Re: [Amps] 4CX1600B

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

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
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