1955 Webcor UL transformer question.

[scrimpus]

So I got this 55 webcor model 1139 amp and I and I'm a bit confused about how the power tubes are biased. The cathode resistor and bypass cap are connected a tap on the OT and I have not seen this before. I want to convert this for guitar but first I need to know what am I looking at here?

[tubeswell]

Those output tubes are cathode biased, and the bias resistor is bypassed, and is going to the centre tap of a winding on the PT, either end of which is connected one each of the output tube cathodes. My guess is this has been done that way to enable some level of cathode current feedback for improved fidelity. Anyway, that's how I see it.

[scrimpus]

That makes sense. I have reworked V1 into a fendery sort of preamp and this thing is really loud. Interesting tone controls on this one as well. I kinda like it but I'll likely just gut most of it and build a combo amp clone out of it. Nobody wants unique guitar amps these days, they all want clones so they can be clones of guitar heroes they will never sound like anyway but business is business I suppose.

[HotBluePlates]

... My guess is this has been done that way to enable some level of cathode current feedback for improved fidelity. ...

Probably cathode feedback.

I thought first it might be distributed loading (half the load in the plate circuit, half the load in the cathode circuit), but there doesn't appear to be big-enough voltage drive prior to the output tubes for that.

[PRR]

It's just a little NFB. Essentially same-as taking a resistor from the speaker to the driver. Enough to take the nasty off the speaker.

I don't see it needs to be tampered. It worked for Webcor. These were fine music machines.