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 Post subject: Volume control pot values
PostPosted: Sep Sun 05, 2021 1:48 pm 
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I’m trying to understand what role the value of a volume potentiometer has in a radios audio design.
I ask this because while troubleshooting a radio recently I tried a number of different value volume pots and none seem to vary the volume range. So why do some radios have 500k pots and others 1.5m or more.
In one case I had a radio that the schematic said 1.3m but the pot fitted was 3.3m and seemed to work fine.
I do understand that at minimum volume the pot is at its highest value, and that at max volume the pot is at 0 ohms. So therefore the maximum possible volume doesn’t seem to be affected by the pot value.


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 Post subject: Re: Volume control pot values
PostPosted: Sep Sun 05, 2021 2:22 pm 
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"pot" is an abbreviation of the term "potentiometer", which loosely translates to "variable potential"....https://en.wikipedia.org/wiki/Potentiometer
A potentiometer is simply the typical method for using a variable resistor to get a variable voltage. For all this to work, one has to pay attention to the ciircuit impedance on both sides. The typical volume control will often be at least 1 megohm to minimize loading on whatever circuit is producing the audio signal. Also, the downstream circuit must have an input impedance high enough that the output of the pot is not reduced by loading.

The other major variable is the "taper', which is thefunction defining how the voltage ratio changes with the physical position of the wiper---which in turn might be defined in terms of angular rotation of the shaft.

For more details, try browsing the websites of control manufacturers---or maybe use the search engine at a parts house such as Mouser or Digikey..

Keep in mind that you CAN have variable potential with a simple variable resistor.....it does not need to be wired as a "potentiometer", but the latter is the most common configuration

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 Post subject: Re: Volume control pot values
PostPosted: Sep Sun 05, 2021 3:10 pm 
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The value and taper depend upon the circuit in which they are used. In many early radios, the volume control is in the cathode circuit of the RF tube, along with shunting the antenna coil. This calls for a low-value control, with a special reverse taper. Some sets, such as the Radiola 18, put the volume control in the antenna circuit, while the Zenith 39A and 40A put it in the B+ line to the RF stages. A good description of these various circuits can be found in an old edition of the Mallory-Yaxley Service Encyclopedia.

In many Superhetrodyne radios, the volume control is used as the diode load, and the AVC voltage is developed across it; those tend to be around 500k. A larger value would develop excess voltage. Radios with a phonograph input tend to use higher values of volume controls, with a separate resistor for the diode load, along with a coupling capacitor.

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 Post subject: Re: Volume control pot values
PostPosted: Sep Sun 05, 2021 8:43 pm 
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Location: Colorado Springs, CO 80917
kevwr wrote:
I do understand that at minimum volume the pot is at its highest value, and that at max volume the pot is at 0 ohms. So therefore the maximum possible volume doesn’t seem to be affected by the pot value.


I think it would be more correct to say that the pot value remains the same while the sliding tap picks off different levels of the audio voltage that is across the pot.

Usually one side of the pot is grounded so that the pot value is a nearly constant load on the source of the audio. Once the pot value is high enough it represents very little load (& max available audio level) & you'd find that any other pot value with similar or higher resistance also represents very little load so the max available audio level is not adversely affected.


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 Post subject: Re: Volume control pot values
PostPosted: Sep Sun 05, 2021 9:38 pm 
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This is the circuit that I tried various values of volume pot. (R17)
The circuit shows 1.3m but 3.3m was fitted.
What I’m failing to understand is that at for example 10% movement of the control the 1.3m pot is at 1m value and the 3.3m pot is at 3m, both seem to have the same audible volume. Doesn’t seem logical as the second pot is applying higher resistance to the audio signal.


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 Post subject: Re: Volume control pot values
PostPosted: Sep Sun 05, 2021 9:50 pm 
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Location: Colorado Springs, CO 80917
The schematic portion doesn't show the rest of the circuit (to the right) so I'm not sure. What are R19 & R24 supposed to be? Bass & Treble?


