[Amps] sb-220 on 17 meters

[Vic Rosenthal]

peter wrote:

> I recently aquired a sb-220.  When I try to tune up the amp on 17 meters
> using the 15 meter band switch I get only 300 watts out and both tubes
> really are working hard.  I get 1kw out on 15 meters and 20 meters.  Is
> my plate choke resonating about 18 mhz ?  Any help would be appreciated.

It's possible.  You can check this by temporarily shorting the choke, 
using the shortest possible piece of wire, and checking for resonances 
with a GDO (amp power off, of course).  In this case you might also see 
signs of discoloration on the choke from heating.  You can raise the 
self resonant point a bit by removing a few turns, or you can buy a 
choke from Ameritron as used in their amplifiers.

It's also possible that the input circuit is presenting a high SWR to 
your exciter, causing it to reduce power.  I'm not sure what you mean by 
'the tubes are working hard', but if it were an input problem it would 
appear as low drive (low grid and plate current).

-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco