Anti--log and 'tapped' pots

[Yorkie]

I’d previously mentioned problems that I’d had in trying to create an antilog pot with an added resistor, in a thread entitled  ‘Anti-log Pot Track Puzzler’ in connection with restoration of a Portadyne 'Princess' back in 2012:

http://golbornevintageradio.co.uk/forum/...p?tid=2177&page=2&highlight=anti+log+pots

I graphed the resistance v degrees of rotation to confirm that my pot was definitely an anti-log track, and tried a new linear pot with external fixed resistor, unsuccessfully; though maybe I wasn’t measuring the resistance correctly or me expectations were different? It certainly seemed an elegantly simple solution! I’d spotted the website ‘Secret Life of Pots which explained how to do it:

http://www.geofex.com/article_folders/po...tscret.htm

Lawrence then pointed out this useful website which covered the same topic:

http://gaussmarkov.net/wordpress/parts/r...rs-6-mods/

In the diagram below of the pot with the added 'taper' resistor, the desired result would of course be for the centre tap to behave like an anti-log track, but when measured from the tap to chassis, all that it's going to do is to act like a linear pot, which is what it is.

‘Tapped pots’:

I restored a Stella 105U which had a tapped pot, which I covered at this link:

http://golbornevintageradio.co.uk/forum/...hp?tid=889&page=2&highlight=Stella+105U

Other sets use a similar pot – the Pye 31 for example, as depicted in the extract below of Trader Sheet 1476. I’d wondered whether a solution for the Stella 105U as depicted in the last diagram below could be two fixed resistors across the pot. I mentioned that somewhere, and was told no – it wouldn’t work.

I’ve added some attachments covering both of these topics.

The older I get, the less I know, but the more I think I know!

[AlanBeckett]

Yes, Pye were quite keen on their Tapped Pots

Alan

[Terry]

I’d previously mentioned problems that I’d had in trying to create an antilog pot with an added resistor, in a thread entitled  ‘Anti-log Pot Track Puzzler’ in connection with restoration of a Portadyne 'Princess' back in 2012:

As pointed out at the time, your measurement method was flawed.

You were measuring only the unmodified (lower) part of the track but completely ignoring the upper part which is where the non-linear change in resistance occurs.

A simple way to prove this is to pass a small current through the pot and measure the voltage between the slider and the earthy end at differing degrees of rotation.

As for tapped pots, I'm sure that, in another forum,  Mike (Crackle) described how he had successfully added a tap to a pot but can't finf it at the moment.

Perhaps Mike can put in a repeat performance?

[Yorkie]

Thanks for reading Terry, and for your comments.

Yes, I realised afterwards of course that all I was measuring was the lower portion of the pot, over which the taper resistor exercises no influence.