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Re: [Amps] Average anode dissipation using SSB ?

Subject: Re: [Amps] Average anode dissipation using SSB ?
From: Steve <>
Date: Mon, 15 Aug 2022 19:27:04 +0100
List-post: <>
Rule of thumb - for a class AB amplifier, device dissipation is roughly constant from 10-100% output power.

Steve G8GSQ

Average anode dissipation using SSB ?
Ok, this in reference to average anode dissipation only, and NOT the
average load on the B+ supply.
On my L4B, using SSB, and a lot of processing, average plate current is
400ma....exactly 1/2 of key down (800 ma). PEP output is 1290 watts on 75m
into a 1:1 DL..or ant. Switch the CD meter to average, and it only averages
375 watts. Average PO is only 29% of pep output.
Average DC input is only 1000 watts (400 ma @ 2500 vdc). Average output is
only 375 watts. Average anode dissipation is now 1000-375 = 635 watts. Is
this correct ?
With a dead cxr (800ma @ 2500 vdc) it's now 2000 watts DC input..and 1290
watts average PO......and 64.5% tank eff). Anode dissipation is now 2000 -
1290 = 710 watts.
635 watts is a little less than 710 watts, but not a huge amount. When
switching from SSB..... to a dead cxr, several times, while watching the
anodes of the pair of 3-500Z tubes, they show less colour on SSB, but not a
huge amount. Idle current, with 7.2 vdc of bias is 100 ma.
I highly suspect the average anode dissipation on ssb is a lot higher than
most folks realize. I may well be wrong, but I don't know how else to
calculate average anode dissipation. Idle current on SSB also plays a part.
OEM drake setup used no bias at all, and typ 220-230 ma of idle current
(575-600 watts of idle power). I added 10 x 1N5408's in series to the
CT..and brought the idle down to 100
ma..and IMD is excellent. Idle power now down to just 260 watts. But the
added bias was done years ago.
Am I on track here ? Of course with any metal tube, you can't see anode
colour like a glass tube. I'll try measuring exhaust air temps next.


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