Hello all,
Been working on an old SB-220 amp and have highly modified the entire
unit. Using vacuum caps for tune and load along with new heavy duty
band switch etc.
One problem that has me puzzled is attempting to tune up on 160 meters.
At 2mhz all is fine. Low coloration of the tubes and efficiency is good.
Amp delivers over 1200 watts into dummy load. As I lower frequency to
1.9 and worse yet 1.8mhz, tubes begin to glow extremely red, amp still
can deliver one kilowatt out but tuning on the load side is sluggish and
I believe something is not happy in the tank circuit. I might add that
all components remain cold, just efficiency seems to go down and output
coupling is not happy.
I suspect the plate choke but have not confirmed this yet. My blocking
cap is 0.01uf and plate choke is 235 uh.
Wondering what a good number might be for plate choke at 1.8mhz.
Impedance at 2mhz shows choke to be around 3000 ohms, but drops to 2500
or so at 1.8mhz.
Suggestions, comments, welcome.
thanks
chris WA3PBL
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