Log/Lin Pots

[Yorkie]

That's an interesting and informative datasheet Lawrence. I've often wondered how that make log carbon track pots. As to wire-wound long pots, coincidentally there's a thread running 'over the wall' about them which brought back memories of Colin's ('Retired') monumental efforts back in 2010 trying to rebuild one.

Wirewound log/inverse log pots were something of an abomination, and very prone to failure for a number of reasons, not least that to simulate a log law, the resistance wire is progressively finer with each segment, and apart from the fact that the welding of the joints being of dissimilar gauges for each segment is a weakness, because the gauges of the higher resistance segments is so fine, it's current carrying capacity is lower than the earlier segments. Then of course there is the wear of the wiper around the track, again problematical on the finer gauge of wire.

Apart from these shortcomings, sourcing small quantities of resistance wire of the necessary gauges is next to impossible. Back in 2010, Colin set about attempting to repair such a pot and went to very great lengths - making a new Tufnol sawtooth former, making a 'hot air' oven to coat and dry the resistance wire with special varnish (sourced from America as I recall). In the end, it all came to nothing.

Anyone who might wish to read of his efforts will find the thread here:

http://www.vintage-radio.net/forum/showt...hp?t=60050

I've attached a rather crude sketch which outlines how a sawtooth former enables each turn or wire to be progressively longer and hence higher resistance than the turn before it, then each of the following segments is of progressively higher (and thus finer), resistance wire.

I think there are rotary switches available with some 30 click positions. I would have thought it a much better idea to use such a switch with the desired wattage of fixed wirewound resistors (5 or 10 Watts for example), with values chosen to replicate a long law pot. Even a 12-way Lorlin rotary switch might suffice. I wouldn't have thought the pot needed to be so precise.

I've read quite enough sagas about AVO valve tester flimsy GM wirewound pots, tired thumbwheel switches and fragile meters to remind me to never attempt to acquire one!