measures wrote:
>>
>>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?
>
>With filament-type cathodes, without glitch (meter protection) diodes, it
>does.
>
>>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.
>>
>When the glitch R is connected to the neg. return, the meter protection
>diodes limit the voltage drop across the glitch R. This prevents the
>glitch R from doing its job of limiting current.
>
>Thus, when the hot end of a B- glitch resistor is held close to chassis
>potential, the meter protection diodes limit the voltage across the
>resistor to a couple of volts. Thus, virtually no current limiting is
>provided.
>
You're absolutely right, it won't limit current if you put it there...
but that wasn't where I meant.
Referring to "Figure 2" on your web site, leave the meters and diodes
exactly where they are. Move the capacitor stack, divider resistors and
rectifiers away to the left to make a separate group of components.
Connect a single wire between those two groups of components - that is
the B-minus rail.
Now cut that wire, and insert the glitch resistor. I'm not recommending
that is the best place for it (see quote below) but it will limit the
surge current just as effectively as if it were in the B+ rail.
>>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.
>>
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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