JCM800 Bass Series problem! Please help!

[Teddy]

Hi all, sorry to make my first post a tech question but I've only just got a Marshall so am new here!

My newly acquired JCM800 bass series - arrived yesterday - is playing up already!!

When the amp arrived I gave it a quick try and everything seemed fine - sounding good, very Lemmy from Motorhead sound! However, just went back to it today to have another play - something weird is going on with it. The only way I can describe it is the amp is producing a 'farty' kind of sound - a muffled, 'thhurp'-like on the low notes that wasn't there before. My first instinct was a speaker cone had ripped, as it had that kind of sound, but I've just tried it with another bass cab and still the same sound.

Anyone have any idea what this might be? Obviously I'm a bit gutted, I'm hoping it's not a serious problem with the head, the seller said the valves were changed fairly recently... any ideas?

 

[Adwex]

First, I would try replacing one preamp tube at a time and see if the problem goes away. That's the cheapest and easiest first attempt.

 

[Teddy]

I've bought a new set of tubes for the amp and I'm taking it in to a tech - I'm taknig no chances with it, I'll get him to rebias it and make sure the old caps etc are still functioning properly. Will let you know how I get on, thanks for your reply!

 

[Adwex]

I've bought a new set of tubes for the amp and I'm taking it in to a tech - I'm taknig no chances with it, I'll get him to rebias it and make sure the old caps etc are still functioning properly. Will let you know how I get on, thanks for your reply!

You can change the preamp tubes easily yourself, no need for a re-bias. You might get lucky, if that's the problem, you can save the cost of the tech. I would try that first.