Author Topic: Changing amp pots (Read 8617 times)

psy · « **on:** April 02, 2008, 08:43:34 AM »

I'm going to change the pre-amp & master volumes pots in a valve amp (nasty scratchy things that they are). Is this generally a passive area of an amp? Or do I need to discharge anything to avoid electrocuting myself?

ta

Fikealox · « **Reply #1 on:** April 02, 2008, 09:08:49 AM »

I would discharge everything and use a multimeter to double check. Getting fried isn't fun :(

jpfamps · « **Reply #2 on:** April 02, 2008, 12:32:08 PM »

Which amp is it?

psy · « **Reply #3 on:** April 02, 2008, 01:54:20 PM »

Quote from: jpfamps

Which amp is it?

Marshall JCM 800 (2203)

jpfamps · « **Reply #4 on:** April 02, 2008, 03:39:04 PM »

I assume you have an original JCM800 and not a re-issue. I've never worked on a re-issue.

Leave the amp in standby mode and this should discharge the main filter caps. I would double check with a multimeter when you open the amp up.

You can get pots from:

http://www.tube-town.net/ttstore/index.php/cat/c53_Marshall-Potentiometer.html

Marshall won't sell you any replacement parts that require the inside of the chassi to be exposed when you fit them.

Kilby · « **Reply #5 on:** April 02, 2008, 03:44:33 PM »

Quote from: jpfamps

You can get pots from:

http://www.tube-town.net/ttstore/index.php/cat/c53_Marshall-Potentiometer.html

I wish I had known about that site when I was looking for pots for my Fender Blues Jr

HTH AMPS · « **Reply #6 on:** April 02, 2008, 08:46:18 PM »

I'll be surprised if the pots are totally screwed and need replacing on a JCM800. Get some Servisol pot cleaner from Maplins and rotate the pots a couple dozen times - if that doesn't shift it, nothing will.

Just make sure the amp is off and unplugged from the wall. There's nothing around the pots that will give you a belt in a 2203.

psy · « **Reply #7 on:** April 03, 2008, 09:53:15 AM »

jpfamps - It is an original one

HTH - I've tried Maplins switch cleaner on it a few times. It does reduce the scratchy sound a little... But it quickly becomes a problem again.

I don't mind changing the pots like. They're only about £3 each :)

jpfamps · « **Reply #8 on:** April 03, 2008, 12:29:03 PM »

Many of the JCM800 era amps I see need their pots changing (which is why we always have them in stock!)

Twinfan · « **Reply #9 on:** April 03, 2008, 01:18:20 PM »

Cheapo Marshall strikes again ;)

jpfamps · « **Reply #10 on:** April 03, 2008, 02:39:15 PM »

Well I don't know if its Marshall being "cheapo" or there just not being any better components available. Much as it pains me to say so, the failure rate of components on British amps in my experience is significantly higher than in Fender amps of the same era. It is very rare to see a Fender amp from even the 50s where the pots need changing, whereas most of Marshall amps I see from the 60s and 70s have had pots changed. My guess is that British-made components just weren't as good as US made ones.

Mind you this could be a reflection on Marshall users...

Twinfan · « **Reply #11 on:** April 03, 2008, 02:55:24 PM »

Could be! I know Marshall like to control costs, so I'm sure they use the cheapest pots they can get away with.

psy · « **Reply #12 on:** April 03, 2008, 04:17:56 PM »

Does 2 new 1meg pots for £10 inc postage sound about right to you guys?

Kind of related to the 'cheapo' remark... I only paid £150 the head off a mate about 8 years ago. So I don't mind too much having the pots. :)

jpfamps · « **Reply #13 on:** April 03, 2008, 04:38:18 PM »

I think all manufacturers try to control costs as much as possible. Fender were notorious for this. For example, the "clay" dots found on early 60's Fenders are actually linoleum, which rumour has it was left over from a factory renovation! There are plenty of other examples of Fender's economizing.

On vintage amps I simply think that the quality of parts (and not just pots) available in the US was higher than in the UK, and probably reflects the amount the respective governments spent on military equipment who where the major drivers behind improvements in component technology.

Twinfan · « **Reply #14 on:** April 03, 2008, 04:52:08 PM »

Quote from: psy

Does 2 new 1meg pots for £10 inc postage sound about right to you guys?

That sounds expensive. I usually pay 2 or 3 of quid for 500k guitar pots, but I don't know how much amp pots are. Are they the ones with long plastic shafts you cut to length?