On Mon, 15 Mar 1999 11:57:38 -0700 "Richard W. Ehrhorn" <w4eto@rmii.com>
writes:
>
>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!
God, I'm glad you responded Dick. I was getting a bit frustrated with the
comments from outer space and maybe even beyond. What is even worse are
comments from those who have not a clue as to the tube and its specs and
just throw out numbers. I and many others have years of experience with
the 3CX800A7 but not with the intricacies of the 87A.
Definitely not an amp for Internet "experts". I defer to you, John, et
al.
73 Carl KM1H
>
>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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>
>
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