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 Post subject: Re: Volume control pot values
PostPosted: Sep Sun 05, 2021 10:01 pm 
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hwhall wrote:
The schematic portion doesn't show the rest of the circuit (to the right) so I'm not sure. What are R19 & R24 supposed to be? Bass & Treble?

Yes, they are the tone controls


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 Post subject: Re: Volume control pot values
PostPosted: Sep Sun 05, 2021 10:52 pm 
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Location: Colorado Springs, CO 80917
Okay, I see it now. Look at the little attached excerpt from the schematic. The audio source goes into R17 (a big resistance) then thru R18 (small resisitance). The pot tap goes to a triode grid which is very high resistance, too.

The incoming audio from V2 is divided across R17 and R18 but almost ALL of the division is in R17 because it's so large compared to R18. No matter what large resistance you substitute for R17, half of the audio is at the halfway point of rotation. Ditto for 1/4, 3/4 etc.

Because the V3 grid is such high resistance it doesn't affect the division of the audio noticeably. So long as the R17 value is very large, it really doesn't matter much for the volume what the actual value is. HOWEVER, because R17 is tapped for part of the tone control circuit, large differences between the design value & a substitution MIGHT make a difference in the tone settings.


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 Post subject: Re: Volume control pot values
PostPosted: Sep Sun 05, 2021 11:33 pm 
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Joined: Jan Thu 07, 2021 10:41 am
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hwhall wrote:
Okay, I see it now. Look at the little attached excerpt from the schematic. The audio source goes into R17 (a big resistance) then thru R18 (small resisitance). The pot tap goes to a triode grid which is very high resistance, too.

The incoming audio from V2 is divided across R17 and R18 but almost ALL of the division is in R17 because it's so large compared to R18. No matter what large resistance you substitute for R17, half of the audio is at the halfway point of rotation. Ditto for 1/4, 3/4 etc.

Because the V3 grid is such high resistance it doesn't affect the division of the audio noticeably. So long as the R17 value is very large, it really doesn't matter much for the volume what the actual value is. HOWEVER, because R17 is tapped for part of the tone control circuit, large differences between the design value & a substitution MIGHT make a difference in the tone settings.

Thanks for such a comprehensive answer, very much appreciated.

To complicate things this radio has a combined volume and on/off switch. The only available pot I can find with a switch is 1m, compared to the 1.3m on the schematic.

Unfortunately it doesn’t have a center tap, but others have added one using conductive paint. I do have the skills to do this sort of mod, but is another alternative to bypass the tap? If so would you have any suggestions on what to do with the current tap wiring?

I do realize that I would lose the “loudness” function but this radio has great tone control via bass/treble pots and also bass/descant/Speech push buttons.


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 Post subject: Re: Volume control pot values
PostPosted: Sep Mon 06, 2021 12:00 am 
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Joined: Jan Sat 25, 2020 5:23 am
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Location: Colorado Springs, CO 80917
I'm not as up to speed on tone circuits. I'd bet that the 1M might be fine subbed for a 1.3M. That's just 30% off & some of those old sets used 20% tolerances from the factory.

Let me rummage in my old pots stash a bit. You need a tapped, 1.3M audio taper with SPST switch? What about the shaft?
Also, you have any pots will work but are only missing the switch? Sometimes they can be added off of a similar pot.

EDIT: No luck on tapped audios of a meg or so. Do have several diff types of switched pots if you want to investigate a transplant.


Last edited by hwhall on Sep Mon 06, 2021 12:12 am, edited 1 time in total.

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 Post subject: Re: Volume control pot values
PostPosted: Sep Mon 06, 2021 12:05 am 
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Joined: Jan Thu 07, 2021 10:41 am
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hwhall wrote:
I'm not as up to speed on tone circuits. I'd bet that the 1M might be fine subbed for a 1.3M. That's just 30% off & some of those old sets used 20% tolerances from the factory.

