Hi all.
I am new to this forum, infact new to any forum, so please be patent with me lol. my question is this.... I have a Marshall tsl122 amp and on the clean volume pot it rumbled and cut out at no. 4. I replaced pot with same value but it was a "b200k" instead of an "a200k". now the amp has practically no volume and no volume control. also I have lost distortion channels. I can hear the power amp working but the pre amp is showing all these symptoms despite swapping pots back to before. I hope you can help, thanks in advance, simon.

 

Hello,

The b version of the potentiometer is a linear one.
The a version of the potentiometer is an "audio taper" one.

See here the difference in the reaction on the turning angle:

This picture comes from the following page with more info on the poentiometers:
http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

Bertus

 

Hi Simon, your new symptoms after repair seem to indicate there is another fault besides the type of pot.

You might have broken a wire, or irritated a bad connection, or even blown an opamp IC in the pre-amp circuit.

The original fault where you has a bad spot on the pot might be a sign of corrosion, so I would first go right over all pots and switches with electronic spray cleaner and work (operate for a few minutes) the pots, switches and also the jack plugs. Even the empty jack plugs often have an audio switch built in that can get corroded and cause very low sound volume or sound crackling/cutout.