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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 5th Dec 2018, 4:48 pm   #1
phykell
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Question 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
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Old 5th Dec 2018, 6:51 pm   #2
David G4EBT
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Default Re: Potentiometer Options

Quote:
Originally Posted by phykell View Post
1. Does anyone know where I might source a 1M potentiometer, ideally with a DPST switch, that's around 30mm wide?
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.
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Old 6th Dec 2018, 7:09 am   #3
Diabolical Artificer
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Default 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.
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Old 6th Dec 2018, 9:18 am   #4
ex seismic
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Default 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?
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Old 6th Dec 2018, 9:10 pm   #5
Paul JD
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Default 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?
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Old 6th Dec 2018, 9:21 pm   #6
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Default Re: Potentiometer Options

can you take a pic so we can see the fitting?
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Old 6th Dec 2018, 11:09 pm   #7
phykell
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Default Re: Potentiometer Options

Quote:
Originally Posted by David G4EBT View Post
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.
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.
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Old 6th Dec 2018, 11:36 pm   #8
phykell
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Default 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
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Old 7th Dec 2018, 12:19 am   #9
Paul JD
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Default 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.
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Old 7th Dec 2018, 7:35 am   #10
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Default 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.

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Old 8th Dec 2018, 5:27 pm   #11
TowerRadio
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Default 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.
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Old 9th Dec 2018, 1:47 am   #12
phykell
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Default Re: Potentiometer Options

Quote:
Originally Posted by TowerRadio View Post
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.
Thanks for the offer but itís an audio/log taper that I need.

Good to hear that Iím not the only one with a 30mm pot
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Old 9th Dec 2018, 1:52 am   #13
phykell
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Default Re: Potentiometer Options

Quote:
Originally Posted by Paul JD View Post
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.
I will look for something suitable but Iíll probably need to get the lug to fit if itís a switched pot - itís bound to work loose if itís 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 ďpot holeĒ. The two channel feature was useful for a typical stereo input though.
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Old 9th Dec 2018, 11:16 am   #14
Paul JD
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Default Re: Potentiometer Options

Quote:
Originally Posted by phykell View Post
Iíll probably need to get the lug to fit if itís a switched pot - itís bound to work loose if itís being switched on and off..
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.
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Old 9th Dec 2018, 12:11 pm   #15
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Default 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

Last edited by Argus25; 9th Dec 2018 at 12:22 pm. Reason: add link
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