JCM 900 tube replacement

ironiguana

Active member
Messages
42
I need new tubes for my JCM 900.  I have never replaced tubes before.  Any suggestions on what tubes or installation techniques? 
 

stratplayer1

Senior member
Messages
298
Im assuming you mean power tubes, I got a set of JJ's in an amp with very similar circuitry, a laney AOR 100, just JJ el34's sound pretty good to me, just pop um in and have it biased, or do it yourself, ive also heard great things about groove tubes, so check those out, alot of guys swear by the old more expensive tubes.
 

ironiguana

Active member
Messages
42
Ok. I am diving into this amp repair thing...

Anyone have either one of these books I found on stewmac?

How To Service Your Own Tube Amp
The Tube Amp Book

If so, does it show you how to change out tubes and bias an amp?

Is there a better book out there?
 

jackthehack

Senior member
Messages
5,630
What is your amp doing that makes you think it needs new tubes? easiest route to go if it needs them is simply to buy a matched replacement set from GrooveTubes/JJ/etc.
 

ironiguana

Active member
Messages
42
jackthehack said:
What is your amp doing that makes you think it needs new tubes? easiest route to go if it needs them is simply to buy a matched replacement set from GrooveTubes/JJ/etc.

A few weeks ago it started sounding crackly almost like a speaker was blown.  I have also noticed that it doesn't seem to get as loud as it used to either (could be going deaf but I don't think so yet).  I have never replaced the tubes and it is over 10 years old.  Talking to a few people, they said it is most likely my problem and tubes need to be changed about every 5 years.  Guess I am lucky they lasted this long.  It was going to cost me $40 just for someone to look at it, so I thought I might be able to do this on my own.  Have had pretty good luck modding guitars...

I would go for a matched set but the tech at Guitar center mentioned something about biasing and that I could screw things up if it wasn't correct.

Still on the book search:

Anyone own this one?

Hal Leonard The Complete Guide to Guitar and Amp Maintenance Book
 

jackthehack

Senior member
Messages
5,630
Never pay ANY attention to anything said by a moron at GC. If you replace the tubes with a like matched set there should be no need to rebias the amp. If you don't have any experience, you're better off paying someone to do that, voltages feeding 5881's can kill you quite dead.
 

jimh

Senior member
Messages
1,344
jackthehack said:
Never pay ANY attention to anything said by a moron at GC. If you replace the tubes with a like matched set there should be no need to rebias the amp. If you don't have any experience, you're better off paying someone to do that, voltages feeding 5881's can kill you quite dead.

A big +1 to jack. It's been said here a million times before.
Have a search around for some CB's posts, he's written plenty on amps, tubes and biasing. I wouldn't go poking around inside an amp if you're not familiar with high voltage circuits. These circuits are massively different from guitar vol and tone controls. These bad boys will kill ya. Do some research here before doing anything else.
 

Volitions Advocate

Senior member
Messages
1,239
As far as reccommending a type of tube. I like JJ's.  Go with them. Unless of course you want to avoid having to rebias your amp, then just get the matched set with what you already have like jack said.
 

Patrick from Davis

Senior member
Messages
2,197
I hesitate to recommend Groove Tubes because they are generally other brands repackaged.  JJ Tesla and Sovtek's.  They cost more and are the same thing.  The matched tubes should drop right in and work fine, but the bias should be set.  Because the tubes are matched doesn't mean the bias is OK.  It means the tubes run at the same current.  The bias sets the current level.  If you are replacing the tubes, EL34 with EL34, you should be close enough to 70% of max plate dissipation that it shouldn't cause a prob, but this by no means is any insurance that something bad won't happen because of this.  If you are getting into tube amps, and want to learn more about it, that is great.  If you need this amp for a gig, get someone to service it.  While a great hobby, you do not want to rush into the amp.  It is deadly and there are a lot of things to learn.  Biasing an amp really is not very tough.  Replacing tubes is easy, just pull one set out and put the next set in, they are keyed.  You need the proper tools and you need to know a couple of equations to bias it properly with a multimeter.  But unless you have tested the proper spots on the amp for high voltage, You do not want to touch anything in the amp.
Patrick

