Old Tube Intercom Amp

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I don't know of any reason why it wouldn't make a decent little guitar amp. You might want to readjust the gain, and if it has negative feedback you might want to make that adjustable or delete it entirely. I've researched, designed and built about 10 guitar amps over the last 15 years, and I prefer no negative feedback in any of my amps. I get a more relaxed sound and much better sounding overdrive distortion that way. The Vox AC-30 has no negative feedback. Most of the guitar amps from the 1950's had no feedback. That was the good stuff.
not sure if it would be the case with this unit but many an intercom amp from the tube era used 45 ohm speakers (so they could do dual duty as microphones) so output x-former in this chassis might not be what you expect.

is it an "intercom" or an old "paging" amp?
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I'd stay with the 6SL7's, which should give you enough gain, and a little realtime compression which means more sustain. 6V6's are in some of the best sounding amps I've ever heard. Measure the DC resistoance of the speaker (unhooked). If it's an "8 ohm" speaker, the DC resistance will usually be just a few ohms lower, may 6 ohms. The nominal 8 ohm rating would be for AC somewhere in the mid frequencies. It's more difficult to measure the AC impedance since it varies with frequency, but that's doable too.

You've got 4 triodes to work with before the output tubes (each 6SL7 is a dual triode). I'd rewire the whole thing so it had a common cathode driving a cathode follower, then the tone stack and volume, then a paraphase phase splitter driving the output tubes, for what it's worth. If you want tons of overdrive, forget the follower, just set that triode up as a common cathode voltage gain stage too.
Thanks for the reply's and thank you kevinkr for moving this to the right section of the forum.
The 5A3 circuit is a good idea. I haven't made a schematic yet of how this unit is wired up. One difference from the 5A3 circuit is that there is a choke between two high voltage filter caps. The speaker measures about 5.6 ohms and the output transformer is a Stancor A-3852 The amp has 2 microphone input jacks, one guitar jack and I believe this was a paging system not intercom.
Bottom line it appears I have many of the components to make a 5A3 and since my tube theory is weak I'd probably just stick with the schematic before trying to modify.
Thanks again.
well that clears up the "intercom" status of this unit.
there's a lot of blues harmonica players and guitarists that love these types of chassis pretty much stock.
harp guys parallel the mic inputs with an old "carbon button" mic and wail away.
same applies to guitar, many use an A/B box and use a "mic" channel for distortion.
See turk 182 note about the speaker impedance. Many of these things had 50V or 70V line outputs. Do the DC measurement of the speaker coil as stated above - that will tell you.
The last Intercom/Paging Amp I rebuilt to a guitar amp had 70V line output.
I substituted one of these output trannies:
86PP output transformer

The one I did had a 12AX7 + 2 x 6GW8 (ECL86) + EZ81 so I did a Tiny Terror Clone. Its lovely. You could do the same with 2 x 6SL7, 2 x 6V6 and a 5Y3 if that "floats your boat".

I would not do the 5A3 design. The grid bias is not the best design. You could easily do the 5C3.


The 6SL7 does not have its cathodes internally tied like the 6SC7 but otherwise a nice little circuit. One thing to look out for on this schematic though, the 1Meg NFB from the transformer might be best to forget. Also the 1 Meg in the divider looks wrong, the value from the 5B3 looks more right.


Now saying all that, you can just use the 5E3 values and just sub in the 6LS7's instead of 12A*7's.
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