Rich Measures wrote:
>>>Which 1 ohm 10w resistor blew?
>>>
>>
>> It is between the - of the HV and ground. The grid current metering
>>taps of the high end.
>
>Thanks. It takes lots of Joules to blow one. I'm guessing that you had
>a large surge in grid current.
Much more likely it was an intermittent short from HV+ to chassis. This
would find its way back to HV- through the 1 ohm resistor. It would
indicate as "grid current" - lots of it - but you'd see exactly the same
without any tubes in the amplifier at all.
Just think about it: that resistor is rated to carry 10 amps continuous,
so it would take a surge of >>10A to blow it.The HV supply has to eb the
prime suspect.
Before you sacrifice any more tubes or meters, inspect the HV+ wiring
very carefully. An arc to chassis would probably leave some kind of
trace, unless it was internal to a component. Even if you don't see
anything, replace the resistor but don't connect the meter. Then try the
amp on standby for several hours with no tubes, and see if there are any
more bangs. It might help to reveal the fault if you used a variac to
increase the mains voltage and stress the insulation (within the ratings
of the rectifier diodes and smoothing capacitors of course).
Please keep us all posted.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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