Potentiometer 'laws' - UK Vintage Radio Repair and Restoration Discussion Forum

TonyDuell · 18th Dec 2018, 6:29 pm

I have a potentiometer here which I need to replace as the spindle has been cut too short for the knob to fit properly. Electrically it seems to be good.

It's the IF gain control in a Trio 9R-59 shortwave receiver (aka Lafayette HE-30), the manuals I have simply refer to it as '10k'. It's wired as a variable resistor.

Anyway, the pot I've removed measures pretty much 10k between the ends of the track. With it set about halfway, I get 9k between the anticlockwise end and the slider and 1k between the clockwise end and the slider. So it clearly isn't linear. Does that sound like 'reverse log law'? 'log law' rises more slowly than linear, doesn't it?

Alas the pot isn't original so it may not be the right law anyway...

Paul JD · 18th Dec 2018, 6:43 pm

Assuming you are looking from the front / control knob side of the pot with the lugs pointing upwards when you refer to anticlockwise then that is a standard log taper pot

TonyDuell · 18th Dec 2018, 6:50 pm

Is it? This page :

http://www.resistorguide.com/potentiometer-taper/

suggests that at half-rotation, a linear pot would (obviously) give 50% resistance, a log pot about 10% and an reverse-log pot about 90%. I assume that' the resistance between the wiper and the 'start' (i.e. the anticlockwise end of the track).

Since I am measuring about 90% resistance, it suggests reverse-log.

Paul JD · 18th Dec 2018, 6:59 pm

And if you measure between the other lug and the wiper you will get 10%. Obviously it depends which way you are looking at the pot but using standard lug numbering which is 1,2,3 left to right looking from the rear of the pot with the lugs pointing up a standard log taper will measure 10% between 1&2 and 90% between 2&3 at half rotation.

ms660 · 18th Dec 2018, 7:04 pm

The DS etc versions appear to use a 10k (C) reverse log for the RF gain control.

Lawrence.

18th Dec 2018, 10:16 pm

You can get a quite close approximation to log from a linear pot using a resistor twixt one end and the centre tap, about half the pots resistance seems to be a good start and will probably suffice for "government work".

Skywave · 19th Dec 2018, 1:14 am

I haven't studied the IF amplifier in the Trio 9R-59, but any RF amplifier that uses valves (and probably semi-conductors too) that has a variable resistor in its cathode (or emitter) to control the gain of that stage must be a reverse log if the adjustment of gain as that resistance is varied is to be anything approaching a 'linear law' to our ears. That law is simply dictated by the variation in the mutual conductance of the valve as its current is varied.

The RF and IF gain controls in nearly all Eddystone receivers use a reverse log. pot. - which makes for a bit of a pain when a replacement is required.

Al.

TonyDuell · 19th Dec 2018, 5:53 am

Merlinmaxwell : As far as I can work out, the 'parallel resistor' trick only works if the component is used as a true potentiometer (potential divider) not (as here) as a variable resistor.

Skywave : The IF gain control of this receiver is in the cathode circuit of the 2 IF valves, so from what you say it should be a reverse log one.

I found a log potentiometer (marked as such) in the junk box and at half rotation it gives less than half resistance between the slider and anticlockwise end. So since my original pot gives over half it would confirm it is reverse log.

But the good news. This random pot that I found is mechanically much the same as the reverse log one. Clearly the same manufacturer. It has a full-length 1/4" spindle (not the cut off 6mm one of the reverse log one) so I think with a bit of care I can use the spindle and bushing of that with the track and housing of the original to make what I need. I'll give it a go, I've got nothing to lose.

Paul JD · 19th Dec 2018, 10:10 am

Quote:

Originally Posted by TonyDuell

I found a log potentiometer (marked as such) in the junk box and at half rotation it gives less than half resistance between the slider and anticlockwise end. So since my original pot gives over half it would confirm it is reverse log.

Referring to the "anticlockwise end" is confusing and misleading, it depends which side of the pot you are looking at and how you are defining the "anticlockwise end" - does it mean the terminal that is in an anticlockwise direction from the centre or the terminal that the wiper is closest to when the pot is turned anticlockwise? Using standard pin numbering 1,2 & 3 would make things much clearer and unambiguous.

