Re:
"Hi,
I hope someone can give me some hints on isolating this problem in a
SB200.
I don't know if there were any symptoms prior to the failure since I
just
got this amp.
The SB200 has excessive grid current and zero plate current. When I key
the
amp the grid current jumps up to more than 200 ma in the grid meter
position
(full scale +). The plate should read about 90 ma but it is zero. I
then
pulled the 572b's and when keying the amp I get zero grid current
(except
for a slight bump of the meter reading while the relay operates). I also
got
zero plate current. High voltage looks OK. I checked the tubes for
shorts
with an ohmmeter and they were OK. Put the tubes back in one at a time
and
with either tube or both the amp reads the excessive grid current and no
plate current. I checked the grid bias and when keyed it was -2 vdc as
it
should be. The six 30 k ohm resistors in the PS run very hot and have
toasted the circuit board, but I guess that is "normal". During this
testing I get almost no output from the amp. With 10 watts drive I get
maybe one or two watts output. I and able to peak the small amount of
output which make me thing the tank circuit is resonating correctly.
Thanks for your help,
Dan AF9R"
Dan,
Sounds like your HV is not reaching the plates. Check the plate RFC,
etc. Measure HV at the plate caps--but only if you have the proper
safety knowledge.
Good luck, Roy K6XK
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