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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...
>>>Sounds like a simple case of internal arcing in the tube. A 10-20 Ohm
>>>resistor in the HV lead would possibly have minimized the damage.
>>>The grid chokes are also way too large in current capacity. Replace with
>>>150-200 ma types which have both extra resistance to provide a bit of
>>>degeneration and will act better as current absorbers.
>>>
>>Chokes make flaky fuses. Ordinary carbon film resistors are easier to
>>blow than chokes, and they are cheaper. Fast-acting 0.25A 250v fuses
>>would probably work.
>
>Are you suggesting he actually drill 2 holes on the rear apron for fuse
>holders ???????
I would solder the fast-acting 0.25A 250v fuses between the grid pins and
chassis ground. .
>Seems you were bitchin' about a similar suggestion last week.
The suggestion of mounting grid fuses on the rear panel was not mine.
>In any cases fuses should be in some easily and quickly accessible location.
>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. .
It definitely is true 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 anomaly eliminated by the arc - if in fact it was parasitic in nature -
what killed the parasitic during the BANG? Especially if parasitic suppressor R
increased as a result?
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?
>>>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? I don't
know the particular circuit you're discussing, but if 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 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 limiting R)
is typically used to protect the bias supply. But you knew that.
Dick W0ID
>What is the frequency spectrum generated from an internal gas arc Rich?
IMO, an arc is like a switch that closes.
>
>>>If the amp failed while on 10/12M best to check PC-1 and PC-2 R
>>values also. Replace with metal oxide.
>>>
>>Good advice, Carl, except that I would not be as hasty to replace the
>>resistors. 10m heating produces changes in the external appearance of the
resistors.
>
>
>... ... Long time exposure to lower dissapation will
>still change resistor values and show no outward change in appearance. At
>DC, radiated heat or RF.
True enough, however this sort of R change is typically less than 50%.
OTOH, vhf bursts seem to produce sudden R changes of over 300%.
>A pair of 2 or 3W metal oxides in parallel will give long term
>reliability if the amp will be used a lot on 10/12M.
Not if Ls is too inductive.
...cheers...
Rich...
R. L. Measures, 805-386-3734, AG6K
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