Peavey Bravo Plate voltage

[rhaigh]

I recently picked up a Peavey Bravo 112. One of the EL84's showed signs of red plating. I think the tubes may be original so I ordered some replacements, but before I pop them in, I want to make sure that the amp is operating fine. Does anyone know the normal plate voltage in one of these. With all the tubes removed, i'm getting 418V B+ and 437V on the plates. Vbias is about -18V. I threw in an old set of EL84s and then got Plate V=420V, Vb+=270, and Vbias=-17V.

I just want make sure that the voltages look about right for this amp. The amp is fixed biased, so i'm planning on adding in a bias adjust pot before the new tubes go in.

Thanks,

Rich

[Enzo]

Hi, welcome.

Just a nitpick. When you modify your amp's bias, it will still be fixed bias, it will just also be adjustable.

With the power tubes removed, the B+ will climb. When they are installed, the voltage will fall into place. B+ voltages are not on schematics largely because they would only be approximate. Your 120v mains moved several volts either way all the time, that is normal. At you B+ levels, every volt the mains changes means the B+ will change about 3.5v. So a 3v mains shift means about 10v B+ shift. In any case, B+ is not likely to be wrong high. There are lots of reasons it could be low, but few things will make it higher than it ought to be. So really, your B+ is fine.

If your other tubes were not red plating, then the amp itself is likely OK. It is dissipation that kills tubes, not voltage.

[Swamp]

I recently picked up a Peavey Bravo 112. One of the EL84's showed signs of red plating. I think the tubes may be original so I ordered some replacements, but before I pop them in, I want to make sure that the amp is operating fine. Does anyone know the normal plate voltage in one of these. With all the tubes removed, i'm getting 418V B+ and 437V on the plates. Vbias is about -18V. I threw in an old set of EL84s and then got Plate V=420V, Vb+=270, and Vbias=-17V.

I just want make sure that the voltages look about right for this amp. The amp is fixed biased, so i'm planning on adding in a bias adjust pot before the new tubes go in.

Thanks,

Rich

Try to measure the bias of the amp.

I recommend mod the bias circuit, pretty similar of this document:

http://www.blueguitar.org/new/schem/...0_bias_mod.pdf

Something like a 15k resistor in series with a 25k pot, to get a range between 15k to 40k. In the place of the 33k resistor.
Here where I live, the wall voltage gos from 230 to 245V, and peavey amps usualy runs prety Hot.

Tubetown have some El84 Peavey C30 matched that works fine without changing the bias.

TT-Tone Selection JJ EL84 Peavey C30 / 50 - Tube-Town GmbH

[rhaigh]

Thanks for the replies. I read somewhere that 400-420V is too high for an EL84 and that it exceeds the maximum on the data sheet. I also read that it is common for EL84 guitar amp builders to put this high a voltage on the plates and that its ok as long as its not biased too high. Any thoughts?

[Enzo]

The data sheet numbers are for engineers designing consumer goods - think mom's old table radio. The number insure a long reliable tube life in such applications. Mom wouldn't be happy replacing tubes once or twice a year. But in guitar amps, we run them hard and we expect to put a set or two in an amp each year. As I said, it isn't voltage that kills power tubes, it is the dissipation. That means voltage times current. And it IS common for guitar amp designers to go over the data sheets. Heck, Fender ran the 6V6s in their amps a good 100v over the "maximum" voltage. Those amps have been running just fine that way for over 60 years.

[Swamp]

Thanks for the replies. I read somewhere that 400-420V is too high for an EL84 and that it exceeds the maximum on the data sheet. I also read that it is common for EL84 guitar amp builders to put this high a voltage on the plates and that its ok as long as its not biased too high. Any thoughts?

Have the same problem with a Gibson GA15R, Plate too dam high. But as says Enzo... "It is dissipation that kills tubes, not voltage."


and after a quick reading:
http://music-electronics-forum.com/t24561-3/

As Enzo (again) says:
"Rather than put footnotes on every page, they probably have a chart of them at the front or back of the book.

Just a guess, but I;d have to say the 300v is the max design voltage they recommend in a circuit. The (b) voltage is perhaps the max voltage they want on it period. In other words design it to run on 300v, but it is OK for it to sit there with as much as 550v on its plate."

Makes sense..

[Jazz P Bass]

It is also good to keep in mind that the very last thing that a designer wanted (for musical/ voice reproduction) was an output section that distorted.