[Amps] sb220

[Pete Smith]

Well, there goes another myth of my youth - thanks for clearing up the 
exciter power issue.

As for power output, my decently-calibrated wattmeter says my SB-220 has 
been producing roughly 175% the design power on CW, in contest service, for 
the last 10 years.  Mine has the Harbach high-volume fan, as well as a 
muffin fan to help power supply cooling, and a vacuum relay antenna 
switching system.  It has been reliable, except for occasional filament pin 
desoldering, and even that seems to be cured (fingers crossed) after the 
last round of fixes.  Bottom line - the SB-220 is a nice conservative 
design, still putting out a lot of watts-per-dollar after all these years.

73, Pete N4ZR

   At 10:52 PM 4/29/2007, W8JI wrote:
...

>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.