[AMPS] Network analysis of suppressors

[Tom Rauch]

Hi Ian,

> Tom, please explain in your view, why tube type amplifiers are fitted with
> suppressors. And when amps with 3-500z's (for example) go band, why do
> they go bang?
> 
> Ian, ZS6BTE

Tubes flash over because something either exceeds the normal 
breakdown voltage, or because something inside the tube lowers 
the breakdown voltage.

It's pretty common for 3-500Z's to gas up a little and flash over. The 
gas comes either from internal material outgassing or seal leakage.

The arc itself can getter the tube, and remove the gas. As a matter 
of fact controlled arcs can be used to getter certain tubes in 
manufacturing. 

The 3-500Z normally getters itself when the anode reaches 
temperatures that show some color. In order to completely getter, 
some 3-500 anode designs have to be heated to a red color.

There is nothing sinister about an arc. 

What amazes me is how people conclude a tube will arc from 
some momentary parasitic that occurs one every year, but the very 
same tube WON'T arc from 8 kV of peak RF voltage at the signal 
frequency. 

What is particularly amazing is the anode impedance and Q of the 
circuit is many times lower at VHF than at the operating frequency, 
and yet some people actually seem to believe more voltage is 
developed at VHF than at the fundamental frequency!

This whole parasitic thing is pretty odd, from a technical standpoint.


73, Tom W8JI
w8ji@contesting.com

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