[Heathkit] Re: parasitic suppression - Better Title

[Kenneth G. Gordon]

On 26 Aug 2007 at 17:23, jeremy-ca wrote:

> The
> rod is for straight neutralization and is used in many circuits thru
> 2M The rod is bent to or away from the tube as needed; it is simply
> one plate of a feedback capacitor and the tube anode is the other. It
> is used when only a very small capacitance is required. The variable
> capacitor performs the same function when more capacitance is
> required. NEITHER has anything to do with parasitics....PERIOD.

Absolutely true.

The type of oscillation caused by lack of a correctly sized, or missing, 
"neutralizing" capacitor is NOT a "parasitic" oscillation. 

This type of oscillation is caused by 1) high gain in the tube or 
transistor concerned, coupled with 2) a feedback path which utitlizes 
the tube (or transistor) INTERNAL, usually grid-to-plate, or its transistor 
equivalent, capacitance and is a completely normal occurrence in 
practically any high-gain circuit, whereas parasitics are abnormal in 
practically all circuits.

In fact, although external grid-to-plate capacitance can accentuate the 
normal, un-neutralized type of oscillation if the mechanical layout of the 
circuit is such that grid-to-plate shielding is not "adequate", in general, 
one can figure out what capacitance is needed for complete 
neutralization by finding out what the published tube internal grid-to-
plate capacitance is.

For instance, for the 6146, the internal g-p capacitance is published as 
2.2 pdf, and if one didn't have to take into account external 
capacitances which might be in parallel with that one, a 2.2 pdf 
neutralizing capacitor would be all that was needed to provide perfect 
neutralizing of the circuit.

In a very well-shielded circuit, this capacitance would be simply an 
aluminum or copper rod in parallel with the plate of the tube....as is 
already built-into some rigs.

Also, this type of oscillation, that is, that caused by lack of 
neutralization, can get worse, or become more apparent, with an 
increase in operating frequency, whereas, parasitic oscillations take 
place at ANY operating frequency.

This is one of the reasons why normally one "neutralizes" a rig at the 
highest operating frequency.

Ken Gordon W7EKB