would "do the trick" @395V if -20V won't do it at @410. I am just left with the idea that...
a) very very bad (non linear) things happen when you push a crappy new production tube anywhere past it's limits
b) this MUST have worked OK at some time (when designed years ago) when the line voltage was lower and tubes were conservatively rated.

It really feels like this circuit may have been just fine for a 7189, but not for a newer EL84 (and I tried 8 different ones).
Only ONE of the newer tubes had a nice idle of 20ma but all the others were pegged at 40ma.

At this point, I think I am going to zener drop the power supply and hand pick some el84's

I will recheck my measurements later tonight.
Thanks all again,
Jeremy

[jhelm_waterw]

So C9 and C10 are definitively ok?
Did you actually measure -21V at pin 2 of the EL84 tubes?
I've seen Traynors with EL84s dimed into attenuators with no problems(405v on plate)
I set my Studiomate to idle the El84 tubes @ 15-20ma at 405vdc so the plate voltage is not the problem.

Dan, sorry for long delay... I was so p***ed off at that amp that I put it aside for a little bit and worked on my Hammond preamp. Ok... so anyway... for your Studiomate running at 405v, do you happen to remember what your bias voltage was? I CRANKED up the bias v to -22v and got the tubes to settle down. BUT... -15v@395 (according to schematic) -22v@405??? That seems like a crazy jump in bias for only a 4% change in B+. Thanks, Jeremy

[g1]

Maybe check for AC volts where you are measuring -15DC. Should be very low, although if the bias filter cap were bad I would expect a hum problem.

[thehoj]

Can you adjust (rewire) your bias circuit to be able to provide more negative voltage?

I have a Traynor YGM-2 which also runs el84's. When I got it, the filter caps had already been replaced, so I haven't really done much with it other than play it..
I haven't even checked to see what sort of voltages are being produced, or how hot the tubes are biased..
It runs fun for extended periods of time with no problem, but I bet if I took some measurements they'd be scary.
I should probably check on it.. Your thread just reminded me that up until now I've been choosing to live in ignorance with this amp .

[Gtr_tech]

Dan, sorry for long delay... I was so p***ed off at that amp that I put it aside for a little bit and worked on my Hammond preamp. Ok... so anyway... for your Studiomate running at 405v, do you happen to remember what your bias voltage was? I CRANKED up the bias v to -22v and got the tubes to settle down. BUT... -15v@395 (according to schematic) -22v@405??? That seems like a crazy jump in bias for only a 4% change in B+. Thanks, Jeremy

Who cares what the schematic says....its meaningless with new production tubes. That amp was designed around US/European tubes that are long out of production. The currently available tubes are merely approximations....they are not the real deal. You don't need to worry about what was spec'd 40 years ago, you need to worry about what is going to keep current production output tubes happy. Static dissapation wattage.....google it.

[jhelm_waterw]

Who cares what the schematic says....
Static dissapation wattage.....google it.

I care about what the schematic says. They are typically very close even using today's garbage tubes and higher line voltages. I'm really not a 6BQ5 guy. I've used herds of garbage big bottle tubes and never ever seen anything this far out of spec. Old Leslies, Marshalls, Fenders... even with modern hot line voltages, they are just never this whacked. Pull any old schematic and you'll typically be less than %10 off today (even with the modern tubes). Now mind you... I'll concede that is my experience with big bottle tubes.

So... are you telling me you've had direct experience with current production EL84's that have exhibited behavior like this? No speculation... give me an example.

[Gtr_tech]

I don't bother to look at published voltage specs that are from the 70s or earlier. They usually have little to do with what we're dealing with these days. I just set the bias where the tube wants it to keep the idle current where it should be. Since current production tubes are so all over the map as far as bias requirements I frequently change the bias circuit to accommodate the tubes. So I guess I can't give you an example. I do what needs to be done on a case by case basis. I have seen EI EL84s redplate in a cathode biased amp where JJs and NOS were perfectly happy. It all depends on the output tube itself.....

[Gtr_tech]

If your amp matches the schem you posted there's no screen resistors. If it were me I'd add some. Screens are often a weak spot with the new prod stuff so its cheap insurance to keep screen dissapation under control.

[jhelm_waterw]

Maybe check for AC volts where you are measuring -15DC. Should be very low, although if the bias filter cap were bad I would expect a hum problem.

Can you adjust (rewire) your bias circuit to be able to provide more negative voltage?

I have a clean bias and I did rewire to provide the extra (up to -24v) bias needed to throttle back the tubes.

Also... I did not think this would be a major bone of contention but it was that fact that it ate a pair of NOS GE 6BQ5 when I got it that started all this so I don't think this is a "damn crappy new tubes" issue.

I bought it from a music store for dirt ($50) because the guy told me it ate through tubes and his tech could not fix it. I thought for sure it was just the phase coupling caps and bought it.

So... sort of what I was looking for was some tech to confirm my suspicion and say...
"Yeah, I work on EL84 stuff all the time and they have ZERO tolerance for over voltage. Keep them away from 400 or they will cook" or something to that extent.

I'm just going to drop the B+. I got the 30v zener in the mail today. Let's see what that does.

[thehoj]

I have a clean bias and I did rewire to provide the extra (up to -24v) bias needed to throttle back the tubes.

Right. I was just thinking you could increase that even further.

[mooreamps]

Most of the custom builders, well the "better" one's anyways, would recommend running the plates of an EL-84 at +300 volts... Bottom lime ; you need to drop abouts a 100 volts....

-g