Peavey Mace Bias Mod?

[raph]

Also some troubles with MACE : 3 Tube plate get hot & red

Hi there,
Could anyone help me with this amp PEAVEY MACE VT SERIES : here is a pict of voltage measurements around the bias.

Thank's
Raph

[defaced]

The voltages in red, are they referenced from ground? Alot of the polarities are wrong, which makes me think the measurements were taken with DMM leads reversed. Usually this doesn't make a big difference, but in a bias circuit, seeing 47v, not -47v, on a grid pin is a BIG difference.

[Steve Conner]

Either way I see something like 15V dropped across those 220k bias feed resistors. That probably means the tubes are gassy and leaking a lot of reverse grid current.

So my magic 8-ball says replace the tubes.

[raph]

sorry for disturbing info

Hi Thank's for prompt reply.
Defaced is right - sorry - I wrote values on an inline photo editor, no possible change in typing errors ... values ARE negatives (-60 / -47 / -49 etc). Reason to be honest = I went to bed too late !
Read same negative values at same line ex -47 V three tubes all measures refer to ground.

Steve, about 15 V drop, can you explain ?
Serviceman told me -45/-47 should be OK. Do you agree for Sovtek 5881 tube and 514V ?
I've read also that Sovtek 5881 is perhaps not the right tube to use Who knows anything About that ? Will a bias adjustment be necessary in case I buy new tubes ?
Raph

[Steve Conner]

Ok. You have -63V at one end of R25, R26. But at the other ends of those resistors you have -47 and -49.

That means that current is flowing through those resistors. The current must be coming from the tube grids. That's bad.

It'll get even worse as the tubes heat up in use, the grid current will increase and so will the volt drop across R25, R26, until the tubes start to redplate. Measure the bias while the tubes are redplating, and you'll see it's even lower than -47 and -49.

This is called thermal runaway, a classic failure mode for old, worn-out power tubes. Replace them and the bias will go up to -60V, which is what Peavey intended.

The Sovtek 5881 should be fine in that circuit. If you're feeling brave you can buy 6P3S-E tubes on Ebay from Russia, they are the same thing, but the quality control might be worse.

[raph]

Sorry did not see all the post - understood.

Last question - those CR16 and CR 17 - diode 2873
Do you know which to use instead as discontinued ?

Thank's again
Raph.

[Steve Conner]

I'd use three 1N4007s in series. Or maybe three 1N5408s if there was room. But you might be able to get proper replacements from Peavey Europe in the UK.

[raph]

I'd use three 1N4007s in series. Or maybe three 1N5408s if there was room. But you might be able to get proper replacements from Peavey Europe in the UK.

OK, I'll try to get in touch with them.
This morning they were not answering.

How about white noise in the amp ? May some replacement of TL072 by NE5532 would fit direclty and reduce noise at 1st preamp stage for both channels ?