I picked up this home-brew amplifier and I'm having a difficult time finding a schematic to match it. It use an EF86 (6267) pre-amp, 12AX7, and four EL-34 outputs. It has a UTC LS-58 output transformer, and the wiring to it doesn't seem to be right. It didn't come with a power supply, so before I go to all the effort to make one, I would like to get the amp working correctly.

The power cord is strange: B+, ground and 6.3 volts for the heaters. NO bias voltage--is this right for EL-34s? The tubes checked good and the output transformer has continuity.

Does anyone have a schematic for this tube complement? If I have to re-wire this amp, that's not a big deal.

Thanks.

Sounds like a Mullard 5-20 but with parallel output tubes.


http://r-type.org/static/5-20.htm

That UTC LS-58 is spec'd at 40 Watt with 2.5k/1.5k primaries and a gaggle of output taps from 500 ohm to 1.2 ohm. UTC lists it 'matches' use for parallel push pull using 2A3s, 6AG5s, 300As, and 6A3s so he may be running those EL-34s triode mode, which should be easy enough to tell by tracing the screen connections.

Those two large brown 250 ohm resistors are the cathode resistors , 250 would be correct for each side with the amp being PPP for the bias

Thanks for the information everybody. Much appreciated.

And those large black/yellow thingies are highly suspect by now!

Looks like whoever built it knew what he was doing.

The power cord seems too thin for supplying 6 volts, may need a thicker pair of wires to get the full 6.3 volts to the tubes. It would be safer to replace the entire cable with one rated with 600 volt insulation. Another possibility is to just bolt a chassis with a power transformer and rectifier to the back of this chassis, behind the output transformer.
Don