Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on November 14, 2019, 01:05:48 am
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My stereo amp craze is not yet over and I have learned some interesting things.
I know that my 50lbs stereo deluxe is loud enough for any of the drummers I play with (and one can be quite loud) in fact it seems to have a fair bit of extra clean volume on top of what I use.
Its seems two 18-22 watt deluxe amps side by side with efficient speakers produce A lot of clean volume.
Even in some bigger spaces it provides plenty of stage volume and I like a totally clean sound.
With the two amp thing in mind I thought something smaller lighter and lesser powered might be interesting and I found the 6BM8 / 6GW8 tube intriguing especially for a small and compact amp.
Less tube sockets
I think I could make a 8 tube stereo push pull amp with one tube fender reverb and some type of 1/2 triode tremelo (either princeton or a variation of the tremor-lator) I was contemplating the amp driven Ampeg type of 'stereo reverb too'
Something like this (rehash of my stereo deluxe., ignore the voltages, ampeg style verb in red)
I was contemplating using a Hammond AO-43 amp as a jumping of point. I did make a successful vibrolux with reverb and trem in an AO-43 with a bigger OT. 5 preamp tubes and 2 6L6GCs.
I might run into current problems but I think the original power supply ran 8 preamp tubes and 2 El84s.
It would also be pretty cool to have both kinds of reverb
That's my idea.
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The AO-43 PT will likely be too hot for 6BM8s.
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I have tried to keep the 6BM8 builds I've done with 310v or less on the plates.
With respect, Tubenit
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FWIW my Bogen CHB-20A PA amp puts 400V, stock, on 6GW8's; 450V may be pushing it. To keep a PT, I've used a bucking tranny & B+ dropping R to knock over 100V off my Traynor Bassmate which put 410V > EL84's. Unfortunately, adding a bucking tranny may interfere with the purpose of minimal layout size, though you may be able to squeeze it in on top or inside the chassis; or mount it to a sidewall of the cab.
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Hmmm I did wonder about that a little but I was hoping having 4 power tubes would suck it down a little. Maybe I'll look see what hammomd has. i do have another PT off of an old single ended stereo hifi/radio that used EL84s It's 385 -0 -385 or 755 Vac which would seem to be similar. It's the same length and width but about 7/8" less thick (less metal plates in the middle) . Can onetest a PTS output under load? And if so how?
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Put a load you will need on the rectifier that can stand the heat? Some power resistors. That's what i would want to do the next time before going the extra circle but there's fancy formulas out there as well.
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I do have access to an AO-44 I could steal a PT and OT out of (I already do have an extra OT though) I also could buy an AO-35 locally. It might be more the ticket
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AO-44 PT only had to provide 2.32A of filament current for three tubes. The B+ will be about 300V with a 6CA4 rectifier. I converted one to a Magnetone M2 by adding two 12A?7 tubes and going to SS rectifier This actually decreased the filament current to 1.92A and increased the B+ to 338V. But I don't know how it will handle 8 tubes. It's a tiny PT. Two power amps will probably suck the B+ back down to around 300V. Can't say about the extra filament load. I guess just try and see. May want to use ss diodes for B+ to free up 1A filament current.
I would probably temporarily string the eight tubes you plan to use and connect to the AO-44 PT just to see if the PT can carry that filament load... Before committing to this PT.