Tone controls and 'anti-log' potentiometers

Chris M. White

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Oct 3, 2021, 12:43:57 PM10/3/21

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Hey gang,

Does anyone know if anti-log pots are more prone to failure? I bought
an old vintage broadcast radio that has separate bass and treble
controls and for some reason they've used an anti-log pot for the
treble - and it doesn't do nothing. Apart from that, the radio seems
fine so I'd like to get it fixed. I've never encountered 'anti-log'
pots before (never even heard of them) and am just curious as to if
they're more likely to fail.

Thanks all,

Chris.

PS: the radio in question is a Hacker Sovereign II from about 1970
made in liddle ol' England.

Peter W.

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Oct 3, 2021, 1:01:43 PM10/3/21

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No more so than any other pot - just a tad more difficult to make. Point of the choice, initially, was to make the pot travel more linear to the human ear, vs. electrically. They are easily obtained in most common values, other than the fact that the Brits, like the French, when they made their electronics copied no one, and no one copied them.

I would first try cleaning the pot. I do not know how the treble-circuit is designed in that radio - other than the pot-resistor wants to be in-circuit in many designs, so it could be as simple as something in the wiper. Further, if the overall pot value and current-handling capacity is correct, the only difference between OEM and a replacement - anti-log or not - would be the response-to-travel perception.

Peter Wieck
Melrose Park, PA

Ralph Mowery

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Oct 3, 2021, 1:11:36 PM10/3/21

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In article <i4njlght4300h0hb0...@4ax.com>, cw9...@gsm.com
says...

>
> Does anyone know if anti-log pots are more prone to failure? I bought
> an old vintage broadcast radio that has separate bass and treble
> controls and for some reason they've used an anti-log pot for the
> treble - and it doesn't do nothing. Apart from that, the radio seems
> fine so I'd like to get it fixed. I've never encountered 'anti-log'
> pots before (never even heard of them) and am just curious as to if
> they're more likely to fail.
>
>
>

Pots like anti-log, log, linear are just ways of how the resistance
changes as the knob is turned.

Many audio type pots will be log or anti-log so that as they are
turned the results seem linear to the ear as the ear is not a linear
device,but more of a log type of device.

A linear taper pot will be turned just a little on one end of the
rotation and a lot on the other end of the rotation to get the same
effect.
The resistance of a linear pot will change the same ammount no matter
where on the rotation it is. A log and anti-log pot will change
resistance very much on one end and not much on the other end of the
rotation for the same ammount of degreese the shaft is rotated.

So the taper should not make any difference in the life of a pot.

Chris M. White

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Oct 3, 2021, 1:16:02 PM10/3/21

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Thanks. I came across the relevant schematic fragment online showing
the tone controls for bass and treble. I can't see how they work
unless they form some sort of elementary network with the 2.5uF C11.
Hard to say with those pesky sockets in the way of the signal path.
It's about a third of the way down the page in this document:

https://www.microscopy-uk.org.uk/mag/artjan13/iw-radio2.html

Ralph Mowery

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Oct 3, 2021, 2:17:25 PM10/3/21

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In article <cvojlgdenm5kj60ja...@4ax.com>, cw9...@gsm.com
says...

>
> Thanks. I came across the relevant schematic fragment online showing
> the tone controls for bass and treble. I can't see how they work
> unless they form some sort of elementary network with the 2.5uF C11.
> Hard to say with those pesky sockets in the way of the signal path.
> It's about a third of the way down the page in this document:
>
> https://www.microscopy-uk.org.uk/mag/artjan13/iw-radio2.html
>
>
>
>

From the looks of things there is no way to tell how the base and tresle
controls work. They are shown to go into the block diagram to the right
of the 2 batteries. There could be anything in that block.It is
doubtful that the C11 has anything to do with the 2 pots.

Phil Allison

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Oct 3, 2021, 4:57:21 PM10/3/21

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Chris M. White wrote:
=================

>
> Does anyone know if anti-log pots are more prone to failure?

** I do.

>I bought
> an old vintage broadcast radio that has separate bass and treble
> controls and for some reason they've used an anti-log pot for the
> treble - and it doesn't do nothing.

** So you can't say if it is a cut or boost ?

> Apart from that, the radio seems
> fine so I'd like to get it fixed. I've never encountered 'anti-log'
> pots before (never even heard of them) and am just curious as to if
> they're more likely to fail.

** Rare to see one in a tone circuit.

But very common the the "gain" control of mic pre-amps.
Here the gain increases as the resistance reduces, so a normal log pot has to operate anticlockwise.

BTW when you have a sample size of one, probability theory does not work.

...... Phil

Phil Allison

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Oct 3, 2021, 5:00:12 PM10/3/21

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Ralph Mowery wrote:
=======================

> So the taper should not make any difference in the life of a pot.

** Don't be so sure.

Most reverse log pots are made by installing the carbon element from a log pot upside down.
That has an effect on reliability.

...... Phil

Chris M. White

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Oct 3, 2021, 5:02:57 PM10/3/21

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Thanks for your observations. That explains a lot. I'll need to order
a service manual for this radio off the Web as there's no free
downloads for this model available anywhere. Until that arrives,
there's nothing more I can do.