safe wattage for bridged t attenuator

What would be the max amp output to safely install a bridged t attenuator on a tube amp? These are Hammond amps, I've got six, four EL84s an two 7591's, I'm guessing they're in the 10-15 and 20-25 watt range, (guesses'll have to do until I get the manual specs.)

Of course the idea is to crank the organ and the Leslie to get good disrotion but not anhiilate my realtionships with neighbors; also recording at reasonable levels to avoid bleed in an open room, etc. I've never built a reactive one, but there are plenty of voice coils around here. Wouldn't know where to start anyhoo.

How about adding a switch to run the power tubes in triode mode, even if this does not drop the volume enough you do not have to attenuate it as much. In my experiance the more you attenuate the more the sound suffers.

adam

[quote author="adamasd"]How about adding a switch to run the power tubes in triode mode, even if this does not drop the volume enough you do not have to attenuate it as much. In my experiance the more you attenuate the more the sound suffers.

adam[/quote]

Fooey, they already run in triode mode, those tricksters at Hammond!

Harry Kolbe's got intersting attenuators, uses transformers rather than a voice coil.
http://www.soundsmith.com/atten.htm

> What would be the max amp output to safely install a bridged t attenuator on a tube amp?

What's the biggest resistors you can buy? 100 Watts? 400 Watts? BTW, a 120V hot-plate is about speaker impedance, will dissipate 1,000 watts steady and could probably waste the speech/music output of a 4,000 Watt amplifier.

Why Bridge-T? That's a constant-impedance network, and a lot of switch contacts. What the speaker probably wants (especially since they run Triode) is the lowest possible impedance. For that you want a potentiometer attenuator, except in this case you want to switch-out the potentiometer when at full-power. Four 2Ω 10Watt resistors in series would give you 20W to 1.25W on various taps, the worst case source impedance would be 2Ω, not too different from the triode. If you were sure the speakers really preferred ~4Ω source, then you would add series resistors and indeed it would be a Tee, just not the conventional impedance relationships.

Also: about 3KΩ 20W in the B+ line would drop these amps from 20W to 5W without any great change of gain or damping or THD curve. This does need a beefy switch, but if you just want 2 or 3 settings, a couple toggles is a lot cheaper than a proper Bridged-Tee affair on the speaker.

You can also buy in-wall speaker volume controls. A lot are for 70V lines and 10W power, and some have dubious transformers. But Radio Shack used to sell a 8Ω "75W" L-pad (constant-Z to amp, minimum-Z to speaker) that would have honestly absorbed amps like yours even working at full blast.

Ah, I don't see the L-pad but they have the generic in-wall switches:

$16.99 Brand: RadioShack Catalog #: 40-993, gives 0dB to -30dB (0.020W out for 20W in) in 10 postitions, says 25W RMS. Must be transformer; check for bass distortion.

[quote author="PRR"]Also: about 3K? 20W in the B+ line would drop these amps from 20W to 5W without any great change of gain or damping or THD curve. This does need a beefy switch, but if you just want 2 or 3 settings, a couple toggles is a lot cheaper than a proper Bridged-Tee affair on the speaker.
[/quote]

Thank you Paul, I like this one. +350 at the 7951s and +300 at the 6BQ5s, it's easy to test the sound and see if it will still produce the, a-hem, lush Hammond overdrive at full power.

Most power soaker box sellers warn of grave dangers and fried amps when using resistive attenuators, don't know how much of that is hype.