Potentiometers for Marshall

[Liam]

Got Marshall BB reissue with the dreaded "over-sensitive volume pot" disease. In addition, channel 2 has got a dead spot between "off" and rehearsal volume, which means it's very hard to be at a volume I really want.

First of all - has anyone come across this problem being anything other than the pots they use? I'm even considering reducing the gain of the first valve stages, but it would seem a shame to mess up any of that lovely assymetric crunch when it's turned up.

Secondly (might be one for Steve), where in the UK can one buy NICE split shaft 1M log pots? Or should I just settle for plastic shaft Citecs and try to squeeze the Marshall control knobs on there?

Any advice at all gratefully received as ever

TIA

Liam

[dai h.]

could you maybe use a resistor to alter the taper to get a more gradual effect? Here's an article by R.G. covering the use of tapering Rs:

http://www.geofex.com/Article_Folder...s/potscret.htm

I have no idea if this applies to your situation but I remember reading something about some "audio tapers" being two linear tapers (I gather basically due to what I presume is the lower expense) making the volume transition abrupt so if that is the case then maybe using an R to alter a bit may help. I was also re-reading some article last night where the author found a lot of cheap single and dual pots not accurately reflecting their taper or tracking sides well, etc. and generally not very tight to spec so possibly a symptom of trying to cut costs perhaps.

[Steve Conner]

Rapid Electronics have 1M log split shaft pots, part no. 65-0845

http://www.rapidonline.com/productin...moduleno=65222

They are the 16mm mini type, if you need full size, try Hotrox or Allparts.

[Liam]

Thanks Dai & Steve, that answers all my questions.

Steve, I hadn't thought of Hotrox, their parts selection is pretty good nowadays! Just got to pop the chassis out and see what size I need.

Dai, I'd completely forgotten about RG's "tapering resistor" article, if replacement pots don't work it will definitely do the trick. I've never really worked out what the fact that the load impedance changes as the pot is turned will do, but at low volume the answer will be "not much" I'm sure.

Anyone care to comment?

[loudthud]

The tapering resistor doesn't do much at the low end of the pot. I was working on a spreadsheet today to calculate the attenuation. A 1meg linear pot at 1/10th rotation gives you -20dB attenuation. With a 220K tapering resistor you get ~-23dB. Using a dual ganged 1Meg linear pot (pots connected in series) gives -40.7dB. A 20dB Audio pot gives ~-34dB.