[Amps] SB-200 Problem

[Gary J. Ferdinand]

Chuck,

Frequently an amp will take off like that only with a particular combination
of load and setting of plate/load capacitors. That it seems to happen with
an arbitrary setting of the caps tells me you more likely have something a
bit more fundamental going on.

Not a bad idea to check tubes for shorts, tho you say 2 different sets of
tubes exhibit the same feature - sounds like low probability.

Not a bad idea to probe for VHF resonances with a GDO.  Look in the
50-150MHz area.  While you're at it, see if you get a resonance on the band
you have it set to.

Have you tried powering on the amp with the tubes removed?  All ok then, I
suppose?

I still suspect you have an issue with the tank circuit - what the tubes are
seeing looking outbound towards the load.  If what you have is a short wire
(ie the pi-network is not connected, or something close to that), you could
be setting yourself up for a dandy VHF resonance.   Thru all this I'm hoping
you had a decent, non-reactive 50 ohm load...

I've had an SB200 get squirrely on me on 10 meters, and only with the plate
tuning cap near the extreme low-C end, and then only in a very specific
spot.  Other than that, I've always viewed the 572Bs in the SB200
(particularly the old vintage tubes) to be rather stable.  New tubes will
likely have more gain, but still with a proper load ought to be better
behaved than you describe.

GL/73

Gary W2CS


> -----Original Message-----
> From: amps-admin@contesting.com [mailto:amps-admin@contesting.com]On
> Behalf Of Chuck Mills
> Sent: Monday, March 04, 2002 9:36 PM
> To: Gary J. Ferdinand
> Cc: amps@contesting.com
> Subject: Re: [Amps] SB-200 Problem
>
>
> Yikes....lots of good info here.  I replaced all of the blown
> components I could
> find (RF Choke, the 33ohm from lug to ground) and threw the
> antenna switch ...
> it still pegs and the 33 ohm resistor will blow again (or throw
> the breaker at
> the panel...).  This is on the bench with NO rf into it.
>
> Chuck
>