Poti value specification in old circuit diagram

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#1

Hello, I have the circuit for an old guitar amplifier. Potis are working erronous. But it seems someone has already manipulated this amplifier…

The diagram includes the Potis with index and value.

The value is like: 100kLOG which certaily means a 100 kOhm Poti with logarithmic behaviour.

But what is meant with: 100kA and 100kB ? It is obviously no instance marking because it comes after the value and double Potis are shown oldstyle with a dashed line inbetween.

Does anybody know whats this means ? Maybe someone from another country ? (Im in Germany)

#2

A is log curve, B is linear curve. The dashed line means the pots are ganged, e.g. stereo volume control.

#3

maybe, though I speculated A is lin and B is LOG ?
But why does the cicuit use the LOG notation additionally ?

#4

Apparently some manufacturers used the reverse convention, maybe that’s why they added LOG to be clear. Anyway the function in the amplifier will tell you, volume pots are log, and others, like trimmers, are linear.

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#5

These are probably old, national or proprietary markings.
For standard code letters, see e.g. the second table on $2037358.
Usually, volume is log, frequency may be any of log, lin, rev log, and rest is often lin (S is usually wah guitar pedal).

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#6

thank you, good link, I should have searched more thoroughly…
the use of LOG AND A/B may certainily be just coincidence.

#7

There are many different kinds of log (and approximations), so they had different additional letters to differentiate them. Also, there are different kinds of lin (to allow for larger manufacturing tolerances, or as an approximation for sin, cos, S-curve, etc., like used for balance circuits). Usually, tolerances are ±20%, so it doesn’t matter that much nowadays (but it does for wire-wound pots, sensing pots used for measurements, some guitar effects or high-end audio gear).

#8

Ah the good old days, when my goal of making amplifiers was to annoy parents with music not to their taste. :cat: Now I can afford to buy amps off the shelf but no oldies to annoy. :crying_cat_face: No fun. :neutral_face:

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#9

OK, to the original author LOG my be something different. I remember the term “pos log” or “+log” from old times.

#10

But you can annoy the youngies with that “old peoples cr*p”

#11

Actually my taste in music has become quite broad. I’ve almost 2 decades of world music recorded off radio. During the pandemic I got around to indexing them:

As the joke goes there are only two genres of music I can’t stand: Country and Western. :wink:

#12

I’d have to add “doof doof” genre to “yipee” and I still data index with a felt tip pen on those 12cm. discs.
I really am quite decrepit and out of technological touch with the world :slightly_frowning_face:

#14

If the taper is wrong then the pots will still work correctly but the “feel” will be off . I suggest you check for other issues first especially since someone has been in there and modified things.

#15

yes, I fear I will have to start from the input and do a complete overhaul.
Thank you all for the fast reaction and all the hints

#16

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