> My impression is that the 1 KW limit was regulatory rather
> than
> technical. The amplifier is capable of rather more - mine
> puts out
> 900-1200 watts depending on band (including the exciter
> power, of course),
> with about 100 watts drive and grid current well within
> ratings.
The SB220 was designed to meet the FCC rules current at the
time it was manufactured. There was never any thought or
consideration given to exceeding those limits. No one ever
dreamed the FCC would change to a power output method and
especially to 1500 watts, and Heath never had any interest
in marketing anything illegal or for illegal use.
The entire exciter power does NOT feed through. The only
part of the exciter power not accounted for in the anode
power input readings is the small additional anode voltage
caused by swinging the filament negative on RF drive
negative voltage excursions at the cathode. 100% of the
additional anode current is metered, it is only the HV that
reads incorrectly and the total error is very small.....so
we shouldn't think we have 100 free watts at the output. It
just does not happen. 20 watts is more like it.
Since the amount of unaccounted for input power varies with
the cathode swing and since it is theoretically possible to
use a tube with a very high cathode impedance and obtain
more output than dc anode input, the FCC had to control the
worse case situation. Worse case is a long way from actual.
The cooling system of the SB-220, the power transformer, and
everything else was sized to run ~1kW dc input on CW. At the
typical anode efficiency of 60-65% that's 600-650 watts out
plus about 20W of unaccounted feedthrough power caused by
the increase in effective anode voltage on negative voltage
drive swings.
73 Tom
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