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Re: Hum pot in DHT preamp ???

lets assume more traditional R/C coupling....the filamnet has a 7.5V dc potential across it.... now tie one side to ground (through the RC) and build two different supplies a +7.5V one and a -7.5V one the two different filament supplies will considerably change where the tube biases up, and for the same operating point two different bias resistors will be needed...

i think the negative supply will require a smaller bias resistor for the same current... the down side is grid current will start to flow when the grid hits -7.5V rather than zero...this is a problem in output tubes, but in drivers and such were we tend to keep well clear of the 0 bias point... maybe this is a place to make a small gain???

in other words with the -7.5V supply when one 1/2 of the filament is at -7.5V the other half is at zero and some grid current will start to flow. but with the +7.5V supply when one 1/2 of the filament hits 0 the other 1/2 is still at -7.5V so the grid current will start flowing with a difference of bias of 7.5V

is this making sense???

dave

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