Gentlemen,
Wanted: an understanding of the actual and true meaning of the plate
disipation limits with respect to duty cycle.
I may have missed it, but it seems the answer is hiding at least from me.
All our tubes have plate dis limits associated with them. Sometimes
there is an associated cooling requirement with it as a footnote, but
beyond that, not much else is said.
Say a guy loves SSB (low duty cycle) and RTTY (100% duty cycle). The
rule of thumb in some cases is to run the RTTY mode at 1/2 the typical
power of SSB. But this is often stated without explaining why the RTTY
power level specified as 1/2 is the right level from a specification or
design standpoint.
I realize that in the greater scheme, there are a host of components to
consider when talking about an amp as a whole. But here i am addressing
the tube only as an isolated case.
Eimac's C&F does not mention RTTY that I recll, but they do talk a lot
about commercial 24/7 FM service - and that's a 100% non-stop mode;
equivalent to RTTY. They suggest in the C&F documents that the tube
will run up to the rated plate dis in CCS. OK. Maybe the
interpretation is that the Pd-max is a hard limit? Valid for all time
and all cases.
And then there are the pulse applications that come along and spoil the
CCS argument. Many tubes have a pulse rating - or in the case of many
of the Russian tubes - a pulse rating spec set only without CCS duty
being adequately specified.
In these pulse duty cases, the time averaged plate dis is below the
published limit, I'm sure. But for the pulse duration, the Pd is going
to be exceeded by a huge margin.
That means, that in some lower duty cycle circumstances, the assumed CCS
Pd can be safely exceeded.
However I cannot find an explanation that ties the duty cycle to the
plate dissipation. Reconciling the two data points. Either on a
derating or pulse basis - even as a rule-of-thumb kind of factor.
The usual sources are not clear on the point. The Eimac literature does
not come out and say it clearly. Bill Orr loves heavy metal for
transformers - but for SSB duty, has no problems exceeding the CCS specs
on many components including tubes. And nothing on the net that I've
seen links a position and some data or logic into something that is more
substantial that would pull it out of the opinion and into the
engineering basis category.
Hoping that one of you guys working in the industry - or having
encountered this question before - may have the magic answer that hooks
the plate dis and duty cycle together...
73/jeff/ac0c
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