[AMPS] Technical Question #1

[Tom Rauch]

> Date:          Thu, 21 May 1998 11:43:40 -0500
> From:          Jon Ogden <jono@webspun.com>

Hi Jon,
 
> I believe there is some neutralization that can be done in GG amps.  The 
> most important aspect I recall has to do with making sure that there is a 
> balance between the two tubes.  A single tube amp need not worry about 
> it, but in 2 tube amps there can be a problem.  Someone please correct me 
> if I am incorrect.  I don't have Mr. Orr's book handy at the moment.

Don't rely entirely on Mr. Orr's book. That is a collection of data 
from many sources, and it was never really peer reviewed.

Many other books do describe neutralization of GG amps, it is no 
major feat. Electronic Designers Handbook contains such circuitry, 
as well as many other engineering textbooks. 

The Heathkit Warrior and Gonset (4) 811A amps were both neutralized, 
and are much more stable at HF than amps that are not neutralized.

In most parallel tube applications it is not necessary to neutralize 
the tubes separately.  You simpler invert the phase 180 degrees in a 
broadband transformer, and couple the filament to the anode.

The AL-811H and AL-1200 both use this technique. Both are 
completely stable with any terminations on input and output and at 
any control setting, unlike amps using the same tubes without 
neutralization.

You can measure the effect of this by disabling the filament supplies 
and feeding RF into the output port of the PA, and measuring RF at 
the input port. The AL-811H has about 10-20 dB less stray input to 
output coupling with the neutralizing system active on upper HF.

Lack of neutralization is why Clippertons,  FL-2100's, and other amps 
oscillate when unloaded on ten and 15 meters (and sometimes lower 
bands).  

RCA neutralized a grounded grid UHF pre-amplifier, to improve 
noise figure.


73, Tom W8JI
w8ji.tom@MCIONE.com

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