? With 33v of cutoff bias, anode-current in healthy tubes should
probably be under 50uA. My guess is that there is a leakage problem
with both tubes. To find out for sure, a high-pot test is the best.
If you need to have the tubes high-potted, I can do.
Well, there could be some leakage, but it wouldn't explain the 70mA Ip
reading in standby. I think if the tubes were leaking that much they'd be
getting pretty soft. But the real clue is that the 70mA reading exists
whether I run with one tube or the other, or whether I disconnect the HV
lead from the HV power supply board completely! With no HV connection to the
plates at all, it certainly doesn't seem possible that tube leakage could
have anything to do with the 70mA reading. Besides, the voltage across the
10 ohm shunt resistor with HV disconnected from the plate circuit is 15mV,
which computes to just 1.5mA of current. That's a far cry from 70mA. So I
still think the primary cause of the problem is in the op amp circuit used
to measure voltage across the shunt resistor.
Of course, there remains the mystery of how any current can be flowing
through the shunt resistor with HV disconnected. Aside from the cathode bias
circuit and the -HV end of the power supply, there are two circuits
connected to the shunt resistor -- the cathode bias supply and the op amp
meter circuit. I don't think any current flows from the cathode bias supply
back through the shunt resistor to the -HV side of the suppy because it
would have noplace to go from there. I haven't quite mapped it out, but
perhaps some current is actually flowing through the op amp meter circuit.
Perhaps whatever is wrong with that circuit accounts for this, too. I'll
have to look at that more closely.
I'll have to confirm it, but with HV reconnected the voltage across the
shunt resistor increases from 15mV to 30mV. As I said, that might be
evidence of a small amount of leakage in the tubes, but not enough to
account for the other mysteries.
So how hard do you have to tap the anodes?
73, Dick, WC1M
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