[AMPS] parasites

[Tom Rauch]

Hi Peter,

> It should be remembered that if the parasitic resonant frequency 
of the
> grid circuit is ABOVE the parasitic resonant frequency of the anode 
> circuit, then it doesn't oscillate - which is an explanation of why
> external anode tubes with relatively large output capacities can be (not
> necessarily are) stable without parasitic suppressors.

3CX5000s, 8877's, 3CX1200Z7's all fit that category IF the grid is 
grounded properly. It seems more a matter of the internal grid 
structure than anything. 

Sweeping the 8877 with a network analyzer in the transmission 
loss mode and with high load impedance selected, the 8877 
doesn't have grid problems until well above 600 MHz if the 
fingerstock contacting the grid ring is laid right on the chassis. 

It would be hard to get the anode path resonant up that high, so the 
tube is generally unconditionally stable with no suppression.
 
> I will agree with Tom in that in my experience, 572B's are a MAJOR pain in
> the  butt to stop parasitics in. Looking at the construction, with those
> long grid leads, it's hardly surprising.

I tried to get Svetlana to use the blank pin for a second lead to the 
grid. That would have made a major difference. In some amps, the 
grid becomes an open circuit at about 40 MHz! There is enough 
feed through in four 811's or three 572's to make them behave like 
a TPTK oscillator above 15 meters unless the PA is heavily loaded.

Don't ever mesh the loading cap too far on on a Dentron amp with 
572's on ten or 15 meters, the thing will oscillate wildly right on HF!!!

The only way to cure it is to neutralize the tubes.


73, Tom W8JI
w8ji@contesting.com

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