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In Reply to: RE: 6SF5 to 45 design help posted by johnsonad on August 23, 2014 at 15:50:47
DanL
Follow Ups:
Floats the 2A3 filament up in voltage to allow direct coupling from the 6SF5 plate voltage.
No, it is R4 and R3 adjustable, being returned to ground, that biases the Finals tube above ground, to set up the direct couple. R3 and R4 is AKA an Rk.
R7 is just a voltage dropper, known as R-Dropper. It acts as a power supply decoupler, ( front end B+ from finals B+ ). R7, along with a cap to ground after R7, it forms a supply for the front end tube's B+, feeding the plate resistor of the front end tube.
Jeff Medwin
Take R7 out and see what happens. R3, R4 & R7 provide the network, but poster asked about R-7.
Agree with you completely - about it being a network.In my two stage amps, I do NOT ever tie R7 to R4, and I have no R3.
The circuit would work well with JUST an R4, a properly-sized Rk, and C3 would not have to serve "dual purpose" as it is shown in the posted 1947 Lipman-Robins schematic. .
With R7 totally unconnected, ( separated from the cathode return to ground ), we can optimize the C ( amount of uFs and type) for feeding the driver tube's plate resistor, and, we can SEPARATELY optimize the C ( amount of uFs and types ) that is bypassing the Final's tube Rk.
Certainly, as originaly shown, there are two different "C" requirements trying to be met by one C3 when R7 is connected to the cathode. These two uses should not be combined into one C3 cap, to pinch a penny and to sub-optimise performance in two areas. Each circuit requirement deserves its own "best solution" of C filtering IMHO!!
Jeff Medwin
Edits: 08/25/14 08/25/14
Hello my friend. I'm not sure. A voltage dropping resistor? I'm open to significant modification. The values posted are from an old post and mirror the Fi design if my memory serves me right.
You have that right Aaron.
In a two stage DC amp, R7 would be called the "R-Dropper", a voltage dropper.
Its value is carefully chosen to "set up the front end" so the front end balances out, voltage-wise, and self-bias-wise, to the finals or rear end. It ALL must balance, and its "lotsa FUN" ( a challenge ) to juggle it in a design - as everything is inter-related when DCed . !!
Jeff Medwin
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