Hi John,
Thanks very much for the info. That's a great bit of data there. And
it's great to have that confirmation.
For the duty cycle, the ARRL if I remember correctly calls out the duty
cycles for unprocessed SSB at 20%, and processed SSB as well as
"conversational" CW both at 40%. And I want to say that PSK runs about
70% - something like that.
So to combine all this together, let's say that we have a CCS plate dis
spec of 100W. And assuming the scaling effect (duty cycle vs plate dis)
is completely linear, then that means we should be able to safely run at
these power levels - assuming here that we are only talking about the
tube to simplify the discussion:
Mode duty cycle plate dis
--------------------------------------------
RTTY 100% 100w
PSK (approx) 70% 140w
Unprocessed SSB 20% 500w
Processed SSB/CW 40% 250w
What do you think?
73/jeff/ac0c
John Lyles wrote:
> You are correct that RTTY is basically 100% on like FM, at least during a
> burst of transmission. So this is limited by the average plate dissipation of
> the tube, not to exceed XXX watts or KW. Thermally, a few seconds on is like
> CW as far as the tube anode structure is concerned.
>
> For pulsed ratings, the average dissipation limit is the same. If you are on
> 10% of the time, then the peak power may go up to the limits of the cathode
> emission current of the tube in some cases, however, the plate dissipation
> still sets the limit for the thermal loading on the anode. If you run 10 X
> the CW power level and are on 10% of the time, its the same dissipation as
> CW. You cannot cheat and get more dissipation on an average than the tube is
> capable of. You can get high peak dissipation, but that really isn't the
> definition of dissipation. Its an average thing. There is a limit to how long
> you can leave the pulse on, however, before the tube makers say no. In other
> words, you cannot run, like 1 second on at 10 X the average dissipation, then
> wait 10 seconds and do it again, and expect long life. Fusion RF systems run
> a blast of RF for 20-100 seconds, and they are considered CW. Particle
> accelerators like where I work run 10% DF, so we can crank up the peak to
> quite
h
> igh. We
> have 250 kW of plate dissipation in triodes, and on a peak basis its like 2.5
> MW. But thermally it is only 250 kW of power into the copper. The pulses are
> short.
> Oxide cathode and thoriated tungsten tubes have their favored regimes with
> respect to peak cathode emission, which is often another unmentioned limit of
> a tube.
>
> As for SSB, I defer to those who practice making SSB amplifiers.
>
> 73
> John
> K5PRO
>
>
>> Message: 7
>> Date: Wed, 11 Mar 2009 21:51:34 -0500
>> From: Jeff Blaine <keepwalking188@yahoo.com>
>> Subject: [Amps] What is the true and actual meaning of a plate
>> dissipation limit?
>>
>> 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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