[Boatanchors] SB-220 in AM service

[Jim Wiley]

I use an old Drake L-4B in AM service, and have used it for more than 20 
years, with no failures - and the same set of tubes.  I usually set it 
for about 350 watts unmodulated  carrier output (the max allowable is 
375 watts, which which with modulation equals 1500 watts PEP).  I tend 
to be long winded, but the L-4B is a continuous duty rated amp, and was 
built properly.  It has a pair of 3-500Z tubes, a squirrel cage blower, 
chimneys on the tubes, tube sockets in a pressurized plenum, plus a 
separate power supply that can really take some guff.  


The SB-220 should be OK at  250 watts  or so out, as long as you limit 
continuous talk time to 10 minutes or less , with no more than a 50% 
overall duty cycle. Keep your eye (or nose?) on the transformer - if it 
starts to smell hot, back off on the talk time.  For a test, I ran the 
L-4B on 20 meters at 1000 watts output  for a hour into a dummy load - 
other than the power supply starting to smell a bit warm,  and the tubes 
getting sort of orange, no problems.


Before the rules for AM changed, I used to run it at about 400 watts 
carrier output, mostly on 10 meters AM, also with no problems.  However, 
I did manage to unsolder the connections inside an external low-pass 
filter doing that.  


Good luck.  Maybe I will hear you on  the band once I get my 10-meter 
antenna back up. 


- Jim, KL7CC


 
>>> Greetings Anchor People
>>>
>>> The Heath SB-220 in the shack is seldom used and I would like to use 
>>> it in
>>> AM linear mode with  low power exciters such as the Heath DX-60, Knight
>>> T-60, HT-32 / 37, HX-500, WRL DSB-100 etc.  As its rated for 1KW CW 
>>> RTTY
>>> (10 Min max) I thought that 250W input on AM should not stress the 
>>> power
>>> supply or tubes, in the low power position (graphite tubes) At 60%
>>> efficiency that should be approx. 150W rf output. Any thoughts from 
>>> others
>>> who have used the Amp on AM ?  Thanks for your input  Bill KE7KK SD
> Bob Bethman/NØDGN/BC-610/Northern Radio SP-600 Manassas, VA - 
> MultiBand Dipoles
>
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