>
>Vic Rosenthal wrote:
>>
>>In the common circuit used for GG triode amplifiers that separates the B-
from
>>ground in order to allow for convenient plate and grid current metering,
there
>>is a 'safety resistor' from B- to ground.
>>
>>Most designs seem to use a value like 10 or 20 ohms. As G3sek
>><http://www.ifwtech.demon.co.uk/g3sek/> points out, if you use the 'voltmeter
>>method' to measure grid current, the high value of the shunt resistor
requires
>a
>>higher value for the safety resistor (he suggests 1000 ohms) to keep the grid
>>metering accurate.
>>
>>My question is: What malfunction is this 'safety resistor' protecting us
>>against? What could happen if it were omitted or opened in service?
>
>The resistor is there to keep the B-minus rail somewhere close to ground
>(chassis) potential, even if the high voltage supply is operated
>separately from the rest of the amplifier.
>
>Without it, the whole rectifier/capacitor stack would be completely
>floating with respect to chassis, ......
Only if one inadvertently forgets to connect the grid-current meter shunt
R between gnd and the positive side of the anode-current meter shunt.
- Rich..., 805.386.3734, www.vcnet.com/measures.
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