Preamp and impedance question

You can put a volume pot on the amplifier, the only reason most amps do not have one is it limits their market to customers with only one source. What you have to take into consideration is what input impedance your source wants to see. Anything above 10k ohms should work fine. I would go with a shunt volume control as it only puts one (series) resistor in the signal path and I would use a series resistor of 15K ohms, high enough for a respectable input impedance through the volume steps and low enough to keep out noise. You could also include a balance control if you want, just place it upstream of the volume pot.

If your amp has a 4-ohm tap, you can use that with no problem. The power output does not change, but the current draw will be higher as the output xfmr tries to maintain voltage at the lower reflected impedance (power is consumed at the speaker, NOT the amplifier). Any limitations on the amplifier will show up here in the form of voltage drop and distortion as the tubes are asked to work harder. However, it should not affect anything if you listen at reasonable volumes.

@gte357s
No, what I meant was that if you have both 8-ohms and 4-ohms on the secondary, using 4-ohms will not be a problem as long as you don’t expect the maximum power the amp is capable of. The secondary will draw more current and if the xfmr and the tubes can handle it, then the amp will perform to spec.

If you are specifying a new transformer, then the above is moot. What you do is look at the impedance graph of your speaker for the entire audio range and look for the average impedance.

For example, my Thiel 3.6 speakers are rated at 4 ohms, however they are at 3 ohms from 100 hz to 10khz. If I were to spec a transformer, I would first see where the tube operates at the optimum power and THD. Say that the tube load is optimum at 3,000 ohms. I would then have the transformer wound to reflect a 3,000 ohm on the primary with a 3-ohm load on the secondary. Then I know what VA rating I need. With those specs in hand, I don’t need to worry about the transformer being able to drive the speaker because it is optimized for most of the impedance range. But in the lower frequencies when the impedance climbs to 8 ohms, the tube will be taken out of its power sweet spot but may still remain relatively low in THD. It's a tradeoff I'll take because it is, after all, a tube amp.