Bob & Linda McGraw K4TAX wrote:
>
>Art:
>Thanks for the info and the references.
>
>Interestingly enough of the 8 or so responses I've received, there seems
>to be a wide variance, according to the respondees, as to what EIMAC
>specs as the proper warm-up time. Some said 3 min, some said 5 min and
>some said 10 min. So as it goes, everybody has an opinion.......not
>necessarily facts however.
>
The Eimac data sheet says "180 seconds minimum".
>It got me to thinking, wonder what other tube makers spec as warm-up
>time. Perhaps different tube folks have different numbers. I don't
>have a clue where the 1 min figure came from as I built the amp and PSU
>some 8+ years ago.
>
>Anyway, this tube has 8043.8 hours on it. I put them all on it myself.
>Still cranks out the same power today as it did 8 years ago. In
>actuality, I doubt that it's ever been hit with RF drive within 10 or so
>min of turn-on. The fan/fil timer does time-out, allowing HV to be
>turned on, in 62 sec however.
>
>Guess I been lucky.
>
The exact warmup time is a judgement call, even by Eimac. If you connect
a high-resolution DVM across the heater, it acts as its own resistance
thermometer for the cathode, and you can watch the cathode heat up.
(This is best seen by powering the heater from a fairly high-impedance
source, eg a transformer of higher voltage, with a dropping resistor in
the primary or secondary, adjusted to give the correct voltage when the
cathode is thoroughly warm.)
For a typical "3 minute" cathode, you would see the AC millivolts and
tens of millivolts changing rapidly over the first full minute, slowing
down over the second minute, and changing quite slowly over the third
minute.
So how does the manufacturer decide the precise moment when the cathode
is "hot enough to run"? It would obviously be stupid to operate the tube
in the first minute, when the cathode is still not up to temperature,
but the difference between say 2.5 and 3 minutes might be something very
subtle, like a shortening of the useful life for that particular tube.
Since that is impossible to measure without a time machine, somebody
eventually has to apply "engineering judgement".
In QEX about 3 years ago there was a very interesting article by Mark
Mandelkern, K5AM about his 4CX1000 amp for 144. To save the heat load on
his mountain-top shack in summer, but still be ready for a sudden
sporadic-E opening on the second day of a contest, his amp had a "warm
standby" mode. The heater was continuously powered, but at reduced
voltage - essentially to save those first two minutes of warmup. This is
obviously uncharted territory, not in the data sheets, but he could
detect no short-term harm to the tube.
Oh dear... I just started to think, "Wonder if I'll get an e-mail
from Bill Orr about this?" Bill was a silent reader of AMPS for a long
time, but would quite often send private e-mails with useful information
and Eimac history. Like so many other AMPS readers, I'll miss him too.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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