 

stratplayer1

Senior member
Messages
298
Also, you dont need to buy a book, you can find everything you'll ever need to know for biasing and replacing tubes for free on the internet, as jimh said, look for some of cb's posts, they've enlightened me a great deal, ill see if i cant find some of them
 

stratplayer1

Senior member
Messages
298
eres a pretty interesting one, doesnt really explain how to bias, but gives insight as to what bias does, it really helped me understand how the tubes work and what things do inside an amp, if you can grasp this, , you should be able to figure out how to bias your amp, btw you need a few pieces of equipment to bias an amp, nothing expensive though.

http://www.unofficialwarmoth.com/index.php?topic=2467.0

edit: read all the pages not just the first, theres some other good stuff in there

Heres some online articles I found useful

http://www.athensmusician.net/archive/2004-04-23_tubes.php - simple basic info

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#howtobias - more in depth

main menu of the last site http://www.geofex.com/tubeampfaq/tafaqndx.htm
 

jackthehack

Senior member
Messages
5,630
Depending on age/origin, the JCM 900 could have shipped with either 5881s or EL84s power tubes and ECC-83 preamp tubes. Are they the original Marshall labeled tubes? 5881s are a 6L5 variant. If you want to replace the set you shouldn't need to rebias the amp. If you want to switch it out to EL34s or some other tube set you will need to. While you CAN do it yourself, the plate voltage levels in that amp are high enough to cause severe burns, nerve damage and death if you screw up; some money is not worth saving....
 

ironiguana

Active member
Messages
42
Thanks for feedback guys.  I think I will leave this one to the experts and stay alive.  My dog would miss me too much :)
 

jimh

Senior member
Messages
1,344
Don't be scared of having a look.  You'll never learn anything that way....... Just keep your hands in your pockets  :eek:ccasion14:
 

GearBoxTy

Senior member
Messages
437
I replaced the tubes in my Mesa/Boogie Mark IIb with mostly Sovteks and love it.  I got them from tubedepot.com.  They have reviews of different brands of similar tubes.  I got some Chinese 12AT7 for the reverb driver.

You can see pics of my tube replacement "process" on Flickr: http://www.flickr.com/photos/gearboxty/sets/72157606715616411/

I had a JCM 900 in the early 1990s and it used EL34s in the power amp section.  Never had to replace them as I didn't have or use that head for long.  I'm sure there are tons of resources on the Internet for what those amps require.

I also own an older copy of Aspen Pittman's The Tube Amp Book. It just has general tube amp information and a shiteload of schematics.  I should dig that book out, it was kinda cool...
 

legend58

New member
Messages
2
I have a JCM 900.  Great amp.  When I first got it I put it on my bench and noticed it had some cross-over distortion.  That occurs when the bias voltage is too high (actually too low because it is a negative voltage).  It's purpose is to keep the tube pretty much turned off until driven otherwise the tube would conduct all the time and there would be no amplification, not to mention it would burn up real quick.  The output tubes are typically class B which means one (or one set) amplifies the postive portion of the signal and the the other(s) amplify the negative portion.  Crossover distortion is caused when the tube shuts off at the point where it swings positive to negative (and vice versa).  You can see it on an o-scope.  The idea is to keep the tube conducting just enough to let the other tube take over the other half of the waveform.  If this doesn't happen the amp sounds knarly.  When it is set properly the amp will sing (if it is a good one).  If the negative voltage amplied for bias is too little the tube will conduct more and sound crappy.  It will also run really hot and die on you quick.  I simply stuck a new set of Groove Tubes in mine and the amp came alive!!  Didn't need to adjust the bias at all.  One thing Grove Tubes recommends is to replace the driver tube as well.  This is the tube right before the output stage.  I didn't bother being pretty broke and all and the amp still kicks butt. 
 
Top