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Re: [Amps] 4CX1600B

To: amps@contesting.com
Subject: Re: [Amps] 4CX1600B
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
Reply-to: "Ian White, G3SEK" <g3sek@ifwtech.co.uk>
Date: Fri, 14 Nov 2003 18:05:23 +0000
List-post: <mailto:amps@contesting.com>
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.

€ 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.



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.


.
--   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.


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...


and we'd better end it there, before the moderators become nervous.


-- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek _______________________________________________ Amps mailing list Amps@contesting.com http://lists.contesting.com/mailman/listinfo/amps

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