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[Amps] 3-500Z parasitic wows

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Subject: [Amps] 3-500Z parasitic wows
From: (2)
Date: Fri, 23 Aug 2002 18:50:40 -0700
>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!
**  agreed

>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!
**  I have seen gassy tubes, but I have never seen one  damage a VHF 
parasitic suppressor resistor..

>>>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."
**  Protecting an already gassy tube makes little sense to me. 

>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
**  I have seen them.  

>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.

**  To prove that an anomaly was due to loss of vacuum or a "barnacle", 
the tube would have to be high-pot. tested immediately after the event. 
>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. 

**  Owners of amplifiers that had a stentorian bang reported seeing an 
arc in the vicinity of the anode circuitry.  

>There are enough reports of blown meter shunts, zeners etc to 
>explain a lot of bangs from unprotected amps.

**  In my opinion, none of these would be likely to generate that much 
>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.
**  The first thing I measure is tube leakage.  The second is the 
resistance of the suppressor resistor.  
cheers,  Ian

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,  

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