Sano 250R bad pots?

[jvm]

I have a Sano 250R that seems to function fairly well except that most of the pots seem ineffective until they are full open. Anyone have experience with these amps? I wonder if this is a Sano quirk or if I have that many defective pots.

[J M Fahey]

Sounds more like the design works that way.

It would be extremely unlikely several pots failed the same way at the same time.

And typical pot failures are scratch, loss of sound at certain settings (very dirty/bad contacts) or open tracks/contacts , not change of response curves.

Chalk it to it being a very old somewhat crude design.

[Mick Bailey]

I think so too. Quite probably the taper of the pots isn't optimized to their location in the circuit. An amp I recently worked on had a Bax-type tone control and 'suffered' from all of the bass/treble control occupying the last 1/5 of the pot rotation. It had audio taper pots. After a little experimentation I ended up with a C taper for bass and linear for treble and this gave a uniform sweep.

If the amp is original I'd leave it alone. We tend to want to 'fix' everything, but often beauty lies within imperfection.

[jvm]

Thanks guys, I appreciate your input. I agree that it most likely is a design quirk. The amp is very clean and original so I will leave it as is.

[J M Fahey]

See that the tone control is fed through 100K resistors, while its own frequency shaping resistors are 100K and 47K.
That alone makes the tone control response "sluggish".
I bet that if a cathode follower were inserted between that 2 x 100K junction and the actual tone control, that response would be much smoother..
Not suggesting you do so, just looking for a reason for less than stellar operation.

If in doubt, go to TSC page (the tone control simulation page) and simulate these controls twice; first with preset assumed low impedance driving the tonestack and then with inserted ariund 70K generator series resistance.

I bet you will find control range suffers.

Also simulate with Log abd with Lin controls, that will also have influence on results.