QQVO6/40 · 19th Dec 2018, 1:23 pm

Just a bit of trivia.
Tonight I have just finished repairing the "gain" pot from a Murphy B40D.
It is used in the cathode circuit of 4 of valves.
The pot in question here is a 3k ohm wirewound linear device. Definitely linear and not a log pot.
I also have a Trio-Kenwood 9R59D waiting it's turn on the bench. I agree that in the parts list it shows a 10k ohm "C" law pot and in the circuit is wired as a rheostat. It stands to reason that it would be a reverse taper because turning the pot shaft clockwise would reduce the resistance in the cathode circuits of the respective valves and so increase the gain of those stages.
Interesting that this topic has appeared as I was working on this equivalent part of my B40D tonight.

Cheers all and the very best wishes for the season.
Robert.

G6Tanuki · 19th Dec 2018, 1:32 pm

I'm reading this thread with interest because I too have a 9R-59 in my "to-do" pile - seems a previous owner has replaced the RF/IF gain pot with a random 10K job which I now suspect has the wrong 'law' - which could be the answer to why I thought the receiver had a lack of IF gain since the first half of the control's rotation did nothing...!

Paul JD · 19th Dec 2018, 1:39 pm

Quote:

Originally Posted by QQVO6/40

It stands to reason that it would be a reverse taper because turning the pot shaft clockwise would reduce the resistance in the cathode circuits of the respective valves and so increase the gain of those stages.

Whether the resistance increases or decreases when turning the shaft clockwise has nothing to do with the taper, it depends on which lugs are used. The taper determines whether the resistance increases or decreases in a linear or non-linear fashion.

19th Dec 2018, 3:58 pm

Quote:

As far as I can work out, the 'parallel resistor' trick only works if the component is used as a true potentiometer (potential divider) not (as here) as a variable resistor.

Yes, it won't do much "logging", however if you connect the resistor other end to a fixed voltage...

TonyDuell · 19th Dec 2018, 7:07 pm

Firstly to clarify, to me the 'Anticlockwise end' is the end of the track that the wiper is connected to when the spindle is turned fully anticlockwise. I'd not come across a standard way to number the terminals on a potentiometer before (despite having used said components for over 45 years).

Anyway, I was lucky. A pot from my junk box was mechanically identical to the IF gain control in this receiver (which is almost certainly not original). A bit of careful bending up lugs and swapping bits round means I had a 10k reverse-log pot (using the track from the IF gain control) with a long spindle (spindle/wiper from the junk box. I then found why the old spindle had been modified, the knob is designed for 6mm (not 1/4") spindles. Nothing a bit of careful drilling couldn't fix...

Anyway, the Trio 9R-59 I have is the original one (no suffix letters). A previous owner had 'got at' it and attempted to replace all the resistors and capacitors. Sometimes he got it very wrong (capacitors where resistors should be and vice versa). So I am basically having to take everthing apart (and removing a lot of solder in the process) and rebuild it (hopefully) according to the circuit diagram.

A few general points on this set. Originally it had a 2 core mains lead with the switch in one wire and the fuse in the other (!). I've kept that arrangement but at least fitted a 3 core lead to earth the chassis. Mine was missing the fusehold when I got it, I have put one back in.

I am not sure if this was original or part of the work done by said previous owner but there were bare wires (component leads) carrying HT crossing over bare earthed wires. I have used a lot of sleeving...

Unfortunately I can't test it yet, I need to get the whole thing assembled and then find what I got wrong. Still got the coil pack to sort out.

One final tip. If you are working on this receiver, download the 'Lafayette HE-30' manual. It's not a full service manual but I found the circuit diagram in it to be clearer than the one in any of the Trio 9R-59 manuals I could find. And the Lafayette manual includes the IF and RF alignment instructions.

David G4EBT · 19th Dec 2018, 9:11 pm

This might be of interest - 'The Secret Life of Pots':

http://www.geofex.com/Article_Folder...s/potscret.htm

I think Lawrence ms660 brought it to my attention some time ago when I needed a reverse log pot for a Portadyne Princess TRF radio.

