Potentiometers in a '68 SL

tWreCK

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A friend of mine has given me some old pots from what I think is a JCM 800. They're all working but need to be cleaned up. The pots are:

Presence 22k - ('68 SL - 5k)
Bass 1M
Mid 22k - ('68 SL - 25k)
Treble 220k - ('68 SL - 250k)
Master 1M
Volume preamp 1M

Compared to a '68 the bass and both volume pots are the same but presence, mid and treble differ. Would these work in a '68 circuit? How would the tone/sound be affected by the differences? Since they're free I'm thinking maybe I could save me some cash. Worst case I'll just have to buy the correct values for the other 3 pots.
 

neikeel

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As the pots are old have you measured them with a meter?
Most of the pots on a 68 will be the RS labelled AB CTS type. Earlier amps the bigger RS type.
Most of the best pots that sound best actually test high.
 

tWreCK

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Haven't measured them yet as my DMM stopped working. Let's say we disregard the effects of age for now (value drift etc) and directly compare the pots in the circuit now (with their stated values), how will the presence/mid/treble controls act soundwise? In other words, will the treble be more shrill, mids more pronounced etc?
 

Pete Farrington

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Can’t really ignore the actual value as typically, old British pot tracks measure somewhat higher than their nominal value, eg 50% above.
If you can visualise the audio effect of changes to a frequency response plot, then it would be best to input the relevant values into Duncan’s TSC or similar.
If not, higher treble pot values will bring the lower corner frequency of the treble control even furrher down into the midrange. Higher mid pot values will mean that when bass is set to minimum, there will be a bit more bass, ie it reduces the bass range of control.
The presence control may be intended for a different circuit arrangement.

It’s worth noting that pots are ridiculously cheap.
Most of the best pots that sound best actually test high
Might that just be a chicken or egg thing though? eg old British pots typically measure high, old British amps, when working right, generally sound great.
 


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