> It shouldn't be that sensitive to a grid resistor value.
> If it is, you must have little phase margin, and I would
> suspect too much phase shift in the tuned input at freq of
> oscillation if it is in band.
That has little to do with it unless it is a solid state
amplifier that is already right at the edge of stability.
Many things that apply in special cases don't apply at all
in other cases.
The problem in the SB200 is the tube design used was
basically intended for audio applications, not RF. It has a
single very thin, very long, grid lead. This means at some
frequency (it happens to be just over 100MHz) the grid is
effectively floating, and the tube can easily become a TPTG
oscillator. The control grid is one primary system, the
anode the other, and the input is largely out of play.
Virtually all VHF problems in tubes involve only the grids
A seconday offshoot is the grid provides very poor
shielding, and the anode to cathode path is involved in
feedback on frequencies below around 80MHz. This is why so
many manufacturers neutralized 4x 811A or 4x 572 amplifiers.
Two 572 tubes are about at the limit of stability on
20-30MHz because of feedtrough from anode to cathode, and
de-Qing the cathode is no help at all unless it adds
significant swamping losses at the cathode. Network phase
shift, unlike in broadband solid state amps, is unimportant
because the exciter and interconnecting cables cannot be
depended on to load the input of the amplifying device.
The only real cure is you have a suppressor that loads the
anode very well at 100 MHz, and either load the anode so
heavy it keeps gain down or do something else to swamp the
amp at the operating frequency with loss. It is absolutely
not a phase delay issue in the networks, as it is not in
most tube PA's.
> You should be able to take a decent linear and swing the
> bandswitch and caps all over and NEVER have it take off,
> especially in grounded grid. It is back to the drawing
> board if it does.
The problem is there is always a tradeoff between cost and
headroom. Heath, like others, decided to use 572's but NOT
neutralize the tubes. It is a very marginal stability design
on 15 and 10, and 572's and 811's are heck anyway on VHF.
The grid is really not worthy of being called a good modern
transmitting tube design.
Now the Japanese tried to neutralize the FL2100, but did
some very poor engineering. They tied the neutralizing cap
from the output of the pi network to the input. That doesn't
give 180 shift, and it isn't stable in feedback level or
phase. So they get ten points for knowing they had a problem
and knowing what was needed, but deduct 9 points because
they didn't do the system anywhere near correctly. Even
Gonset in the 50's and Heath with the Warrior knew how to
> Is not a SB-200 GG ?
Yes, and the grids are not grounded at all at 100 MHz or so.
They are only poorly grounded at 30MHz, but they are
grounded very well at 1 MHz. So it is only a "sorta grounded
This is why the SB200 is sensitive to anything you do in the
grids or anode.
Now the disappointing part. If you fiddle with the grids and
anode using some kind of Edisonian cut-and-try engineering
rather than really fixing the actual problem, the cure you
get will be totally unreliable because of the instability
that was never really properly addressed. It is an
unrepeatable or unreliable cure to just fiddle around with
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