Preamp Volume Controls

[10west]

I've got a bit of a dunce question. I've been making the odd valve power amp since I was 13-14 (25 now). I realise that I've never made a valve pre-amp, I've got enough parts in the old parts drawer to make something I think, but I don't understand about fitting a volume control. I see alot of designs on the net for preamps that have tone controls but thats it. Am I right in thinking that I can pretty much just put a variable resistor on the line source prior to it going into a preamp circuit such as the one below?

[DF96]

A volume control is not quite a variable resistor (that would be two connections) but a potentiometer (three connections) acting as a potential divider. Just look at what other people do - there are plenty of preamp circuits on the web. Even the bad circuits usually manage to get the volume control right.

[10west]

Hi! Thanks for your help,

So What I need to be doing (using the above circuit as an exampe) is to put my potentiometer ont he input side, so the wiper feeds the input and the other two connections are connected to ground and the source signal respectively. I suppose the next question is what value pot to use.

I seem to remember reading that the ballpark figures were something along the lines of 10k for SS input or from a valve device you would use around 100k. Is is this at all valid? Or are there any simple calculations or rules you can follow to choose the correct one?

Thanks again for your help!

[DF96]

Yes, you've got it. The value is not too critical. It needs to be equal to or higher than the minimum load of the source. It also needs to be less than four times the maximum source inmpedance for the next stage. If in doubt, use 50k - a bit more for valves, a bit less for SS.

You normally need a log law pot, so that it roughly matches the way our ears respond to volume changes.

[Hi-Q]

Interesting that your schematic of a Pentode strapped as a Triode shows both G3 and G2 connected to the plate. This is the way I always expected it to be done but appears to be frowned upon in output stages. I can only assume it is because of the higher currents expected and the more fragile G3 is returned to cathode instead. Just to keep on thread, I think that all that requires to be said on the volume control values has already been stated. I recently came across a Guitar amplifier that had 3 volume controls fed from a Tone Ladder Network and then summed together. All controls were 470k LOG which always was a standard value on older tube equipment, in fact most volume controls were either that or 1 Meg Ohm. Nowadays I find much lower values in use, more probably due to the scarcity of the higher value controls, especially in Tandem configuration. 50k LOG and up seems to be the more common.