Re: [Amps] 4CX1600B

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

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.

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.

The surprising good news was that the tube seemed completely unharmed.

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.

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.


--
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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