Randall RD50 Bias

[Wes]

I have one of these I'm looking at and I'm wondering about the bias on this thing.
I know I post from time to time about bias on cathode biased amps; I think there's
something I'm missing. I always see these amps that have tubes biased what seems
to be way too hot.

After a few minutes of running there is red plating on this particular amp.

This particular amp has the following voltages:

2x EL34

A- 430 (K to A)

S- 417 (K to S)

G- -30 (K to G)

K- 30.9

Each tube has it's own 330R cathode resistor. Each tube is pulling 85ma!

There is nothing that has been changed in the circuitry around these tubes and all resistors fall in tolerance.

RD50 Sch0514.pdf


What is it with these companies setting these amps up this way?

[Chuck H]

Not sure what to say except that I agree with you. If the red plating is on both tubes then I vote to cool it off a bit. If it's only one tube then swap them and see if the problem is with the tube or the socket. If it's the tube then you can TRY a new pair of tubes. IME the Ruby EL34b seems happy with a few less bias volts than some other current production tubes. If the problem is with the socket check the cathode caps. Probably not the case since you seem to have an accurate grid to cathode reading.

[Wes]

Thanks for the reply, Chuck!

Both of the tubes were red plating. I went ahead and bumped up the K resistors to 500. The tubes are idling about 80% now.

I just can't believe that these companies send these products out setup like this.