Enzo! Your advice is needed!

[Indyryder02]

Have a Peavey Road Master, came in with a burn mark under the blue 400 ohm resistor. I’ve read a few threads on these and seen you’re very familiar with these amps. This seems to be a common issue on these. But haven’t read anywhere the cause of it. Or is it normal and just needs raised from the board?
the resistor still measures about 400.. all tubes have proper voltages etc.. raised the resistor up from the board.
E Caps have been replaced. Don’t see any other issues. Just want to make sure I’m not missing something.
thanks in advance!

[Jon Snell]

The burn mark is due to the heat generated from the resistor dropping the power and the board is a budget board.
Lift the resistor 1/2" off the board on standoff turrets or at least spacers to stop this from happening again.

[TomCarlos]

Not Enzo but.... :-)

You can also consider using a terminal strip.

[Enzo]

The resistor measures 400? Then likely it is OK. The burn could be from a previous episode.

It is the screen node resistor for the B+ supply, and while it does get hot normally, failed power tubes also can damage it.

[glebert]

Since there is high voltage there you need to make sure that the scorched part of the circuit board is removed as it can become conductive. You may be able to take a dremel or something similar and grind out the burned area until you are down to clean board material. FYI, I have the same amp that had the same problem.

[Indyryder02]

Ok. This is what I was suspecting. But wanted more confirmation. Owner said he was playing it. Smelled electronic burning smell. And had a slightly reduced volume but still had sound. Tubes all seem to be working fine.
just wanted to make sure something didn’t cause it to overheat all at once, that I missed. But seems to play fine. The resistor gets hot but I raised it from the board. Don’t see anything else out of wack.

the bias spec seems extremely cold, but I Assume this is normal as well? Only making about 5 watts idle per tube it looks like with the factory spec bias of -58v

[Jazz P Bass]

It sounds to me like one of the output tubes was going bad & pulled too much current through the resistor.
It may have been an internal short in the tube that subsequently 'vaporized' & is now gone.
I would throw in a new set of tubes.

[The Dude]

Agree with Jazz. If you smelled a burning smell and amp volume dropped, there's likely problems other than just a hot resistor (bad tube or tubes).

[Enzo]

PV amps are usually biased "cold" from the factory.

[jack briggs]

Smelled electronic burning smell.

Or burning phenolic, maybe?

[Indyryder02]

I read on another thread making the bias hotter on these made the sockets melt! Haha. I don’t know about that. I’m sure it’d sound better hotter, but then longevity of 6 6L6’s is a thing too. Not a cheap set to replace. Matched groove tubes currently in. It’s just so cold from what I’m used to in most amps or anything with 6L6. Usually closer to -50v for around 70%ish
usually use a scope along with meters to find a happy balance of safe running and just a hair of crossover at peak.
so it doesn’t sound sterile and cold.. least for most amps this works great.

[J M Fahey]

the bias spec seems extremely cold, but I Assume this is normal as well? Only making about 5 watts idle per tube it looks like with the factory spec bias of -58v

Biasing to 60/70% of max plate dissipation is a dangerously and sadly persistent Myth, probably initiated by some unwashed Guru and repeated ad nauseam by others without justfication or measurements.

FWIW *Engineers* bias "as cold as possible without getting crossover distortion" for two very good reasons:

1) it provides MAXIMUM power out, period, because you MAXIMIZE current swing.
Just-look-at-the-datasheets.

Current can swing *only* between Maximum (which for a given plate and screen voltage is constant) and Idle which you set yourself.

So the higher the idle Current, the lower the current swing possible, hence lower power out, at the speaker voice coil where it counts.

Practical and very real example:

2) tubes run cooler and last longer, what´s not to like?

You´ll read everywhere "Factory bias is too cold" and for most brands (except VOX, Matchless and a couple Boutique ones), now you know why.