Roy, and the group: It's entirely possible the tubes are at fault because
they may be shorted only when the filament is hot (energized) the only way
to know is with known good tubes.!!Also, you better check all the metering
resistors, grid and plate, and also , as mentioned, the plate voltage, with
the proper meter and ALL safety precautions.!!!I would bet that either the
tubes are bad or you've had a parasitic osc. that killed the metering
resistors. Check the continuity from the pwr. supply to the plates. I'm not
the ultimate expert on these amps but I have repaired a few.I think the
SB200 is a very good amp, but let's face it, it needs to be up-dated as any
amp from way back when needs to be.!!! there several things that need
up-grading.
carl / kz5ca
----- Original Message -----
From: "Roy Koeppe" <royanjoy@ncn.net>
To: "Amps Reflector" <amps@contesting.com>
Sent: Wednesday, November 26, 2003 8:34 AM
Subject: [Amps] Re: SB200 Grid Current...
> 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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