[Amps] Average anode dissipation using SSB ?

Steve g8gsq72 at gmail.com
Mon Aug 15 14:27:04 EDT 2022


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.
> 
> Jim VE7RF



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