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previous: M Cook Do plate resistors have any other f... -- 6/18/2000 10:12 PM View Thread

Re: Plate resistor question?

6/27/2000 12:30 PM
Ray Ivers
Re: Plate resistor question?
As usual, Ken's comments are right on the money. I just wanted also to mention that it might help to think of the plate resistor as the 'plate load resistor', as it is so often (and more correctly IMO) referred to. It is AC grounded to the power supply and (usually) serves as the primary load on the tube output; the higher the value, the less loading occurs (an oversimplification, but true to a point). The choice of plate load resistor also greatly affects output impedance and maximum signal swing. Unless you're using a 12BH7 or a low-mu triode, 15K sounds pretty low to me, but you may be running a low B+ voltage. The lower the value of plate resistor, the higher the voltage on the tube's plate, all else being equal; I would think that unless you need a lower output impedance from the stage to drive a tone stack or something, you might be better off choosing your PLR value for gain and signal-swing considerations, and varying your plate voltage by adjusting the value of cathode bias resistor.  
These are just a few off-the-cuff comments on choosing a plate load resistor. Generally, the higher the tube's mu, the higher the tube's plate resistance, and thus the higher the range of plate load resistances usable for 'normal' operation.  
'Normal operation' - is that what we want?  
Ray Ivers