>? 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.
? This is not normal. If I did not have a high-pot., I would remove the
tube, connect a 1k ohm resistor (shunted by 1N5400-series glitch
protection diodes) in series with the hv neg. return which I would
connect to the grid collet. I would connect the anode to hv pos. By
measuring the v-drop across the resistor with a DMM, the leakage current
can be calculated by sliding the decimal point. .
>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?
>
? enough to make strong men weep. . I use the flat side of a 2oz.
ballpeen hammer, striking flat against the top of the anode connector.
By watching leakage current, one can see what works. I hammer until the
leakage current stops decreasing. My guess is that the force amounts to
c. 40Gs. After the gold meltballs have been moved to the base, one
should keep the anode end up. .
- cheers, Dick
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|