OT protection

[mooreamps]

Kevin O'Connor strongly recommends this practice. He says he test-drives all his new amps with no speaker plugged in, just a 270 ohm resistor over the secondaries.

Kevin is correct.
I strongly recommend it as well. For myself, I use a 100 ohm resistor tied to the 16 ohm tap, but 270 works.

-g

[jrfrond]

To protect against the case where nothing is plugged into the jack, you can use a shorting jack with a power resistor wired between the inner tab (the one that contacts the hot lug when nothing is plugged in) and ground. Might be tough to find a spot for the appropriate size resistor in a big amp, but if you can pull it off, you've always got a dummy load ready for testing!

- Scott

This is what I do in my amp designs. It's a 25W/4Ω Arcol chassis-mount resistor, bolted to the rear-apron of the chassis. It's the "buy me time" resistor, and also acts as a dummy load when troubleshooting. I use the same series of resistor in a 250Ω value for cathode-biasing purposes.

You just CANNOT guard against every boneheaded move an amp owner will make. At some point, if a user is THAT stupid, then they almost deserve to have a toasted OT. Perhaps THEN they will learn.

[pdf64]

I'm working on a buddy's Traynor YB1A Bass Master at the moment, that has a 180 ohm 10 watt wirewound across the (8 ohm only) speaker output, as well as a shorting contact on the main speaker socket.

[R.G.]

At some point, if a user is THAT stupid, then they almost deserve to have a toasted OT. Perhaps THEN they will learn.

What do you think the chances of learning are, in reality? An owner who toasts his OT will blame the bartender, his drummer, his girlfriend, the amp tech who last touched the amp, the guy who sold him the amp, his parents, his traumatic childhood, the weather, and the phase of the moon before thinking "Hmmm. I bet I did something that made that burn out." Maybe many other things between the moon and him. I just got tired of typing.

Remember, you can lead a horse to water, but you can't make him think.

[Steve Conner]

RG makes a good point here about the difference between different kinds of amp owners. There are tech geeks like him (and me and probably the rest of you ) who have a mental model of how the thing works inside, and use it with that in mind. If amps were cars, we would be building kit cars and racing them at the weekends.

But I'm sure the vast majority of owners, while they might well interact with the thing just as deeply on a sonic level, on the technical level they just want a reliable grocery getter. They don't know or care how it works, any more than the average car owner knows how the grimy stuff under the hood works.

[Enzo]

And there are the guys who don;'t plug a speaker in, and there are the guys who don;t have a speaker on the other end of the cord. Now what about the speaker cord plugged in at both ends like it ought to be, and something backstage like a piece of scaffolding cuts through the cord? or a pissed off union guy takes a knife to it.

[Enzo]

You can lead a horse to drink, but you can't make him water.

[lowell]

You can lead a horse to drink but he may develop a drinking problem... just thought I'd continue this ridiculousness.

[tubeswell]

Wow!, I just did this 270R resistor trick with my 5F6something amp, and it cured a high-pitched ringing I was getting when I play loud. (I had previously thought it could've been the pan feeding back and I was hunting around for ages trying to figure it out). Many thanks to lowell for starting this thread.

[lowell]

Cool, though I started the thread, I cannot take credit for everyone's suggestions and replies.