Every amp I build has a filament volt meter calibrated against a
true RMS meter while measuring at the tube pins. Although my
line voltage is a very stable 243 volts, it is just good sense to glance at the
meter
occasionally to make sure everything is within specs. I adjust my 8877
filaments to no less than 4.95 volts and no more than 5.0.
It is also a good idea to look at the meter while keyed down at full power.
I have seen many ham shacks that were under-wired, creating a drop of
several volts between the service entrance panel and the wall socket.
Long, small line cords from the amp to the wall socket will also cause
the same problem. All my sockets here are wired with #4 wire to the
breaker panel. The voltage drop is less than .75 volts with a 40 amp load.
Over-engineering in this area is a good investment!
Season's Greetings to all on the reflector.
(((73)))
Phil, K5PC
Subject: [AMPS] [Fwd: {Collins} Tube Filament Voltages]
>
>
> 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
>
> Sponsored by the Collins Collectors Association
> http://www.collinsradio.org
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>
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