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 ######
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|