Carl wrote:
>
>
>
>>>
>>>Sounds like a HV breakdown to me that is external to the tube.
>>
>On Fri, 19 Mar 1999 00:35:31 +0000 "Ian White, G3SEK"
><G3SEK@ifwtech.demon.co.uk> writes:
>>There could have been a silent arc inside the tube, with the bang
>>coming
>>from the resistor. IMO a 2W resistor standing in the way of a
>>near-short
>>across the HV supply is very likely to exit with a bang!
>
>
>I take it that you are not familiar with the SB-220 circuit. The
>resistor in question is not in the HV path at all.
>
It's been years since I worked on an SB-220, so I didn't have the
circuit handy, but have now downloaded it from Rich's web site
(thanks!).
R3 is not involved in the normal anode-cathode HV current path. But if
there's an arc from B+ to a grid or the chassis, the whole arc current
flows up through that 0.82 ohm 2W resistor (R3) to reach B-. (The same
current also goes through the 1 ohm plate meter shunt R1, but that's a
5W component.)
However, you may be right after all about it being an external flashover
to chassis, Carl, because an internal anode-grid arc would probably have
taken out the grid RFC.
It seems like a lot of damage to resistors, meters and RFCs could be
avoided by bridging R3 and R1 with big diodes (one across R3, and two in
series across R1, all cathodes pointing "up" towards B-) and also adding
the B+ surge limiting resistor that is used in all modern amps... what
do the SB-220 experts say?
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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