[AMPS] [Fwd: {Collins} Tube Filament Voltages]

[KB7WW Art Moe]

Comments ???


Art
KB7WW
-------- Original Message --------
From: "James C. Garland" <4CX250B@miavx1.acs.muohio.edu>
Subject: {Collins} Tube Filament Voltages
To: collins@listserve.com


Hi Gang,
I've enjoyed the messages on the relationship between tube life and
filament voltage, and would like to add just one small caution: it is
important never to run inadvertantly an indirectly heated cathode
transmitting tube BELOW its ratings. 

Here's a sad, real-life story to illustrate this point. A few years
ago, I
homebrewed an HF amplifier, using an 8877 triode.  Having shelled out
$400+
for a new Eimac tube, I was naturally interested in prolonging the
tube
life in my design. The 8877 is nominally rated at 5V filament voltage,
and
the data sheet says that the voltage should be maintained between
4.75V -
5.25V.

Reasoning that longest tube life would result from the low end of the
range, I carefully set the voltage, measured at the tube socket, to
4.80V.
The amp worked great and I was confident that I wouldn't have to shell
out
another wad of cash for another tube.

Then I moved to another QTH, where the line voltage was subject to
some
daily fluctuation. After a few months I noticed that my amp wasn't
loading
as easily to full output, and a few months later I began to suspect my
costly 8877 was going soft.  A few more weeks and I had to face the
truth.
I had a bad tube.

 How was this possible? In diagnosing the problem, I learned that at
my new
QTH the filament voltage often hovered around 4.6V.  Running my tube
only
0.15V below the minimum tube rating had wrecked the tube in fewer than
a
hundred hours of operation!  I had outsmarted myself by trying to run
the
tube too close to its lower filament rating. 

Now I run my amp at its rated 5.0V, and have experienced no problems
in
several years of operation. Most hams  wreck their amplifier tubes by
drawing excessive grid current, exceeding the rated cathode current
limit,
or exceeding the plate dissipation rating. Few hams lose their tubes
because they have exceeded the normal filament life. Based on my
experience, the risk of running an expensive vacuum tube too close to
its
lower filament rating outweighs any possible benefit.

73,

Jim Garland W8ZR

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