[Boatanchors] PA tube biasing?

[jeremy-ca]

Most of the dual tube tetrode amps that I am familiar with use a fixed 
screen voltage and have individual bias controls to set up the static 
matching.

There are tolerances in tubes, especially the ceramic/metal type which are 
very high gain, that make for the need to match.

As far as if one tube is weak then run one tube at a time at half drive 
level and block the air flow thru the empty socket with a rag. Im "assuming" 
that the Harris doesnt have all sorts of interlocks to prevent such a simple 
test.

Carl
KM1H


----- Original Message ----- 
From: "J Forster" <jfor at quik.com>
To: <eugene at hertzmail.com>
Cc: <Boatanchors at mailman.qth.net>
Sent: Sunday, December 16, 2007 8:38 PM
Subject: Re: [Boatanchors] PA tube biasing?


>I personally don't know, but there is a book, "The Care and Feeding of 
>Power
> Grid Tubes" published by Eimac that will, most likely, answer your
> questions.
>
> I suspect it's available on the web, but the Eimac site didn't work for 
> me.
>
> Best,
> -John
>
>
>
> eugene at hertzmail.com wrote:
>
>> Hi there,
>>
>> Working on my harris RF-110A (urt-23) and had a question about biasing.
>> I am reading the procedure to properly set the bias of the two finals
>> (4CX1500B).  The instructions explain that the difference in idle plate
>> current of the two tubes should not exceed 40ma.  To assist in getting
>> the two plate currents to within 40ma of each other, there is a diode
>> voltage divider that allows you to change the screen voltage of each
>> tube from 250v to 320v. There are two jumpers with 6 positions to allow
>> each tube's screen voltages to increase or decrease based on the
>> position chosen.  The two tubes are currently set for the first position
>> for V1 and the next to last position for V2.
>>
>> I have noticed that the two tubes are not identical, meaning they must
>> have been acquired at different times (one says eimac, one does not,
>> etc).
>>
>> My question is what causes the requirement of different screen voltages
>> to produce the same plate current? Is it simply differences in
>> manufacturing? Or does it indicate that one tube is older than the
>> other? If age is the cause, my concern is that with only one jumper
>> position left on V2 that I might be running out of life on the tube. I
>> suspect a 4CX1500B is very $$$.
>>
>> So, is age the primary cause of the need to change screen voltages in
>> order to balance the tubes? Or is it mostly mfg differences in which
>> case, I could conceivably keep these screen voltages for a very long
>> time to come.
>>
>> Thanks!
>> Eugene
>>
>> _______________________________________________
>
> _______________________________________________
>