Hi Dick...
You're absolutely right - when HV is physically disconnected from both
tubes' anodes, it's just really improbable that those mystical little gold
balls we so often hear about -- the ones that infest the tubes of every amp
not fitted with one specific type of equally-mystical parasitic suppressors
-- it's just really improbable that they can cause the tube(s) to draw 75
mA of Ip!
In fact, through 29 years of shipping thousands of amps using 8874/
3CX800A7/8877 class triodes, I've personally NEVER seen a tube draw more
than a few hundred uA of Ip under "cutoff" bias conditions. For a while
about 25 years ago, an occasional 8874 (old or new) developed
heater-to-cathode leakage, which in turn pulled down the bias supply so
that the tube(s) would draw Ip in standby. But of course the tubes weren't
actually under cutoff bias, and problem was defective cathode construction
- not gold balls.
I'm no expert on electrical minutia of the 87A control system, but my guess
is you're right about the Ip indication being an artifact, probably related
to the op amp or related circuitry - an annoyance but nothing to worry
about. And definitely not a tube problem. Carl is the right guy to talk to
at Alpha/Power - but W0UN owned two 87A's long before taking over the helm
there, so he's quite familiar with the amp first-hand, too. John is in BV-
land this week but should be back at work in Longmont by next Monday or
Tuesday.
Good luck & 73, Dick W0ID (formerly W4ETO)
-----Original Message-----
From: Dick Green [SMTP:dick.green@valley.net]
Sent: Monday, March 15, 1999 8:32 AM
To: Rich Measures; Amps
Subject: Re: [AMPS] 87A Ip Meter Problem (long)
? 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.
Dick Green replied.... 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...
--
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