Paul J: ef86 cathode servo circuit questions

[arveedub]

Could you explain the design behind the servo circuit on the ef86 cathode?

On a (I think) related note: does the servo lessen the need for tube matching? I.e. does the circuit provide some leeway for different ratings on both side?

Thanks!

[Paul Joppa]

There are actually two feedback loops, both sensing the second stage cathode voltage. The first feeds that voltage to the EF86 screen grid. This was in the original design, and is good enough for EF86s that meet their specified characteristics closely - i.e. mostly NOS from the better manufacturers. We use that in the Repro with a quiet LED in the cathode.

Our experience with the wider range of modern manufacture EF86s is that they vary much more widely, and many will not bias up properly in that circuit. So I added the transistor, whose base goes to a voltage divider on the cathode of the second stage. When that voltage reaches about 0.6v, the transistor conducts, and it controls the EF86 cathode current. That transistor has high impedance and must be bypassed; we use a low-voltage polymer electrolytic which has excellent high-frequency performance.

In both cases, the dominant pole of the negative feedback loop (which controls stability) is set by the second stage cathode bypass capacitor.

[paswen]

Could one remove the self biasing circuit if one were to use only high quality NOS EF86 tubes? Would this involve removing the 2n222 transistor, jumpering the 237 ohm resistor that goes to ground, replacing the 4v capacitor with a LED and leaving the Ef86 screen grid as connected? Or is this more involved?     Thanks, Paul Swenson

[Paul Joppa]

It's late Saturday night and I have a bad cold, but I think that is all it takes. Check the 6922 plate and cathode voltages; they should be 70 volts apart, +/- 10v or so.