Let me rummage in my old pots stash a bit. You need a tapped, 1.3M audio taper with SPST switch? What about the shaft?
Also, you have any pots will work but are only missing the switch? Sometimes they can be added off of a similar pot.

I appreciate your offer, but I’m in Australia so postage is an issue. I can buy a 1m locally very cheap so I think that is the best option. My on/off switch is fine but despite cleaning the pot itself is very scratchy and cuts out at times.


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 Post subject: Re: Volume control pot values
PostPosted: Sep Mon 06, 2021 12:13 am 
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Ah, yeah, the postage becomes an issue even for very lightweight bits.


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 Post subject: Re: Volume control pot values
PostPosted: Sep Mon 06, 2021 2:41 am 
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Negative feedback is being applied across R18, through the RC network, for some bass boost and distortion reduction. It would be best to use a replacement control with the same value and tap; a 1.5 megohm would be the closest. Measure the value of the tap from the end connected to R18.

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 Post subject: Re: Volume control pot values
PostPosted: Sep Mon 06, 2021 3:16 am 
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Tim Tress wrote:
Negative feedback is being applied across R18, through the RC network, for some bass boost and distortion reduction. It would be best to use a replacement control with the same value and tap; a 1.5 megohm would be the closest. Measure the value of the tap from the end connected to R18.

Thanks Tim, sorry for the newbie questions, I have a basic understanding about the negative feedback, but the RC network you mention, is that R22 and C37 on my schematic.
Also, I may only be able to source a 1m pot, is this a problem? Could I compensate in any way by changing the value of R18?


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 Post subject: Re: Volume control pot values
PostPosted: Sep Mon 06, 2021 4:47 am 
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I used a 1 meg pot in place of a 500 K in a Philco recently and it works well, no taps, typical circuit. With the higher resistance the knob may need to be turned a bit more to increase volume but not noticeable.

In your case, there seems to be more involved, and you are wanting to use a lower resistance. You may find a 300 to 500 K-ohm resistor is needed in series to reduce the volume completely, but it does depend on the taper. No harm in trying it.

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 Post subject: Re: Volume control pot values
PostPosted: Sep Mon 06, 2021 5:15 am 
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One other note.... if you are using a pot that has a very different taper than the original, it's going to act and "feel" different than the original part. They are available, and from one of our vendors here if you want, but yes, you'll have to pay something for it, and postage.

Try https://www.oldradioparts.net

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 Post subject: Re: Volume control pot values
PostPosted: Sep Mon 06, 2021 2:16 pm 
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Tim Tress wrote:
The value and taper depend upon the circuit in which they are used. In many early radios, the volume control is in the cathode circuit of the RF tube, along with shunting the antenna coil. This calls for a low-value control, with a special reverse taper. Some sets, such as the Radiola 18, put the volume control in the antenna circuit, while the Zenith 39A and 40A put it in the B+ line to the RF stages. A good description of these various circuits can be found in an old edition of the Mallory-Yaxley Service Encyclopedia.

In many Superhetrodyne radios, the volume control is used as the diode load, and the AVC voltage is developed across it; those tend to be around 500k. A larger value would develop excess voltage. Radios with a phonograph input tend to use higher values of volume controls, with a separate resistor for the diode load, along with a coupling capacitor.

A bit of clarification....

Once the (modulated)IF signal goes through the detector stage, you have a signal with 2 components:

DC, proportional to overall signal strength, and typically used for AVC.
AC (audio), determined by the original modulation scheme at the transmitter
Every radio has a different method for developing and using these components.......

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"Voltage is fun to watch, but it's the CURRENT that does the work."


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 Post subject: Re: Volume control pot values
PostPosted: Sep Mon 06, 2021 10:26 pm 
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Joined: Jan Thu 07, 2021 10:41 am
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hwhall wrote:
Okay, I see it now. Look at the little attached excerpt from the schematic. The audio source goes into R17 (a big resistance) then thru R18 (small resisitance). The pot tap goes to a triode grid which is very high resistance, too.

