Potentiometer Options
 

Hi,

I have a Harverson 10/14 amplifier from the 50s (I think) and it uses 4x 1M log potentiometers. One of them incorporates a DPST switch but, curiously, it's integrated with the treble tone control and, as I recently discovered, it's actually 25K instead of 1M which then explained why it didn't really do much as a treble control!

I'd like to replace the 25K unit with a 1M version, ideally with a switch (to avoid having to drill an additional hole) and solder lugs, but these seem impossible to source if you also need the potentiometer to be larger than the usual 16mm or 24mm units (to fit securely in the chassis' already-drilled mounting holes - the incumbent units are more like 30mm wide.

1. Does anyone know where I might source a 1M potentiometer, ideally with a DPST switch, that's around 30mm wide?

2. Would it be possible to use the 25K poteniometer as-is but with some additional circuitry to increase the effective maximum resistance up to 1M?

Any suggestions welcome :)

Re: Potentiometer Options
 

You might find that Bore Edwards can help, but it would be quite a lot more expensive than a standard 1 Meg switched pot from the likes of Bowood Electronics, who could supply a 25mm diam one in either linear or log law for £2.45 plus £2.85 P&P (No VAT). IE:

https://www.bowood-electronics.co.uk...bca7ebd3d42435

Blore Edwards Tel: 01685 882043

Website: https://www.blore-ed.com/

Good luck in your quest.

Re: Potentiometer Options
 

"Does anyone know where I might source a 1M potentiometer, ideally with a DPST switch, that's around 30mm wide?" I might have a used one somewhere, I've been saving them for years, doubt I'll use them.

As regards the 25k being useless as a tone control pot; 25k should be more than enough for even a passive control or simple H/LPF, why not change the caps instead?

Andy.

Re: Potentiometer Options
 

Can you use a 24mm pot on a bit of sheet metal glued behind the front panel? Hopefully the knob would hide it?

Re: Potentiometer Options
 

Why does it need to be 30mm wide? Typical 1950's pots mount in a 3/8" hole so a modern pot with a 9.5mm thread will fit snugly in the original hole, or am I missing something?

Re: Potentiometer Options
 

can you take a pic so we can see the fitting?

Re: Potentiometer Options
 

Thanks David - that's really useful. It's just typical that the one I need is a weird size and I'll probably need to replace them all as the existing ones also have metal shafts - a more modern one will probably be plastic.

Re: Potentiometer Options
 

Thanks everyone - I've attached a couple of pictures so you can see the issues. One picture shows the amplifier with an original, fitted pot, a modern replacement pot, and the original switched pot (now removed). The other picture shows three pots, one is the 30mm switched unit and I then have a modern 1M log pot together with an older pot.

1. The modern unit has a smaller nut size than both of the older ones

2. The lug in the original unit, that stops the unit rotating in the chassis, is set further out than the lugs on the other two.

https://1drv.ms/u/s!AEB_A0SO1G6dlG0

https://1drv.ms/u/s!AEB_A0SO1G6dlGw

Re: Potentiometer Options
 

Difficult to tell from the photo but the modern pot in your second pic appears to be a "miniature" 16mm dia pot designed to fit a 7mm dia mounting hole. If you look for a 24mm dia pot that will be designed to fit a 9.5mm mounting hole and should fit the original hole. The locking tab not lining up should not be a problem, just bend the tab back or break it off on the new pot, as long as the nut is done up tightly it shouldn't work loose.

Re: Potentiometer Options
 

If a pot is loose enough to need the support of the lug in use, the nuts will soon loosen further and the whole thing will rock around. The lug really only serves as an assembly guide.

David

Re: Potentiometer Options
 

I have a vintage pot(Morganite),NOS, with switch, says 1 Meg on the side so I assume its linear. 30 mm across,20 mm long shaft. 35mm deep from front to end of switch contacts. I haven't got round to checking it but if it seems good would you like to buy it for £5,this includes postage(£2.95) Les.

Re: Potentiometer Options
 

Thanks for the offer but it’s an audio/log taper that I need.

Good to hear that I’m not the only one with a 30mm pot :)

Re: Potentiometer Options
 

I will look for something suitable but I’ll probably need to get the lug to fit if it’s a switched pot - it’s bound to work loose if it’s being switched on and off.

It just occurred to me that an option would be to simply remove one of the two input pots, shift them all along, and put a switch in the vacant “pot hole”. The two channel feature was useful for a typical stereo input though.

Re: Potentiometer Options
 

I can assure you that this is not the case, I have fitted a great many pots both switched and un-switched without a locating lug and it is not a problem. As someone else pointed out the job of the locating tab is not to prevent the nut coming loose, that is the job of the washers, the lug is just there to locate the pot and prevent it turning while the nut is being tightened during assembly.

Re: Potentiometer Options
 

I'm not sure if this will help but over the last 20 years I have gone to TOCOS (Tokyo Cosmos) potentiometers.

They come in 24 or 31mm diameter (and there are smaller 16mm ones too), with & without switches and dual gang. I got totally hooked on them after buying them in Akihabara in Tokyo and being impressed with the quality. I've used them in practically every radio home brew project, amplifier repairs and restoration project. Their quality and construction is outstanding/unmatched. The only disadvantage is they are Metric, 6mm shaft and 9mm mounting hole. The range is 50R to 2Meg. I have worked around the metric issue with thin shims and use a type of slim line shaft extender where required. They have a sealed construction.

I noticed recently that the manufacturers have an English website so it looks like it is possible to buy them now directly from the factory. Many Japanese companies now have overseas sales department and accept paypal.

The type I'm talking about is near the bottom of the page on this link:

http://www.tocos-j.co.jp/e/catalog/pot/pot01.html

After using these it is just about impossible to go back to most other products.

If you scroll to page 10 of this article you will see an array of the 16mm diameter versions I recently used to modify a video monitor:

http://worldphaco.com/uploads/Autode...e_monitor..pdf