Protection?

[daz]

OK, so now that i'm done tweaking my JCM800 type build, (close to a 800 anyways) what should i do to protect the amp, tubes and tranny especially WITHOUT altering tone? I have a mains fuse, but thats all i have as far as protection goes.

[Don Moose]

A B+ fuse is not a bad idea.

A 10-ohm, 10W resistor across the speaker jack, arranged so that if no speaker is connected, the amp sees the resistor as a load.

Some people stack up reverse-biased diodes between the output plates and ground as another no-load protection (speaker wires fail occasionally).

I think RG or Randall Aiken have an article about 'bulletproofing' an amp.

Hope this helps!

[Enzo]

And keep in mind that really, fuses are more about preventing a fire than protecting the amp.


people sometimes wonder why a resistor can burn to a crisp without blowing a fuse. If there is a 4 amp mains fuse, at 120VAC mains, that means the fuse won't blow even when 480 watts are being drawn by the amp. Unless a problem sucks up more than 480 watts, the fuse won't blow.

Got a 1A HT fuse? At a B+ of 480v - not unreasonable and also a convenient number - again, whatever the problem is must use more than 480 watts to pop that fuse.

SO a 1/2 watt or a 1 watt resistor will not challenge the fuse much while they burn.

[daz]

Ok, so a 500ma at the OT pimary center. (got that from a search in a post by Bruce) But what about the B+? Inline right after the rectifier i take it, but how do i determine value?

[Enzo]

Just put the fuse over at the rectifier. The vast majority of B+ current flows through the OT CT to the power tubes. The preamp tubes account for a couple ma for each plate, and the screens a few ma, that's all. SO instead of having the fuse oversee just the OT, I'd let it oversee everything. It will still cover the tranny there. This like MArshall does it. I don't see the point in having two fuses in the B+ line, the OT CT is part of the B+ distribution.

[daz]

You mean right AFTER the rectifier, or on one leg of the PT secondary between the PT and diodes?

[Enzo]

Yes, after the rectifier. You could also put it in the center tap to ground path.

[daz]

Any suggestions as to the value, or should i post a schematic for that?

[Enzo]

You didn't mention a model, only "JCM800." SInce it is a Marshall-like amp, I'd use Marshall-like fuse values. T500ma for 50 watt and T1A for 100 watt. Those are as good a set of values as any.

[daz]

Thanks. I'll use a 500ma then.

[Steve Conner]

Don, surely you want a 100 ohm resistor, not a 10 ohm? The whole idea is that it needs to be connected all the time, not just when the speaker jack is unplugged. And 10 ohms would eat quite a lot of your power.

[Don Moose]

I didn't think I described that right. The jack is a switching type that, with nothing plugged in, connects the 10-ohm resistor. Plugging in a cable opens the switch and disconnects that resistor - it doesn't guarantee there's a load on the far end of the cable, though.

[Paul P]

I did both. An 8 ohm dummy load for when nothing is plugged in and
a 500 ohm resistor for when there is a cable plugged in but nothing
at the other end, or a bad cable.

Paul P

[Tubis]

Fuse the filament as well. Fuse everything.

[Pedro Vecino]

For "not speaker protection".., Rivera uses in certains amps a method that disconnects the power cathodes if a speaker jack has not connected...
To protect tube fails the better system for me is to use fuses in the cathodes (fast 180/250mA for unit, for example).

Regards