[AMPS] neutralization

[w8jitom@postoffice.worldnet.att.net]

> >>If you look at the Heathkit Warrior and Gonset 811A PA, you'll see 
> >>they are neutralized. So is the AL-811H.

> The Warrior and the Gonset were thought to be neutralized.  However, they 
> were not and they were not stable.  The SB-200 and the SB-220 were not 
> "neutralized" because someone at Heath came to their senses.  

The SB-200 was not neutralized because it was only two 572B tubes. 
Two 572B tubes are on the edge of stability, and should probably be 
neutralized. The FL-2100 suffers from instability that could be cured 
by neutralization, as does the 30L1.

The SB-220 used 3-500Z's. 3-500Z's have much better isolation 
between input and output, although the grid chokes and capacitors 
were a mistake.

> The reason why Collins Radio Co. did not neutralize the 30L-1 is because 
> there is o way to neutralize a single ended g-g amplifier.  [see *Care 
> and Feeding of Power Grid Tubes*]. 

Untrue. Care and Feeding has a section on neutralization, 
and states (on page 83) that neutralization "generally IS necessary 
when grid leads have high inductance". It discusses only two "common 
methods" of neutralizing tubes, but clearly includes bridge 
neutralization. "Care and Feeding" plainly states its' text 
describes only TWO of the more common methods.

June 1967 QST contains an article describing neutralization in single 
ended grounded grid PA's ("The Cathode Driven Linear  Amplifier").  
One method uses a 180 degree phase inversion and a feedback capacitor 
between the tube anode and filament. The result is a simple and 
very common balanced bridge neutralization circuit.

Heath and Gonset obtained bridge neutralization through use of a 
third winding on the filament choke. This allowed use of the 
filament choke as a broad-band phase inverting  transformer. 
Ameritron uses a separate 180 degree phase inverting transformer in 
the 811H, 1200 and 572 amplifiers. 

Bridge neutralization is simple and easy to adjust, and works over a 
very wide frequency range. It works as well with single ended 
grounded grid stages as it does with tetrodes.

Any PA having maximum output outside the plate current dip on 
higher frequencies is a candidate for neutralization.

73, Tom W8JI 

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