Randall RG50TC Biasing

[goldtop5]

Does anyone have instructions on how to set the bias in a Randall RG50TC combo amp?

[52 Bill]

Adjust the two bias pots for 50mV across the 1 ohm cathode resistor of each output tube.

Check each tube for red plating and readjust as needed. 50mA might be a bit too hot for some modern EL34s.

[TimmyP1955]

With a B+ of 444 (per the schematic), that's a lot. I'd think 34ma would be more like it.

[The Dude]

Weber Bias Calculator

[52 Bill]

With a B+ of 444 (per the schematic), that's a lot. I'd think 34ma would be more like it.

Yeah, that's why I warned about red plating. The 50mV figure is also from the schematic.

[goldtop5]

Ok, I'll check it out. There was a little bit of red-plating on one of the tubes.

[goldtop5]

Problem fixed. I measured across to the 1 ohm resistors and one of them was about 20mV too high and the other was 10mV too low. No red-plating. Thanks a bunch!

[The Dude]

Check your screen resistors. Of course it's possible that your tubes are just that mismatched, but you may have a screen resistor that has gone towards open. 30mA is a pretty good sized mismatch. Another way to tell if it's amp or tube related is to swap the tubes into opposite sockets and see if the variance follows the tube or the socket. I can tell you that it would not leave my shop this way. IMO, that is not an acceptable match. Either the amp is still "broken" or the tubes are seriously mismatched.

[goldtop5]

I took amp off the limiter and variac and plugged it straight into the wall. I left it that way for an hour. So far the 50mV is holding stable and there was no red-plating. I'll check it again when our new shipment of tubes come in.