Hello group. I'm trying to find the answer to why I'm having so much
trouble with my amplifier, but not having much luck.
The SB-200 performs well on 80, 40 and 20. However, on 15, it will not tune
properly - I may have a problem with the input circuit. My biggest problem
is on 10. Yesterday, I tried to tune on 10, but I can get only 50 watts out
for 100 in. I had the AG6K parasitic suppressors installed, took them out,
and re-installed the original Heathkit suppressors which resulted in only
200 watts out. A big increase, but not near what the amp should be
producing. Now, while I could tune for 200W, if I turned the "load" past 6
on the scale, the amp would begin to peg the forward and reflected power
needles on my SA-2060 tuner. The tubes I'm using are Taylor brand from RF
Parts - received in August. I also had a pair of Chinese tubes which had a
failure (filament went open). Is this "pegging" of the needles an
oscillation? I can actually get the amp to do this without any RF input. The
Taylors failed last night on me while sitting idle - taking out the power
supply. My first set of Taylors had failed too, but there wasn't any further
damage. Are the Chinese making tubes of such poor quality lately? While
tuning up, if I saw any more than a dull red glow from the tubes I'd stop &
let them settle for a couple of minutes. My plate voltage is 2250V. On the
lower bands, the plate current was 400ma, grid current was 250 - 300ma. On
10, the plate current was 400-450ma, but the grid current was only 150-200ma
with 200 watts out (if I remember correctly). The bias voltage is -133V
standby, and -1.99V keyed. I even dropped the filament voltage to 6.15V from
6.45V to try to keep everything in check.
When I get the oscillation & watch those needles move, I can "play" with
how much they deflect using the "load" knob. If I do this with the amp
keyed, but no RF input, there's an increase in receiver noise, and the more
the needles are deflected, the more noise in the radio. This noise has also
taken out an automatic antenna tuner on me that was between the radio and
amplifier, so whatever it is, it's harmful! Hoping someone here can help.
...Larry
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