Marshall JCM-2000 DSL - do I need to replace my tubes?

nukem2k5 · #1 10-25-2004, 11:00 PM

I'm not getting any sound through my amp anymore. Sometimes if it's left warming up for a long time I can take it off standby and it'll work, but I'm assuming that my valves are starting to die.

I've taken off the back panel and my power amp valves look fine. They all look about the same when they glow, but my pre-amp valves look pretty weak. I don't have any reference as to how much they should be glowing, so I don't quite know what to do.

Anybody have any experience with replacing the valves in a JCM-2000 and care to share some input?

I didn't do anything to my amp to make this start happening so I'm almost positive it's valve-related.

Thanks.

nukem2k5 · #2 10-25-2004, 11:04 PM

Also, I can hear some whining/buzzing/whatever coming through my speakers. It sounds somewhat like a mosquito buzzing around my ears. I am assuming this is that "microphonic" (?) sound that happens when the valves start to die.

I checked my fuses and they both looked fine. How can I tell which valves need to be replaced - pre-amp or power-amp?

Gresh · #3 10-26-2004, 12:36 PM

If it is valve related, it is most likely power amp tubes. Preamp tubes basically last forever...at least they last a very long time. People who replace them are looking for a tonal change primarily. Power amp tubes are the ones taking all the current.

To do this properly, you'll need a multi-meter so that you can get the bias right. You'll also need to know what the -mV bias settings are for your amp as recommended by Marshall. If yours is a 100watter, then I think it is -90mV per side, but please check that with Marshall....

On the back of the amp there will be a little white dial and a three prong connector. The dial is the bias adjust, the prongs are where you connect the multi-meter. The middle prong is neutral, the two outer ones are hots. Black lead goes to the middle, red lead goes to which ever side you are adjusting at the time. In order to adjust properly, the amp must be powered on and in "ready to play" mode...ie, not in standby, and you must have a cabinet connected to provide proper load.

If you don't understand any of this, you'd better take it in somewhere and have a tech do it. It's really not hard at all...but you need to understand what you are doing or you risk screwing it up and messing up the tone of your amp by running it cold, or shortening the life of your new (and not cheap) tubes but setting it too hot.

nukem2k5 · #4 10-26-2004, 06:20 PM

So you think it's most likely the power amp valves?

Also, when I'm adjusting the bias with the multi meter, if -90mV is correct, I just adjust both sides to that? Could you please explain it in a little more detail? Thanks a bunch.

Gresh · #5 10-26-2004, 06:49 PM

If it is a tube issue, it is probably a power tube that's to blame...at least that's where I'd start. Power tubes are the cause more often than not, preamp tubes are pretty durable and they don't wear out as easily/frequently.

Yes, the -mV levels are for each side of the power section. I may have the marshall settings at home in my PC somewhere...I scanned them in at one time. If I find the spec sheet I'll let you know and send it to you.

The dial on the back of marshall, the bias trim pot, is very touchy and it's difficult to get it exactly at the desired setting. Close is good enough in this game. You just don't want to be way off. Also, some people seem to like amps with a hot bias...I could never tell much difference, so I figure the factory levels exist for a reason.

Gresh · #6 10-26-2004, 06:52 PM

Sorry,

You also will want to replace one tube at a time to try and identify which one is the culprit if you can. If you find that same location continues to go bad repeatedly, then you have another problem that a tech will have to deal with...but at least you'll know which socket to focus on.

Not that you asked...but I'd go with Svetlana or JJ/Tesla EL-34 tubes as replacements...and get a matched quad. Those and a decent multi-meter will probably run less than one trip to the tech.

nukem2k5 · #7 10-26-2004, 07:14 PM

Are the ones you suggested better than the Marshall brand valves?

Elysian · #8 10-26-2004, 07:17 PM

Quote:

Originally Posted by nukem2k5

Are the ones you suggested better than the Marshall brand valves?

i don't think marshall makes their own tubes... i think most amp manufacturers just buy em from other companies... could be wrong, but i sure know crate does(stupid sovteks)

nukem2k5 · #9 10-26-2004, 07:46 PM

I haven't looked very closely at my valves but I did see Marshall written on them, so that lead me to believe Marshall had specific valves that they resold to go with their amps.

Gresh · #10 10-26-2004, 10:56 PM

Are you sure you're not confusing them with Mesa Boogie...they brand their own valves, which are actually groove tubes, which are actually something else....long story involving many mark-ups. I've never seen a Marshall valve in my life....but I'm not all that old...yet.

Yes I would recommend those valves over anything...unless you want to pony up for some new old stock mullards....which you don't, trust me on that one.

Elysian · #11 10-26-2004, 11:33 PM

Quote:

Originally Posted by Gresh

Are you sure you're not confusing them with Mesa Boogie...they brand their own valves, which are actually groove tubes, which are actually something else....long story involving many mark-ups. I've never seen a Marshall valve in my life....but I'm not all that old...yet.

Yes I would recommend those valves over anything...unless you want to pony up for some new old stock mullards....which you don't, trust me on that one.

aren't groove tubes relabeled sovteks? thats what the other guitarist in my band seems to think

Gresh · #12 10-27-2004, 10:20 AM

Could be, I know they don't have their own factory so someone is branding them for them.

Above when I said" I recommend those tubes over anything..." I was referring to the JJs and Svetlana power tubes...not Groove Tubes or Mesa or whatever the hell they are.

I've heard that Sovtek makes great preamp tubes, but for power amp tubes go with the JJs or Svets.

Elysian · #13 10-27-2004, 02:33 PM

Quote:

Originally Posted by Gresh

Could be, I know they don't have their own factory so someone is branding them for them.

Above when I said" I recommend those tubes over anything..." I was referring to the JJs and Svetlana power tubes...not Groove Tubes or Mesa or whatever the hell they are.

I've heard that Sovtek makes great preamp tubes, but for power amp tubes go with the JJs or Svets.

i got sovtek 6l6gt's right now... they r good until u get into the gain and the volume... i looked up the JJ's, and a matched quad 6l6 set it 55.90, not too shabby

nukem2k5 · #14 10-28-2004, 12:31 AM

Just so I know, about how much should I expect good valves to glow? Mine are somewhat dim, as in I can pretty much see the insides while they're on...should they be glowing a lot? Perhaps compare it to something, please? Thanks again.