How much does tone/volume pot ohm value matter?

[SteveInMN]

I have been working on an old 1968 Kalamazoo (Gibson) 30 Watt bass amp. The volume and tone pots are super noisy and do not respond to Deoxit, so I want to replace them.

The original values were 2Meg for the treble and bass pots, and 1Meg for the volume. Finding those today, particularly the 2Meg,  is hard, harder still if you are trying to use the original knobs, which take a 24 (fine) spline 6mm split shaft. No idea if they were audio or linear.
 
I am considering going to 1Meg for all three pots, or perhaps 1Meg for the treble and bass, 500K for the volume (to maintain the original ratio). What impact might one expect a change to these more common values to have?

The schematic is attached if that would be helpful. Thanks!

[AlNewman]

I'm not sure you'd want to change the value of the volume pot, but if you were to reduce the treble and bass to 1M, you'd probably want to reduce the 220k and 47k resistors by 1/2.  The treble and bass are likely linear, and the volume audio taper.
I may be wrong though. 
You could try just changing the pots, but I think there'd be some adjustment beyond that.

I'm sure you could check the taper of the pots at least with a few tests.

[HotBluePlates]

I am considering going to 1Meg for all three pots ... What impact might one expect a change to these more common values to have? ...

Based on the Tone Stack Calculator, it won't have much impact.

   - My link ought to apply the 1MΩ values to your amp's circuit.
   - Click "Edit" then change RB and RT to "2M".  Click "Apply" to get the original-circuit response.

The original circuit seems "okay" though the stock values for the TSC if you navigate away, then select "James Passive / Dual Bass Capacitor" will show circuit-values for a more-effective tone control setup (and no unusual pots!).

[glass54]

+1 with HBP. I wouldn't change the 1M Vol Pot. A 500k value will marginally load the Tone Stack.
I was going to suggest Duncan's Tonestock Calculator BUT HBP's link give better results.
You may even be happy by just changing pots to 1M, then fine tune if required, (using the link as a guide).
Regards
Mirek

[SteveInMN]

I am considering going to 1Meg for all three pots ... What impact might one expect a change to these more common values to have? ...

Based on the Tone Stack Calculator, it won't have much impact.

   - My link ought to apply the 1MΩ values to your amp's circuit.
   - Click "Edit" then change RB and RT to "2M".  Click "Apply" to get the original-circuit response.

The original circuit seems "okay" though the stock values for the TSC if you navigate away, then select "James Passive / Dual Bass Capacitor" will show circuit-values for a more-effective tone control setup (and no unusual pots!).

Very cool, thanks. Going from 2M to 1M shifts the peak toward the guitar range, which is the end goal for this amp.

I had swapped in a Princeton Reverb stack and the result was not good. The bass/treble had almost zero effect on the output. I had trouble seeing that I had done anything wrong, so switched back. This circuit is odd, but the effects it produces are as good as any I've ever experienced.

[SteveInMN]

I'm not sure you'd want to change the value of the volume pot, but if you were to reduce the treble and bass to 1M, you'd probably want to reduce the 220k and 47k resistors by 1/2.  The treble and bass are likely linear, and the volume audio taper.
I may be wrong though. 
You could try just changing the pots, but I think there'd be some adjustment beyond that.

I'm sure you could check the taper of the pots at least with a few tests.

Super, Al, thanks. Just to be clear -- all three 220Ks, right?

[SteveInMN]

+1 with HBP. I wouldn't change the 1M Vol Pot. A 500k value will marginally load the Tone Stack.
I was going to suggest Duncan's Tonestock Calculator BUT HBP's link give better results.
You may even be happy by just changing pots to 1M, then fine tune if required, (using the link as a guide).
Regards
Mirek

Thanks Mirek. Looks like we have consensus!

[AlNewman]

Super, Al, thanks. Just to be clear -- all three 220Ks, right?

I'm not sure, but that's what I figured at a quick glance.  You have the tone stack calculator, and basically you have a James tone stack.  Punch in whatever values you want, and compare them to what you have.  I'm sure you can find the right recipe. 

You're lucky, your amp works, and you get to tweak.  It's fun.  Rock n Roll.

[AlNewman]

It MAY be slightly strange that your pots can't be cleaned.  You may want to look for DC voltage, IE a leaky cap on the input...

[sluckey]

If you are still considering using 1M pots you may consider the Ampeg Gemini II tone stack. The component values are tailored for guitar. I have two amps with this tone stack. Very guitar friendly. I've modified your schematic to include the Ampeg tone stack.

AES has the 1M pots you would need...

    https://www.tubesandmore.com/products/potentiometer-cts-audio-knurled-shaft-38-bushing

[SteveInMN]

You're lucky, your amp works, and you get to tweak.  It's fun.  Rock n Roll.

We have an "AMEN!"

[SteveInMN]

It MAY be slightly strange that your pots can't be cleaned.  You may want to look for DC voltage, IE a leaky cap on the input...

Al, I owe you a chicken dinner. 'Cause you're the winner! Replaced the cap, all good, total silence. Never would have thought of that myself, thank you. So that I may learn something -- what made you go there? Pots just get unfixably noisy that infrequently?  Why does DC make noise, but AC doesn't?

[SteveInMN]

If you are still considering using 1M pots you may consider the Ampeg Gemini II tone stack. The component values are tailored for guitar. I have two amps with this tone stack. Very guitar friendly. I've modified your schematic to include the Ampeg tone stack.

AES has the 1M pots you would need...

    https://www.tubesandmore.com/products/potentiometer-cts-audio-knurled-shaft-38-bushing

Sluckey, thanks for this idea, I have put it in the notebook for future experimenting. As I learn about these old amps I am surprised at how modular the sections are. Who would have thunk?

[tubeswell]

You can halve tone and vol control pot resistance values to reduce the noise floor but you will also increase signal impedance (I.e. attenuate the signal more). However, the noise reduction benefit of going from 2M to 1M is not that noticeable (all other things being equal), you’ll still have hiss with 1M pots. And to get the same frequency response out of the circuit, you’d want to double all the capacitances in that part of the circuit to balance out the f=1/(2Pi.R.C) equation. However, inter-electrode capacitance is fixed for each tube (and assuming you’re not changing the tubes, f will change (slightly) anyway). Will you like the result? Try it and see - it’s a guitar amp after all is said and done.