Question on Da Geezer`s Bypass cap attenuator

[punkykatt]

I have a request to knock power down  (full/ half/ quarter) on a AC-30  Hoffman style amp.  What wattage resistors are needed that go on the switch 100k and 330R ? The cathode voltage is 11v and .050A on each el84 cathode.  Thanks much . Punky

[HotBluePlates]

Do you have a picture or link? I can't seem to find Da Geezer's circuit anymore.

So without more info, I would do the simple math to calculate ratings: 11v²/100kΩ = ~1mW (so any practical resistor size is good); 11v²/330Ω = ~0.4w, better use 1-2w minimum.

[sluckey]

Before you pursue this mod, just disconnect the bypass cap on the output tubes. No cap is the lowest power output. If the volumes drops enough for you it's worthwhile to do the mod.

1/2 watt on the switch are OK. Here's a pic...

[punkykatt]

thanks for the replies Guys.  I must have put my thinking cap on backwards this morning.  I will try lifting the cathode bypass cap to see how much the volume drops.  Sometimes my brain farts and just keeps on farting.  Thanks again.

[HotBluePlates]

Do you have a picture or link? I can't seem to find Da Geezer's circuit anymore.

So without more info, I would do the simple math to calculate ratings: 11v²/100kΩ = ~1mW (so any practical resistor size is good); 11v²/330Ω = ~0.4w, better use 1-2w minimum.

After looking at Sluckey's drawing, there is no d.c. across the resistors unless the bypass caps is shorted (which I've never seen before, but I suppose is possible). Given that, each resistor can be low-wattage. Only the actual bias resistor needs to be rated for the power implied by the standing d.c. voltage.

[jojokeo]

I've not tried this on a PP amp but have with single ended and to me it doesn't seem to affect the power level so much as written. But maybe it's different in PP situations? The other thing you'll definitely notice is a definite change in tone and feel. You loose a lot of high end for a much more brown sound, but then this also occurs when in triode mode.

I'm curious on anyone detailing their experiences in performing the phase inverter mod?

[sluckey]

I'm curious on anyone detailing their experiences in performing the phase inverter mod?

I tried it in my Marshall 18W but didn't leave it in place. It should work to strangle any LTP PI. I can't find the original thread where we were playing around with this but here's a later thread with some comments.

     http://el34world.com/Forum/index.php?topic=9645.msg87571#msg87571

[jojokeo]

Thanks Steve, I always thought of the pi being modded for more or less headroom as was mentioned also in that link, but also it's about balance too. After messing around I've usually found that a simple Vol put before the pi is a nice way to control things best IMHO. Then a ppimv and you can control things pretty nicely however you feel for that gig or day. Lastly, I'll just change the tube if more or less gain is wanted as I usually like an at7 for a pi. Then I'll go either with an 5751/ax7 or ay7 either way if something more/less needs added response-control.