FW: [AMPS] smoked TL922

[richard w. ehrhorn]

Hello all...

Apologize for unconventional format and incomplete text on some of my postings -- local gurus tell me to dump MS Exchange and get Eudora.
Hope this one is all here.

#### My comments are marked ####

Dick W0ID
----------

From: 	Rich Measures[SMTP:measures@vc.net]
Sent: 	Monday, December 08, 1997 11:38 AM
To: 	km1h @ juno.com; amps@contesting.com
Subject: 	Re: [AMPS] smoked TL922

>
>On Sun, 7 Dec 97 17:31:47 -0800 Rich Measures <measures@vc.net> writes:
>>>
>>>On Sun, 7 Dec 1997 11:26:57 EST JW KIMBALL <JWKIMBALL@aol.com> 
......snip...snip...snip
 ----------------- 
>Personally, I prefer the chokes since as current absorbers of minor arcs
>from gas, etc they will not blow or collapse. Once the event has passed
>the amp works as before. 
>
It might be interesting to high-pot such a tube immediately after the 
suspected gas arc, just to see whether the tube is gassy or not -- unless 
you subscribe to the rauchian vanishing gas theorum.  .  

#####
>From our experience over 25 years and many thousands of tubes, it's very clear that in the vast majority of cases, after one or two BANGs early in life, the tube continues on for a normal lifetime of normal performance, just as if nothing had happened.  If the cause wasn't gas or some other physical anomaly that is basically eliminated by the arc - if in fact it was parasitic in nature - what killed the parasite during the BANG? Especially if parasitic suppressor R increased as a result? Would seem that gain at the parasitic freq would be higher, not lower....?
 
#### Also, it's much more common in our experience for a new tube to BANG when in standby with full cutoff bias applied rather than when running RF, keyed or key-down.  How to explain that? Seems like most these are most unfavorable conditions for spontaneous start of a VHF parasitic - or any other sort of oscillation. #####

>>>In extreme cases when the tube arcs or shorts you will also take out the
>>>Zener D-2, and Bias diode D-1 and caps C-3 and C-26.
>>
>>If a gassy tube arced between the anode and the grounded grid, how could 
>>current flow in the cathode bias Zener?  
>>-  With zero volts bias between the grid and the cathode, the tubes draw 
>>around 300mA of cathode current.  I don't see how 300mA could blow the 
>>cathode bias Zener, Carl.  
>
>Which goes first...the grid choke or the zener?  

There is typically a big bang, and subsequently the blown choke and the 
shorted Zener are discovered during the repair process.

>And if C3, C26 and D-1
>also blow would that not assume a rather high voltage thru that path?

Agreed, Carl.  However the current path of an anode to grounded grid arc 
is not through components in the cathode.  

######
How about the fault current return path from ground back to HV neg? Don't have a schematic of the 922, but if the HV supply negative isn't bolted to ground (i.e., if the bias supply or zener is between HV- and ground), the natural fault current return path is from ground via the bias supply (zener or whatever) back to HV neg. A 200 or 400 surge-amp shunt diode (big enough to survive several discharge cycles of the HV filter cap through the I- limiting series Z) is typically used to protect the bias supply.  ####

##### Dick  W0ID   ######



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