Parasitics in Some Beam Power Tubes Explained

[Tom WA3KLR]

I just came across this dissertation on the negative resistance in the screen circuit of pentodes.  Some of you may find this an interesting article:

http://mysite.verizon.net/dpnewkirk/ej/beam_power_tube_parasitics/

[AB2EZ]

Tom

Thanks!

It's interesting to see the mix of engineering theory and practice that folks were stuggling with in the 1930's... and to compare that to the present.

Until I read this, yesterday, I didn't know about any of the effects being discussed.

Best regards
Stu

[ka3zlr]

Myself I like Reading what Bill Orr has to say on the subject W6SAI.

73

[Tom WA3KLR]

A bit of ambivalence on the situation didn’t help.

From The Radio Handbook, Twelfth Edition, 1949, Chapter 5 - High Frequency Power Amplifiers, page 103 - 104:

“Shielded 807 Amplifier

The versatile 807 unquestionably is the transmitting tube most frequently used by amateurs.  But it also is a very frequently maligned tube due to its tendency toward spurious signal generation and parasitic oscillation,  Nevertheless, the low price and unusual capabilities of the tube overweigh any misgivings about using the tube as a result of its unfortunate tendency toward self-oscillation.  It has been more or less established that the 807’s enthusiasm for parasitic oscillation is the result of two factors:
(1) the extremely high power gain of the tube, and
(2) the sizable screen-lead inductance due to the rather long piece of small wire between the tube socket and the screen element within the electrode structure.”

[AB2EZ]

Tom

Yes...

It is interesting that there are often (usually?) multiple (candidate) explanations  for some sort of undesired or unexpected bahavior. We try one solution ... and it may or may not work. It might work even if the explanation that led us to that solution is wrong.

"The difference between theory and practice is larger in practice than it is in theory" Anon

Stu

[KM1H]

When I first made the transistion from hollow to solid state I thought the days of parasitics and neutralization were over, that was a sad assumption   And you didnt even have the benefit of watching a runaway display

Carl

[w7fox]

cant get to the article, it is "locked".  anyone know how to unlock it? thanks.

[Pete, WA2CWA]

cant get to the article, it is "locked".  anyone know how to unlock it? thanks.

This might be the same article:
http://dpnwritings.nfshost.com/ej/beam_power_tube_parasitics/

[w4bfs]

thanks Pete .... I also was 'locked' .... 73  John

[W3RSW]

Fascinating.  Thanks for the link.  Read also the Frostburg  links, the Wingfoot 6cl6/ 813 stuff and on and on.

Also Makes you want to run out and buy a T-60.

One of the lab experiments in my 1963 Marietta college electronics course was making up a negative resistance Osc. on purpose with I think a  6au6.

I always thought 6146's were squirrelly but that 807's were super stable in comparison.
Well.  --
Learn something every day. 

Heck, even my Citation II with KT88's has 100 ohm grid suppressors.

Whatever it took to tame the beasts, empirical or otherwise over the years.