QQVO6/40 · 19th Dec 2018, 10:45 pm

Quote:

Originally Posted by Paul JD

"Whether the resistance increases or decreases when turning the shaft clockwise has nothing to do with the taper, it depends on which lugs are used. The taper determines whether the resistance increases or decreases in a linear or non-linear fashion".

If this were the case nobody would have bothered to manufacture reverse log pots.
I can assure that they do exist and I have a small number of treasured ones here. One in fact is a reverse log tapered wire wound pot which I believe is quite a collectors item.
The non linear human ear.
When used in a cathode current gain control circuit the resistance has to decrease in a logarithmic curve when the control shaft is turned in a clockwise direction when looking from the point of view of the knob or radio operator. Somewhat intuitive for a human that to turn up the volume you turn the knob clockwise.
This use of a reverse log pot is opposite to a conventional pot that would be used as a voltage divider such as a volume control in an audio amplifier.

Cheers.
Robert.

Paul JD · 19th Dec 2018, 11:08 pm

I'm not disputing that reverse log taper pots exist, I have used many of them and fully understand the reasons why they are used. What I was pointing out was that whether the resistance increases or decreases when the pot is turned a particular way is not a function of the taper it depends on which lugs you measure the resistance between. For example if you measure the resistance between the wiper and lug 1 the resistance will increase as the pot is turned clockwise, this will be the case regardless of pot taper, the rate at which the resistance increases will vary dependent upon the taper but it will always increase, it wont decrease. Conversely if you measure between the wiper and lug 3 the resistance will always decrease as the pot is turned clockwise.

Pamphonica · 20th Dec 2018, 12:42 am

As an aside, it is not that hard to take a piece of metal shaft from a scrap pot and centre drill both it and the too-short shaft and tap both 4BA or equivalent. Chop the top off a long 4BA bolt and use the studding to screw them together with some araldite smeared around and you have a really good shaft extension for your existing pot.
Might work for you, it definitely has worked for me.
Send me your pot and I'll have a go! (No guarantees).
Jeremy

TonyDuell · 20th Dec 2018, 6:05 am

Unfortunately the spindle was nylon, to which Araldite doesn't stick too well (and nor do the threadlock type adhesives). In any case a pot from the junk box supplied a long spindle with the same type of wiper on the business end, so I transplanted that into the reverse-log pot and cut it to a more sensible length.

I do still have the bits of the junk box pot and the short spindle. I never throw anything away...

18th Dec 2018, 6:29 pm	#1
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,214	Potentiometer 'laws' I have a potentiometer here which I need to replace as the spindle has been cut too short for the knob to fit properly. Electrically it seems to be good. It's the IF gain control in a Trio 9R-59 shortwave receiver (aka Lafayette HE-30), the manuals I have simply refer to it as '10k'. It's wired as a variable resistor. Anyway, the pot I've removed measures pretty much 10k between the ends of the track. With it set about halfway, I get 9k between the anticlockwise end and the slider and 1k between the clockwise end and the slider. So it clearly isn't linear. Does that sound like 'reverse log law'? 'log law' rises more slowly than linear, doesn't it? Alas the pot isn't original so it may not be the right law anyway...

18th Dec 2018, 6:43 pm	#2
Paul JD Pentode Join Date: Oct 2018 Location: Coventry, West Midlands, UK. Posts: 220	Re: Potentiometer 'laws' Assuming you are looking from the front / control knob side of the pot with the lugs pointing upwards when you refer to anticlockwise then that is a standard log taper pot

18th Dec 2018, 6:50 pm	#3
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,214	Re: Potentiometer 'laws' Is it? This page : http://www.resistorguide.com/potentiometer-taper/ suggests that at half-rotation, a linear pot would (obviously) give 50% resistance, a log pot about 10% and an reverse-log pot about 90%. I assume that' the resistance between the wiper and the 'start' (i.e. the anticlockwise end of the track). Since I am measuring about 90% resistance, it suggests reverse-log.

