The original 2061 ran the EL84s way over data sheet spec. too. With nearly 400V B+ and a 125R cathode resistor it's almost guaranteed to red plate. That's how they were able to get 20W out of two cathode biassed EL84s.

There's no easy way to calculate the sag resistor value because it's between the rectifier and first filter cap where the current waveform is a series of narrow pulses. I found the B+ voltage drop to be quite a bit more than 20V when the resistor was between the rec. and first cap. It's not easy to calculate the resistor power dissipation either. I measured it (true RMS) and it was 6W with the amp at 15W output so a 10W resistor should be OK. I'd try a 220R sag resistor to start with then adjust it (if required) to make the plate dissipation 12W or lower.

There's no easy way to calculate the sag resistor value because it's between the rectifier and first filter cap where the current waveform is a series of narrow pulses. I found the B+ voltage drop to be quite a bit more than 20V when the resistor was between the rec. and first cap. It's not easy to calculate the resistor power dissipation either. I measured it (true RMS) and it was 6W with the amp at 15W output so a 10W resistor should be OK. I'd try a 220R sag resistor to start with then adjust it (if required) to make the plate dissipation 12W or lower.

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