[Amps] neg screen I in tetrodes-- TSPA

[John T. M. Lyles]

Bill and Rich commented:

>  > The 4- series does not have problems with the negative screen current
>>  like the 4CX series. I just checked the data sheets.
>
>Interesting observation.  Any theories on why this is the case?

I assume that you looked at 'islands' on the screen current 
characteristic curves, usually ripples, circles,  and reversals in 
current which show up at lower plate voltage. Yes, the 4-250, 4-400 
and 4-1000 don't seem to show those.

Here are some possibilities, that I could think of:
1) The data taken for those glass tubes in the 1950s was not as 
carefully done, and was  'smoothed' to present nice curves. Maybe 
Eimac missed the operating points with negative current...

Proof would be to see if 4-400 or 4-1000 has exhibited neg screen I 
in anyones circuit?


2) The electron optics design for the smaller more compact tubes used 
in 4CX- designs causes more intercept of primary electrons 
(cathode-anode flow) causing secondaries to come off the screen.


3) The screen material and coatings is different in the 4CX- designs, 
and causes more secondaries when struck by primaries from the main 
beam.


4) There is more heating of the screen in the 4CX- designs for 
whatever reason (different cooling, running at higher screen power) 
causing it to emit primaries.


Or it may be a combination of these factors. There are probably more 
possibilites to add here. I wonder if the guys at Eimac have the 
answer?

Somewhere I read some old papers from Proc of IRE (from RCA and 
others) where they explained how to test for primary (thermal) and 
secondary emission in grids. I can't recall them now. We have tubes 
here (RCA/Burle 4616 tetrode for instance) which are pulsed and can 
see the screen emission come up as the pulse lengthens, due to 
heating. Initially the current is neg, then goes positive. Difficult 
to build screen power supplies that hold their voltage constant.

One more thing, the advent of pyrolytic graphite grids in bigger 
Eimac and Thales tubes has made negative screen current much less of 
a problem for RF circuit designers.

73
John
K5PRO