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[Amps] 3-500Z parasitic wows

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Subject: [Amps] 3-500Z parasitic wows
From: Ian White, G3SEK" < (Ian White, G3SEK)
Date: Fri, 23 Aug 2002 16:56:00 +0100
2 wrote:
>>>**  RCA's Rocky Point Investigation was somewhat specious  since there is
>>>no such thing as disappearing gas and vanishing "barnacles".
>>Well, so much for RCA then! And so much for the G-E Research Labs,
>G. W. Flyer of GE (1935), H. F Dittrich of Phillips (1958), and ARRL's F.
>E. Handy (1926) arrived at a different conclusion than RCA's Rocky Point
>investigation.   [Dittrich, H. F.; *Tubes for R. F. Heating*; N.V.
>Phillips Gloeilaampenfabrieeken--Eindhoven, the Netherlands.  Section
>5.8, ''Parasitic Suppression Circuits'' begins on page 96.]

THE WHOLE POINT is that there's more than one possible explanation!

Sometimes it's parasitic oscillations, sometimes it's vacuum arcs, 
sometimes it's arcs outside the tube, and so on... it very hard to tell 
from the evidence left behind.

Anyone who believes in only one explanation will be wrong at least some 
of the time - that much we do know for sure!

>>and all the R&D work that went into their book on 'Vacuum Arcs'; and
>>Gossling's work with the UK General Electric Company (no relation) and
>>the BBC; and all the other workers who have found ample evidence for
>>arcs... including Eimac.
>Eimac reference, please.
The same Eimac reference I have given you at least three times before! 
Bulletin 17 on 'Fault Protection', a copy of which is at

Eimac says: "An arc is a self-sustained discharge of electricity, 
between electrodes in a vacuum environment... The arc supports large 
currents by providing its own mechanism of electron emission... Since 
any high voltage vacuum device may arc at one time or another, some 
means of protection is advised in all cases."

So Eimac clearly understand that vacuum arcs are one possible 
explanation of current surges.

>The problem with Rocky Point Effect is that there is apparently rarely
>evidence to support it during kaput tube/valve autopsies.  I have yet to
>autopsy a tube with an anode arc-mark and that includes tubes that are
>gassy due to a leaky seal.

You may have led a sheltered life. There are some excellent autopsy 
photos of arced Russian tubes (again, referenced here before) at

These are corresponding marks on the anode and mesh grid of the same 
tube. Note the multiple arc marks (very much provoked by running the 
tube above 4.5kV and at well over 2kW out on 144MHz).

Each time that the tube arced, it recovered its vacuum and ran OK for a 
while. Note that carefully: tubes do recover their vacuum after an arc. 
This cycle was repeated five times (count the marks) until the last big 
zap, when owner decided to scrap the tube and open it up.

Obviously these are very severe arcs, but much less noticeable arcs 
would be enough to do a lot of damage in an unprotected PA.

> Another problem is that it seems unlikely
>that (since sound travels in air) an amplifier could produce a stentorian
>bang from an arc inside the vacuum.  I have not yet found an amplifier
>that produced a loud bang that did not have a damaged VHF parasitic
>suppressor resistor.   This tells me that there is a quite likely a VHF
>connection with loud bangs.

Vacuum arcs and runaway parasitic oscillations are both possible - and 
both result in a very large current surge. I believe it's the current 
surge in the external components that causes the audible bang, if there 
is one. There are enough reports of blown meter shunts, zeners etc to 
explain a lot of bangs from unprotected amps.

To repeat my main point: NOBODY can tell what happened on any particular 
occasion, without a detailed post-mortem of the whole amp... and maybe 
not even then.

73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'

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