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FW: [AMPS] 87A Ip Meter Problem (long)

To: <amps@contesting.com>
Subject: FW: [AMPS] 87A Ip Meter Problem (long)
From: km1h@juno.com (km1h@juno.com)
Date: Mon, 15 Mar 1999 17:32:26 -0500


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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