18th Dec 2018, 6:59 pm	#4
Paul JD Pentode Join Date: Oct 2018 Location: Coventry, West Midlands, UK. Posts: 220	Re: Potentiometer 'laws' And if you measure between the other lug and the wiper you will get 10%. Obviously it depends which way you are looking at the pot but using standard lug numbering which is 1,2,3 left to right looking from the rear of the pot with the lugs pointing up a standard log taper will measure 10% between 1&2 and 90% between 2&3 at half rotation.

18th Dec 2018, 7:04 pm	#5
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Potentiometer 'laws' The DS etc versions appear to use a 10k (C) reverse log for the RF gain control. Lawrence.

18th Dec 2018, 10:16 pm	#6
Guest Guest Posts: n/a	Re: Potentiometer 'laws' You can get a quite close approximation to log from a linear pot using a resistor twixt one end and the centre tap, about half the pots resistance seems to be a good start and will probably suffice for "government work".

19th Dec 2018, 1:14 am	#7
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Potentiometer 'laws' I haven't studied the IF amplifier in the Trio 9R-59, but any RF amplifier that uses valves (and probably semi-conductors too) that has a variable resistor in its cathode (or emitter) to control the gain of that stage must be a reverse log if the adjustment of gain as that resistance is varied is to be anything approaching a 'linear law' to our ears. That law is simply dictated by the variation in the mutual conductance of the valve as its current is varied. The RF and IF gain controls in nearly all Eddystone receivers use a reverse log. pot. - which makes for a bit of a pain when a replacement is required. Al.

19th Dec 2018, 5:53 am	#8
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,214	Re: Potentiometer 'laws' Merlinmaxwell : As far as I can work out, the 'parallel resistor' trick only works if the component is used as a true potentiometer (potential divider) not (as here) as a variable resistor. Skywave : The IF gain control of this receiver is in the cathode circuit of the 2 IF valves, so from what you say it should be a reverse log one. I found a log potentiometer (marked as such) in the junk box and at half rotation it gives less than half resistance between the slider and anticlockwise end. So since my original pot gives over half it would confirm it is reverse log. But the good news. This random pot that I found is mechanically much the same as the reverse log one. Clearly the same manufacturer. It has a full-length 1/4" spindle (not the cut off 6mm one of the reverse log one) so I think with a bit of care I can use the spindle and bushing of that with the track and housing of the original to make what I need. I'll give it a go, I've got nothing to lose.

19th Dec 2018, 1:23 pm	#10
QQVO6/40 Hexode Join Date: May 2009 Location: Newcastle, Australia Posts: 321	Re: Potentiometer 'laws' Just a bit of trivia. Tonight I have just finished repairing the "gain" pot from a Murphy B40D. It is used in the cathode circuit of 4 of valves. The pot in question here is a 3k ohm wirewound linear device. Definitely linear and not a log pot. I also have a Trio-Kenwood 9R59D waiting it's turn on the bench. I agree that in the parts list it shows a 10k ohm "C" law pot and in the circuit is wired as a rheostat. It stands to reason that it would be a reverse taper because turning the pot shaft clockwise would reduce the resistance in the cathode circuits of the respective valves and so increase the gain of those stages. Interesting that this topic has appeared as I was working on this equivalent part of my B40D tonight. Cheers all and the very best wishes for the season. Robert.

19th Dec 2018, 1:32 pm	#11
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Potentiometer 'laws' I'm reading this thread with interest because I too have a 9R-59 in my "to-do" pile - seems a previous owner has replaced the RF/IF gain pot with a random 10K job which I now suspect has the wrong 'law' - which could be the answer to why I thought the receiver had a lack of IF gain since the first half of the control's rotation did nothing...!

