• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Potentiometer placement in tube gear.

I've been experimenting with a tube preamp designed to add some tube "color" to a solid state system, and as part of this, tested the effects of an input potentiometer vs an output potentiometer, and where you place the actual volume control depends on what you are looking for the pre-amp to do.

Color Pre-amp-final.png
One interesting thing I learned was: there was a lot of conjecture on some forums about the impact a potentiometer placed on the output would have on the frequency response. Several people insisted that with the pot on the output, there would be large changes in the frequency response at different volume settings. After building the amp and testing it, this output pot placement actually had no impact on the frequency response at any setting.

Anyway here is a short video showing the effects on distortion, testing with an actual tube pre-amp, of input vs output potentiometers.

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Thanks for actual measuring stuff instead of just "imagining" results (what 99% reviewers/posters do)
This "colour adding" gain block actually adds VERY LITTLE if at all (between 1.2% and 0.4%), and doubt can be picked in a blind test.
The output pot will NOT change distortion, which comes from the tubes.
I see many more than just second order harmonics; all other ones, even and odd, are present.

As of varying frequency response by output pot, your test is flawed.
Pots by themselves are linear and flat, just a couple resistors at any setting, the treble mloss because of pot source impedance and cable capacitance, 100 to300pF per meter, depending onnquality.

I guess you measured straight at pot wiper or after 10-20 cm cable, which of course does nothing within the Audio band, specially with a 10k pot to begin with: maximumm source impedance at 50% setting: 2k5 (1/4 total).
But again thanks for measuring.
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"I guess you measured straight at pot wiper."

You guessed wrong.

I measure with 20K load both on the RCA jack and with 3ft cable. Still no change. Also tested listening to it with an amp with various input/output levels and detected nothing. That's the reason for using a 10K pot with a big coupling cap. It would have to be something extreme, like 30ft cables etc to ever be an issue.