550V B+ line

[Shottky]

So I picked up this 4x EL84 watt amp, turns out it's pretty much identical to the 36W. Only difference is that the plates have 535V on them (by design) and the bias is cranked down to about 15mA / tube.

The power tubes that are currently in there:

Marked in white silkscreen on the top part of the tube:
6BQ5/EL84
International C
Servicemaster

in fainter purple near the base it says
EL84
USSR

I'm wondering if these are military spec EL84s. I can't imagine how the EL84s in here have stood up to 535V plates and screen. For what it's worth the amp sounds pretty rough when the gain starts to increase. I was thinking of dropping in some new output tubes, but I don't want to put some NOS stuff in there just to have it roast on the high voltage.

7189s? EL84Ms?

[stoo]

Is the PT rated for 120 VAC?.
Stew

[Shottky]

It sure is. I couldn't believe how high the voltages were. Everything seems to check out though and nothing is blown or ruptured.

[rjgtr]

The screens are likely at a lot less than 500V, more like 300V. See if each screen has a dropping resistor. You can have the plates pretty high but I would think the tubes would die with high screen voltages.

[krx]

With zero current, I'd bet the plates in those military tubes would handle 650V and the screens 550V. So with a lean bias they wouldn't necessarily go into meltdown at the voltages you're seeing. They wouldn't necessarily sound good either though . What amp is this anyways??

[zaphod_phil]

There's a Seymour Duncan amp that supposedly gets 50W out of a quad of EL84s, running them at 560V. It uses a split power supply to run the screen grids at 280V. Like Richard said, keeping the screen grid voltage relatively low is the trick. You can exceed the rated maximum voltage on a lot of tubes if run the screens at or below the rated limits. I'm told that the SD amp doesn't sound particularly great, though...

[Shottky]

Thanks all . I just did a quick visual and thought that the screens were connected high. It's a big rats nest and the wiring is hard to read, so I'll pull out the DMM again and check the screens this time.

Interestingly, the board is "Designed by Seymour Duncan" .... and unfortunately it's not the greatest sounding at cranked volumes.

[Shottky]

Phil hit it right on the head.

280V screens, 550V B+.

It's probably the exact amp he was describing as the board is made by Seymour Duncan. Amp is named T50R.

The current bias has one of the tubes running 16 mA on one side and 10mA at the other side. I haven't put current sensing resistors in on the partners for either side yet as the leads don't really reach well. I might have to anyways when I get some new tubes to rebias it with.

I'm still thinking that I may get the higher voltage EL84Ms. They're pretty cheap and this is not an amp that I intend to keep, so hopefully the tone improves a bit. I'm pretty sure the old power tubes are baked, but I'll have to give them a shot in my currently working 18Ws to see how they stack up.

[zaphod_phil]

Oh, I thought you were planning to convert it into a 36W...