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Re: [Amps] SB-200 DATA

To: <>
Subject: Re: [Amps] SB-200 DATA
From: "jeremy-ca" <>
Date: Tue, 17 Jul 2007 19:48:09 -0400
List-post: <>
The 811A is decsended from the pre war 811 and the much earlier 211 with 
most internal problems still intact. However the USA tube is factory rated 
to 54 or 60 MHz depending on the manufacturer and where/when you read the 

>From what Ive seen of some commercial ham amps over the decades Edison is 
alive and well.
The much touted Collins 30L1 is another example and is well known to be 
squirrely on 10M even with Mil Spec RCA 811A's much less the Chinese and 
Russian versions.
The Dentron Clipperton L, & Hunter Bandit 2000 are examples of 4 x 572B amps 
that have issues.

Converting the SB-200 to a 6M monobander is simple and straightforeward when 
basic design principles are followed. Ive been doing those conversions since 
the late 60's and it is completely stable and repeatable. Power output and 
efficiency is the same or better than a good stock unit on say 20M.


----- Original Message ----- 
From: "Tom W8JI" <>
Cc: <>
Sent: Tuesday, July 17, 2007 5:49 PM
Subject: Re: [Amps] SB-200 DATA

>> 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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