Dear David,
In my amplifiers I mount a pair of pin jacks to the apron
in the RF input plenum and connect them to the tube socket,
with a suitably-sized bypass cap on the insides.
For a.c. RMS volts I use a Fluke 89-IV DMM with a.c. RMS
display. Most any panel meter will be useless because of inaccuracy,
because of residual magnetism inside it screwing up the accuracy,
and mostly because fifty millivolts either way might mean hundreds
of hours more life for your tubes.
Hal
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of David C. Hallam
Sent: Sunday, January 28, 2007 2:51 PM
To: Amps@contesting.com
Subject: Re: [Amps] 8877 Longetivity
At the risk of starting another long thread, I will ask a question.
Given the relative low accuracy of normal AC panel meters, how can you be
sure that you are actually within the filament voltage tolerance specified
by the manufacturer? A high accuracy AC meter is a rather large and
somewhat expensive animal.
David
KC2JD/4
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]On
> Behalf Of Nelson Wittstock
> Sent: Sunday, January 28, 2007 2:36 PM
> To: Amps@contesting.com
> Subject: [Amps] 8877 Longetivity
>
>
> FWIW I am using an 8877 in my amp that I built back in 1977. It
> is still the
> original tube. My filament transformer is run off a Variac and I have a
> panel meter to monitor and see that it is five volts. I also have a timer
> interlock so that the HV cannot be turned on until the tube has had three
> minutes to warm up. When I shut down I run the Variac back down
> to zero and
> leave the blower on for a few minutes for cool down. The amp was
> built from
> a QST article in the September 1971 issue. It has seen a lot if use. I
> added 160 meters to it and it has served me well.
>
> Nelson, K8DJC
>
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>
>
>
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