Peavey Roadmaster Filament Problem

[Mr_bibbles]

Hay doods, here's a riddle for you.

Got a Peavey Roadmaster on the bench giving me some symptoms I've never seen before. None of the tubes glow- Checked 15a fuse, Removed all output tubes and switched pre-amps one at a time, no change. Voltage off the transformer (Yellow wires, ran with limiter bulb) 6.3VAC. 23vDC present on heaters, matches well with per 25vdc on schematic.

I resoldered the fuse clip to the board and also the few components in the filament line, the tubes lit up! 6.3VAC across the board. Now though, the fuse itself is getting so hot that it's causing smoke from the board, WITHOUT blowing the fuse.


So my question is, what the hell could cause this? Mechanical components around the fuse? Bad... fuse clip? Molex? Shorted 6L6 pulling the 6.3?

[The Dude]

A schematic would help.

[Enzo]

First replace the fuse clips themselves, once they get hot like that, they have an oxidized surface and they lose their tension, their springiness. When they become resistive, they generate heat.

The heavy yellow wires carry the 6v from the transformer to the power board, then another connector connects the power board to the 6L6 board. Then another cable carries the 6v from the 6L6 board to the preamp board. All of these molex connectors can get loose and resistive. Tighten their pins, and in sopme cases it might be better to hard wire them.

[Dave H]

peavey_roadmaster.pdf

[The Dude]

Agree with Enzo. Something has likely become resistive. I'll add. It could also be the fuse itself. The ends could be corroded or it could be internal. It's rare, but I have had fuses themselves develop resistance.

[Mr_bibbles]

Woops! sorry here: http://juggernauthxc.free.fr/roadmas...ter-schema.jpg.

Thanks, just wanted to make sure this wasn't some rare electronic issue before I rewire the board. I'll let you know if I run into any trouble!

Enjoy your weekend boys and girls

[The Dude]

Well, as it turns out, there's nothing extra fancy about the filament supply, so a schematic probably wasn't necessary. That said, it's still a good idea to post one, if available, when you start a new thread. Good luck!

[Randall]

Hold on. A 15A fuse? In a tube combo amp?

[Mr_bibbles]

Hold on. A 15A fuse? In a tube combo amp?

Yessir, except it is a head and not a cab. This thing is a beast.

[Justin Thomas]

I would expect at least a 10-12A in there, given the inrush current for all those preamp & power tubes - 7.2A at full operating temp. 15A May have been the smallest value that would consistently evade "nuisance blows," especially given that modern tubes have been shown to have all-over-the-map specs on just about everything, including heater current...

Justin

[Tom Phillips]

Hold on. A 15A fuse? In a tube combo amp?

And remember that we are talking about the 15 A fuse in the heater circuit. See the schematic of the beast linked in post #6.