Potentiometers: can the values be changed when replacing old control pots

sdk

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I have a stack of all Indian solid state amps. As and when the tracks wear out and cleaning does not help, my repair tech replaces them. Recently when I opened an amp i found that the values of the pots don't match the original, for eg a 250K log pot was replaced by a 50K linear pot.

What happens to the sound and to the circuit when such replacements are done? Is it safe to do so and replace pots with the nearest value he gets his hands on?
 

OM_2K19

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Most solid stare amplifiers are designed with a 5% tolerance level. As long as the replaced pot fails within the tolerance level, I don't see any issues. If it's off by the way too much, there are high chances that it may impact the amplifier behavior and thus alter the sound signature.
 

reubensm

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Practically speaking, the pot will work and the amplifier will sing, but we have to keep in mind that a pot is a variable resistor.

The resistance across the 2 end terminals engaged with either end of the resistance track (horse shoe) inside the pot, constitute the resistance value of the pot. Of course, the shaft moves the wiper along this track varying the resistance between either track-terminal and the wiper terminal.

This resistance value is of vital importance to the circuit in which it is operating. Changing the value of the POT literally means, changing the value of a resistance in the circuit. The designers of the amplifier may have settled at a certain pot value, keeping in mind the circuit design and requirements. Hence it is always good to use the manufacturer-specified value. For POTs used in tone control stages, this change in value can impact the frequency range of the tone control. If the POT is used for gain control, one must be careful as change in value may impact the input or output impedance of the circuit. Some amplifiers have POTs with loudness taps for the loudness control. Replacements for these tapped POTs are not easy to find as well. Then there is another debate of replacing log pots with lin pots and the other way round, Its a lengthy debate :)
 

sdk

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Practically speaking, the pot will work and the amplifier will sing, but we have to keep in mind that a pot is a variable resistor.

The resistance across the 2 end terminals engaged with either end of the resistance track (horse shoe) inside the pot, constitute the resistance value of the pot. Of course, the shaft moves the wiper along this track varying the resistance between either track-terminal and the wiper terminal.

This resistance value is of vital importance to the circuit in which it is operating. Changing the value of the POT literally means, changing the value of a resistance in the circuit. The designers of the amplifier may have settled at a certain pot value, keeping in mind the circuit design and requirements. Hence it is always good to use the manufacturer-specified value. For POTs used in tone control stages, this change in value can impact the frequency range of the tone control. If the POT is used for gain control, one must be careful as change in value may impact the input or output impedance of the circuit. Some amplifiers have POTs with loudness taps for the loudness control. Replacements for these tapped POTs are not easy to find as well. Then there is another debate of replacing log pots with lin pots and the other way round, Its a lengthy debate :)
Thank you so much for a lovely explanation!

The main problem i face is that for all my cosmic amps, they have passed through so many repair techs that almost no pot is still original.

In the absence of circuit diagrams or markings on the pcb i can only look at the one present right now and replace with the same value.

If there are any Cosmic fans here who own any model of amp, i would be grateful if you could put up the make, model and the pot values of their unit.
 

reubensm

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With cosmic it is actually quite easy. If there are willing FMs out there with cosmic amps in good condition, they should be able to help. Old generation Indian amplifiers used metal shaft large potentiometers (the valve era lasted in India, way into the 1970s and hence, these were compatible with both) that were really long lasting and hence, they are likely to still be in use if properly maintained and not abused. The value can be ascertained by just taking a look inside the amp.
 
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