>
>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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