Wilbert Knol wrote:
>
>
>> With the glitch R in the negative return, the cathode circuitry may be
>> driven to several kV by a B+ to chassis arc.
>
>Good point.
>
>I don't like the idea of most of the HT appearing across the filament
>transformer secondary...
>
Does it? The current around the loop is the same, regardless of whether
the glitch resistor is connected to the top of the capacitor stack (B+)
or the bottom (B-minus). If we keep chassis potential as the voltage
reference, the only difference in the second case is only that the B-
minus end of the resistor spikes to a high *negative* voltage. The other
end of the resistor should be held close to chassis potential by the
usual meter protection diodes.
The practical problem about having the glitch resistor in the B-minus
return is therefore the need for high-grade insulation at *both* ends of
the capacitor stack.
As far as I can see, there isn't any difference in the grid-cathode
region around the tube, is there?
>Perhaps the OE5JFL circuit should have string of diodes in parallel with
>the sensing resistor in the B- line, to limit the voltage drop to a few volts.
>
>And a separate glitch resistor in B+, where it belongs.
>
>Even better: get rid of the B- sensing & glitch protection completely.
>
>Use a glitch resistor in B+ to also trigger an opto SCR like the H11C4.
>They are good for 7.5 kV of isolation and don't cost much.
>
Sensing in the B+ is probably the best option for an add-on crowbar
board for existing amplifiers, because it avoids messing around in all
the various grid and anode current metering schemes that can exist
around the B-minus rail. (At one time I considered making a crowbar
board to complement the Triode and Tetrode Boards, and would have used
B+ sensing for that reason.)
Thanks for the pointer to the H11C4... that would nicely solve the
latching problem. The only thing that worries me about DIL opto-couplers
at high voltages is the idea of several kV between pins only 0.3in apart
- or less than 0.25in between edges of pads. Does anyone here have
experience of such high voltages across short spans of fibreglass PC
board? It sounds like it may be less than the UL minimum creepage
distance, so do you have to slit the board or what?
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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