The incoming audio from V2 is divided across R17 and R18 but almost ALL of the division is in R17 because it's so large compared to R18. No matter what large resistance you substitute for R17, half of the audio is at the halfway point of rotation. Ditto for 1/4, 3/4 etc.

Because the V3 grid is such high resistance it doesn't affect the division of the audio noticeably. So long as the R17 value is very large, it really doesn't matter much for the volume what the actual value is. HOWEVER, because R17 is tapped for part of the tone control circuit, large differences between the design value & a substitution MIGHT make a difference in the tone settings.

Just wondering what effect the value of R18 has on the volume. Could it effect the maximum volume achievable in any way? Could you vary the range of the volume control by increasing the value? This radio is very loud even at 1/4 turn of the control, would be nice to lower the volume a bit to give more range.


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 Post subject: Re: Volume control pot values
PostPosted: Sep Mon 06, 2021 11:43 pm 
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Location: Colorado Springs, CO 80917
Quote:
Just wondering what effect the value of R18 has on the volume. Could it effect the maximum volume achievable in any way? Could you vary the range of the volume control by increasing the value?


R18 is very small compared to R17 so any small changes to it will probably go unnoticed. Increasing it won't increase the max volume but can raise up the min volume level. I wouldn't decrease it - there's some reason they didn't want the end of R17 to be too close to grounded.

Quote:
This radio is very loud even at 1/4 turn of the control, would be nice to lower the volume a bit to give more range.


There's really no reason but "aesthetics" for the control to be higher than 1/4 for comfy listening (unless maybe that was a noisy spot on an otherwise okay pot). On weak stations you may need the rest of that vol control range.

If I were deadset on lowering max volume, I think I'd try:
(a) increasing R14, but that might affect the tone control operation. Would have to experiment. Kinda messy.
(b) changing the 1:1 ratio of R11 & R12. Presently they divide V2's output in half. Might change R11 to 100K & R12 to 100K - that'd still be total 300K but with the signal now reduced to 1/3 instead of 1/2. Might have to adjust that ratio more to get what you want for range of control at R17. This approach has an advantage in that it won't affect the volume from the "Pickup" phono input.

EDIT: Might change R11 to 200K & R12 to 100K - that'd still be total 300K


Last edited by hwhall on Sep Tue 07, 2021 12:07 am, edited 1 time in total.

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 Post subject: Re: Volume control pot values
PostPosted: Sep Tue 07, 2021 12:00 am 
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Joined: Jan Thu 07, 2021 10:41 am
Posts: 87
hwhall wrote:
Quote:
Just wondering what effect the value of R18 has on the volume. Could it effect the maximum volume achievable in any way? Could you vary the range of the volume control by increasing the value?


R18 is very small compared to R17 so any small changes to it will probably go unnoticed. Increasing it won't increase the max volume but can raise up the min volume level. I wouldn't decrease it - there's some reason they didn't want the end of R17 to be too close to grounded.

Quote:
This radio is very loud even at 1/4 turn of the control, would be nice to lower the volume a bit to give more range.


There's really no reason but "aesthetics" for the control to be higher than 1/4 for comfy listening (unless maybe that was a noisy spot on an otherwise okay pot). On weak stations you may need the rest of that vol control range.

If I were deadset on lowering max volume, I think I'd try:
(a) increasing R14, but that might affect the tone control operation. Would have to experiment. Kinda messy.
(b) changing the 1:1 ratio of R11 & R12. Presently they divide V2's output in half. Might change R11 to 100K & R12 to 100K - that'd still be total 300K but with the signal now reduced to 1/3 instead of 1/2. Might have to adjust that ratio more to get what you want for range of control at R17. This approach has an advantage in that it won't affect the volume from the "Pickup" phono input.

I might try the option (b) first as my pu volume is already lower. Your description shows changing both to 100k, should one be 200k?.
This volume control has a tap for loudness so being at full volume at 1/4 turn is probably already affecting the tone to some extent.
I’m happy to experiment to find the correct levels for the best sound quality.


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