[Hammarlund] SP-600 NuvistaPlug & more

[Kenneth G. Gordon]

On 10 Nov 2007 at 11:31, K9FFK wrote:

> Has anyone ever looked at this schematic and asked why there is a 50pf
> (fifty) on the output (pin 5)? The 6BA6 Cout (shielded) is given as
> 5.5pf. The 6CW4/6DS4 Cout is given as 1.8pf. The difference is 3.7pf.
> So a replacement would need an additional 3.7 pf on pin 5 (5pf is
> close enough). I've looked at my Nuvistaplug and indeed there is a
> 50pf on the output. Why? Would/does this not seriously detune the
> tuned circuit? It certainly is quieter! What gives?

I think Carl nailed it. 

Normally, most cascode triode circuits with which I am 
familiar use some form of neutralization to make them more 
stable. 

After all, the active elements are triodes. Triode amplifiers 
really require something external to make them completely 
stable.

However, there are several methods, other than straight 
neutralization, for stabilizing such circuits, at least in 
receivers.

One is very similar to that used here: i.e. loading the circuit 
so that it cannot oscillate, especially at VHF.

It is simple and it works, although maximum possible gain is 
NOT realized.

In this case, that is just fine. We don't really need more 
gain, especially at HF. What we want is a better signal-to-
noise ratio, and we get that with this circuit.

Ken W7EKB