[AMPS] Re: YC156 heater/cathode question

[Phil Clements]

Thanks for the excellent info, Dave!
I have printed out your post and added it to my tube data book.

(((73)))
Phil, K5PC

> A couple of points here.
> 1)  You can apply HV any time you want.  Just don't
> draw any cathode current during the six minute warmup.
>  In other words, keep the cathode biased negative with
> respect to the grid until after six minutes.
>
> The first company who used the YC-156 (Erbtec, now
> Colorado Med-Tech) kept the HV turned off until after
> the heater warmup time.  That was their choice, and it
> was merely convenient for Eimac to write it into the
> specs.
>
> Likewise for the once per day on/off cycle - the first
> user of the tube planned to use it in that manner, so
> it was written up in the spec sheet.  You can believe
> during amplifier development that I saw heaters cycled
> a lot more than that, and the tubes still lasted as
> long (18,000 hours) as those operated with a once per
> day cycle.
>
> 2)  An earlier post suggested that the six minute
> warmup is because the heater is unusually small for
> this size tube.  Actually, this is one of the largest
> heater/cathode arrangements that Eimac makes in the
> oxide coated family.
>
> The heater and cathode have a vacuum for insulation
> between them.  Therefore, the thermal transfer is
> mostly via radiation, and is quite slow.  Oxide
> cathode tubes (separate heater and cathode) always
> have relatively long warmup times.  But there's a
> benefit.
>
> "Instant-on" tubes with thoriated-tungsten filaments
> (3-500Z, 3CX1200A7, others) suffer every time the
> filament is turned on and it passes through the
> crystal realignment temperature region.  (There's a
> wonderful app note on Burle's website about this).
> Therefore, "instant-on" tubes have a finite number of
> turn-on cycles more or less built into them.  In
> broadcast service, where the filament is left on
> continuously, the lifetime is remarkably higher than
> in amateur service.
>
> Hope that helps!
>
> Dave W8NF




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