19th Dec 2018, 7:07 pm	#14
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,214	Re: Potentiometer 'laws' Firstly to clarify, to me the 'Anticlockwise end' is the end of the track that the wiper is connected to when the spindle is turned fully anticlockwise. I'd not come across a standard way to number the terminals on a potentiometer before (despite having used said components for over 45 years). Anyway, I was lucky. A pot from my junk box was mechanically identical to the IF gain control in this receiver (which is almost certainly not original). A bit of careful bending up lugs and swapping bits round means I had a 10k reverse-log pot (using the track from the IF gain control) with a long spindle (spindle/wiper from the junk box. I then found why the old spindle had been modified, the knob is designed for 6mm (not 1/4") spindles. Nothing a bit of careful drilling couldn't fix... Anyway, the Trio 9R-59 I have is the original one (no suffix letters). A previous owner had 'got at' it and attempted to replace all the resistors and capacitors. Sometimes he got it very wrong (capacitors where resistors should be and vice versa). So I am basically having to take everthing apart (and removing a lot of solder in the process) and rebuild it (hopefully) according to the circuit diagram. A few general points on this set. Originally it had a 2 core mains lead with the switch in one wire and the fuse in the other (!). I've kept that arrangement but at least fitted a 3 core lead to earth the chassis. Mine was missing the fusehold when I got it, I have put one back in. I am not sure if this was original or part of the work done by said previous owner but there were bare wires (component leads) carrying HT crossing over bare earthed wires. I have used a lot of sleeving... Unfortunately I can't test it yet, I need to get the whole thing assembled and then find what I got wrong. Still got the coil pack to sort out. One final tip. If you are working on this receiver, download the 'Lafayette HE-30' manual. It's not a full service manual but I found the circuit diagram in it to be clearer than the one in any of the Trio 9R-59 manuals I could find. And the Lafayette manual includes the IF and RF alignment instructions.

19th Dec 2018, 9:11 pm	#15
David G4EBT Dekatron Join Date: Feb 2003 Location: Cottingham, East Yorkshire, UK. Posts: 5,763	Re: Potentiometer 'laws' This might be of interest - 'The Secret Life of Pots': http://www.geofex.com/Article_Folder...s/potscret.htm I think Lawrence ms660 brought it to my attention some time ago when I needed a reverse log pot for a Portadyne Princess TRF radio. __________________ David. BVWS Member. G-QRP Club member 1339.

19th Dec 2018, 11:08 pm	#17
Paul JD Pentode Join Date: Oct 2018 Location: Coventry, West Midlands, UK. Posts: 220	Re: Potentiometer 'laws' I'm not disputing that reverse log taper pots exist, I have used many of them and fully understand the reasons why they are used. What I was pointing out was that whether the resistance increases or decreases when the pot is turned a particular way is not a function of the taper it depends on which lugs you measure the resistance between. For example if you measure the resistance between the wiper and lug 1 the resistance will increase as the pot is turned clockwise, this will be the case regardless of pot taper, the rate at which the resistance increases will vary dependent upon the taper but it will always increase, it wont decrease. Conversely if you measure between the wiper and lug 3 the resistance will always decrease as the pot is turned clockwise. Last edited by Paul JD; 19th Dec 2018 at 11:16 pm.

20th Dec 2018, 12:42 am	#18
Pamphonica Nonode Join Date: Mar 2008 Location: Virginia Water, Surrey, UK. Posts: 2,877	Re: Potentiometer 'laws' As an aside, it is not that hard to take a piece of metal shaft from a scrap pot and centre drill both it and the too-short shaft and tap both 4BA or equivalent. Chop the top off a long 4BA bolt and use the studding to screw them together with some araldite smeared around and you have a really good shaft extension for your existing pot. Might work for you, it definitely has worked for me. Send me your pot and I'll have a go! (No guarantees). Jeremy __________________ Jeremy, G8MLK, BVWTVM Friend, VMARS, BVWS Secretary. www.pamphonic.co.uk www.bttt.org.uk

20th Dec 2018, 6:05 am	#19
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,214	Re: Potentiometer 'laws' Unfortunately the spindle was nylon, to which Araldite doesn't stick too well (and nor do the threadlock type adhesives). In any case a pot from the junk box supplied a long spindle with the same type of wiper on the business end, so I transplanted that into the reverse-log pot and cut it to a more sensible length. I do still have the bits of the junk box pot and the short spindle. I never throw anything away...