Tone controls, presumably logarithmic?

[RichTheDude]

Hi all,

Just a quick question. I know volume controls should be logarithmic potentiometers and I presume Treble and Bass controls are too ?

First time building an amp with bass and treble controls (the kit I have brought has pots with splined shafts, I would like to replace them with pots with 6mm D shaped shafts such that I can fit set screw mounted knobs nicely). The schematics (found online) do not state the law of the pots...

If no-one is sure I can always measure the pots when I get the bits

TIA,
Rich

 

[Nigel Goodwin]

Hi all,

Just a quick question. I know volume controls should be logarithmic potentiometers and I presume Treble and Bass controls are too ?

No, most tone controls are active and use linear pots, passive ones usually use log ones though.

First time building an amp with bass and treble controls (the kit I have brought has pots with splined shafts, I would like to replace them with pots with 6mm D shaped shafts such that I can fit set screw mounted knobs nicely). The schematics (found online) do not state the law of the pots...

Show the schematic.

 

[RichTheDude]

Hi Nigel

Here is the tone circuit (found online)

https://i55.tinypic.com/29z37so.jpg

It's passive tone control, with a single triode pre-amp per channel so I am guessing they are logarithmic. I think I will wait until I have all the bits and measure them to be sure but thanks for the information!

I am predominantly an FPGA/embedded system/instrumentation engineer, but I did build a small practice amp a few years back (6j7 and 6v6) so the realm of audio design and valves is still fairly new to me.

 

[Nigel Goodwin]

That's a crude passive network for a valve amp, you need log pots.

 

[RichTheDude]

Cheers, need to order some odds and ends for it might as well get the pots now whilst ordering.

It's the Mullard 5 10 tone control design topology. Such design decisions surprise me, it always makes sense to do amplification first, as close to the source as possible rather than amplifying the noise introduced by the passive components.

 

[JimB]

The schematics (found online) do not state the law of the pots...

Actually, if you look at the schematic, adjacent to PT1a it does state "Audio Taper Dual Pots".
"Audio Taper" is another expression for what is generally known as "logarithmic".

JimB

 

[RichTheDude]

Actually, if you look at the schematic, adjacent to PT1a it does state "Audio Taper Dual Pots".
"Audio Taper" is another expression for what is generally known as "logarithmic".

JimB

Doh! Thanks for pointing that out, I must be quite sleepy today!