[AMPS] smoked TL922

[Rich Measures]

>
>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.  .  
>
>>>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.  

>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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