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Re: [Amps] SB-200 DATA

Subject: Re: [Amps] SB-200 DATA
From: "Tom W8JI" <>
Date: Tue, 17 Jul 2007 17:49:45 -0400
List-post: <>
> 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 
and anode.

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 
neutralize properly.

> 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 
grid" amplifier.

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 
the grids.

73 Tom

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