Yes, that's one expensive but fantastic performing tube. I was looking
at using it in 1993, for 2 KW CW at 805 MHz, when RCA/Burle pulled the
plug on manufacturing the 8501 Cermelox tetrode, which was a beauty:
gold plated and having the pink Frenchtown NJ ceramics. Instead of
switching to the TH, we made a last purchase, and commenced on
developing a solid state amplifier to do the job instead. It worked
well, using a bucket full of MRF899 (early cellphone basestation BJTs),
and continues to run today. I still have a number of 8501 "trophys" at work.
I think that if you really ramp the filament slowly from zero, following
the exact recommendation of the mfr, you will not have filament
failures. Black heat works too, but thats only if you want to keep it in
standby mode, and switch on quickly. Depends on your operating style.
I've never built one with black heat circuit, using ramped current
instead, as we run them 24/7 until they need maintenance, and then shut
'em off.
73
John
K5PRO
TexasRF@aol.com wrote:
> Carl, that was a price quoted by Freeland Products, a tube rebuilder in
> the New Orleans LA area. New tubes were around $6K from Richardson
> Electronics the last time I checked. There will be no new TH347 tubes in
> my future!
>
> I have seen pulls for around $200 in Europe but they typically will run
> a maximum of 1.2A plate current where a good tube will run 1.9 to 2 A.
> Even at 1.2A they will output 1500W or more so a pull my be ok depending
> on what you are looking for.
>
> A recent batch of four pulls yielded one tube that did 2000W out, one
> that did 1500W out, one that did 900W out and one that arced and
> sparked, not recoverable.
>
> At this rate, a good pull is worth $400 to $800 depending on how good it is.
>
> I have, as suggested, considered running the tube with low filament
> voltage when off. My operating style may keep me away from 1296MHz for
> several weeks at a time so I am not sure I would get a benefit from the
> low voltage.
>
> Too many bands, not enough time!
>
> 73,
> Gerald K5GW
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