[Amps] sb220

[Tom W8JI]

> 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