input impedance question

dspear99ca

dspear99ca

Super Member
I'm curious as to whether adding a volume control to a tube amp will create an impedance matching problem for me.

The grid leak resistor on the input stage is 470k. My volume control, installed just upstream of this resistor, is 100k, basically putting the two resistors in parallel. The way I see it, my input impedance is effectively 100kx470k/(100k+470k) = 82K given that the far side of my volume pot is connected to ground. Should I move the 470k resistor to between the far end of my volume pot and ground? Or remove the grid leak resistor and put a 370K between the pot and ground to add to the 100K to make 470K? Is this an issue or am I over-analyzing things?

Dave
 
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Your input impedance is somewhere between 82K and 100K, depending on position of the volume control. Whatever your source, it should have no problem driving this... it's fine.
 
Shouldn't be a problem.

The grid leak resistor is chosen for a MAXIMUM impedance, that can control the grid voltage. Nothing wrong with lower impedance. Shouldn't change the operating point of the circuit, at all.

If you're worried, you can always put a DC blocking cap (say, .22uf or so) between the pot and the grid resistor. Then, the bias will DEFINITELY be AS ORIGINAL. But, I don't think that'll be necessary, in any realistic case...

My take, though, is that I'd use a HIGHER IMPEDANCE pot than 100K. Something like 250K would be good, IMHO... you'd be pulling less load on the preamp (which MAY help THE PREAMP'S low frequency response- I've had this kind of thing make a READILY AUDIBLE DIFFERENCE in bass extension, sometimes, with tube preamps and some source devices such as tube tuners and such).

Regards,
Gordon.
 
Given that the volume pot is already installed, could I accomplish something close to what you suggest by adding a resistor between the pot and ground?
 
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