>R. Measures wrote:
>>
>>
>>>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.
>>
>>?Ǩ 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.
>
>So "all that its cathode can possibly deliver" in this case is 3A...
>which proved sufficient to make the power supply go bang.
>
** Proved? Power supplies typically don't bang during an overload
condition, the output V sags until the circuit breaker opens.
>>>
>>>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.
>
>So I learned from that experience!
>
>>>The surprising good news was that the tube seemed completely unharmed.
>>>
>>** He tried the tube again?
>>
>No, still talking about my amp.
>
>>>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?
>
>Yes, he did.
** As is the case with 3-500Z shorts, the hat trick is to find out
what caused the short. A similar thing exists with fuses that blow.
Fuses are typically not defective, so when one blows there's a good
chance there will be an encore unless one does some detective work before
replacing the fuse.
>
>>.
>>-- 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 an interesting statement to interject at this point - but in a
>real inquest, you'd be challenged to prove both its truth *and* its
>relevance to this particular dead amplifier. Otherwise it would be
>excluded as irrelevant.
>
** iThe chain of events heretofore described are based in science.
Provng electro-magnetism is a piece of cake. EMF is relevant because
electric-current undoubtedly flows from the cathode to the grid.
Current and EMF are related. The Cu in the grid-gnd RFC melts at 1083ºC.
Tungsten can be bent at 1820ºK.
>And so:
>>>
>>>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.
>
>You and I have very different ideas about the valid uses of evidence,
>and indeed about what "evidence" even is...
** I base such evidence on what is observed in the post-mortem. I got
this shocking idea from an engineer at Eimac® who autopsied (1986) a
pair of kaput 8875s I sent in, explained the procedure Eimac uses,
described the damage observed, and explained the science behind the
damage. The first 8874 I opened displayed a landscape littered with
thousands of tiny gold meltballs. This event made me a believer. I
plead 100% guilty to plagiarism. // On the other hand, one of
Hamdom's "recognized amplifier experts" maintains that nothing can be
learned from a tube autopsy.
>
>and we'd better end it there, before the moderators become nervous.
>
** My guess is that the moderators/censors are either being bored to
tears, or they are sawing logs..
cheers, Ian.
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|