>R. Measures wrote:
>>>The bang would have come from the external circuit, where that same
>>>large current was also flowing (for example from a fuse blowing).
>>>
>>** I don't buy it because with 0V on the grid, the HV drops to a low
>>potential because the 1600B is drawing c. 3A, until the mains breaker
>>opens uneventfully.
>>
>Why only 3A? With no grid bias, the tube will draw all the current that
>its cathode and the HV supply can possibly deliver... and more if it
>could.
? All cathodes are emission-limited. According to Svetlana's
characteristic curves, a 4cx1600 will draw c. 3A with 0V on the grid and
350V on the screen.
>
>Been, there, seen it and heard it. This was only on a little VHF amp
>using a 5894 but the white-hot anodes lit the whole room, until
>something in the HV supply failed with a very loud bang! The cause was
>very simple: the grid bias zener had failed short. The solution was
>equally simple: replace it with a larger one that didn't run so hot.
** Zener ratings should be divided by three, with the exception of
Antarctica in the Winter.
>
>The surprising good news was that the tube seemed completely unharmed.
>
** He tried the tube again?
>RF-wise, the amplifier worked fine before the zener failed, and fine
>again afterwards. KL7RA's amp did the same after his component problem
>was fixed. He told us that his problem was due to the bias rectifier
>going short, so the original cause was probably something perfectly
>simple like a mains spike.
** Did he report a shorted rectifier?.
-- 1Ampere pi-section 3-500Z grid to ground chokes are known to implode
at the same time that a filament-grid short occurs. Some have speculated
that the tube shorted, which blew the choke, but when a clip-lead is used
to short a filament pin to a grid pin on a healthy 3-500Z's tube socket,
nothing extraordinary happens, and the normal 0V-bias anode-I
uneventfully flows. One clue to the cause of the implosion and short
lies in the VHF parasite suppresor resistor because it typically
undergoes a large change in R during a tube-shorting event, but without
exhibiting any signs of external damage.
>
>That's why I say there's not a scrap of hard evidence or logical reason
>to suspect *any* RF-related cause in this particular case.
>
** The evidence may well be found in the autopsy.
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