Attenuator build problems

[Chuck H]

Thanks guys. In the head cabinet the chassis is hung like an older Fender type. I have a space that is about 3.5"W x 5"H x 8"D between the power tubes and the power transformer. So I'm as far from the preamp as possible. Which isn't very far. I'm using a 3" x 3" x 6" aluminum project box. There's just enough room in there for a 50W attenuator with a strip of 1" deep aluminum channel on top as a heat sink. Leaving me a whole inch between the heat sink and the bottom of the chassis Fortunately the attenuator barely gets warm even after a half hour of pounding on it at high attenuation levels. So I'm not concearned with heat. I'll probably try to shield the inductors themselves inside the aluminum project box. I'll be on the lookout for suitable small ferrous boxes or I may fabricate something. I have tried re-orienting the inductors and reversing the wind polarity on one and both. Results are slightly different and equally bad. I think shielding is the best option. I can put the attenuator in it's place in the amp without incident (though a scope read might tell a different story). Until I get my guitar within 4 feet of the amp. With the attenuator outside the amp I can't get the guitar any closer to it. It only happens with my single coils. Is this a common problem with attenuators?

Chuck

[Steve Conner]

Hey Chuck

I sometimes use a big wirewound rheostat that I found on Ebay as a power soak. It's a ceramic tube about 10" long with iron wire wound round it. There is a terminal at each end, and also a sliding wiper with a big carbon brush. I connect one end to ground, the other to the amp output, and the speaker goes between ground and the slider.

It works well, but I also found that if I got my guitar within a few feet, it would burst into high-frequency squealing feedback. I guessed that the coil of wire was radiating a magnetic field that got picked up by my pickups. (Humbuckers would buck this out, which explains why you only see it with single coils.)

So yes, I've had the same problem. Putting the inductors inside an aluminium box might help, or better still, brass or copper. Or a copper box inside a steel one, their shielding properties complement each other. (The copper box works like the copper "hum band" you sometimes see on power transformers. It shields by acting as a shorted turn, repelling the flux away from it, while iron shields by attracting the flux into itself.) I've heard tales of highly sensitive equipment that used a series of alternating copper and mu-metal shields, one inside the next like Russian dolls.

Or use toroidal inductors, they don't leak as much.

[dai h.]

(4 feet is about 122cm) FWIW I had to get MUCH closer to get my load (PB) to squeal--very obviously doing it because you could see the fan start turning suddenly). The gtr. was my PRS EG, an older one with those humbucking pickups made to try to sound like single-coils.

re: the inductor orientation, I tend to think that's more to do with how the reactive load works within itself (inductors in the load affecting ea. other or not) rather than whether you get more or less radiation(but maybe I am wrong here). (Seems you'd still get the radiation anyway.)

oh I and I also remembered a couple of Aiken amps have built-in attenuation, and (from looking at a chassis pic) it appears to be made up of resistors only, perhaps some L-Pad like situation. And the THD Univalve if memory serves has an attenuator(not exactly the same as the indiv. Hot Plate unit?) inside the amp too.