Considering the source, believe it. There is a very good reason
the manufacturers specify a warm-up period for indirectly heated
cathode tubes ... to make certain that the cathode does not run
too cold.
Most tubes are rated for at least 10,000 hours of emissive life.
Assuming an active amateur averages two hours a day of filament
time (even a contester won't run the amp 48 hours every weekend),
that's a rated lifetime of 15 years. In amateur service one is
more likely to lose a tube from due to an open filament from the
turn on transient than "old age" as long as the tube is operated
within its ratings.
Keep the filament voltage at "nominal" and check it when there
are significant changes in power (new transformer, new QTH,
new/repaired wiring, etc.) and whenever you open the case to
clean(!).
73,
... Joe, K4IK
> -----Original Message-----
> From: owner-amps@contesting.com [mailto:owner-amps@contesting.com]On
> Behalf Of KB7WW Art Moe
> Sent: Sunday, December 24, 2000 11:12 AM
> To: Amps
> 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
> Nets: Tues: 3.805 Mc-2000 Central / Thur: 3.875 Mc-2000 Central
> Fri: 3.895 Mc-2000 Pacific / Sun: 14.263 Mc-2000 UTC
>
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