[Amps] SB200 Excessive Grid Current

[Dan Baker]

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
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