New tubes for JCM 900

itsevolutionbaby

Hey all,
I got this head used about 2 years ago,and I am about to change the tubes on it. Any advice? It has 5881 tubes from the factory, and I would assume replacing it with something similar so that I dont have to have it re biased (right?) Forgive me for my ignorance, I am new to tube amp maintenance. Also, should I replace all tubes (power and preamp)? I don't want to spend a ton, but I would love to be able to improve the tone. Thanks again....

Pacomc79

http://www.tubedepot.com


http://www.thetubestore.com

5881's Eh? I think I might be tempted to rebias to EL34's that's just me If you like it, don't F with them.

If you replace the preamps the reverb will be a 12AY7 and the preamps are 12AX7. You can get Sovtek 12AX7LPS's for maybe 12 bucks a piece and they are very good and reliable. I like the design of the Electro Harmonix 5881's (RCA Blackplate copy) Some people say Tesla JJ's are great in Marshalls some don't. It depends on the people. You probably don't have to change the preamps now, but it probably would sound better.

If you go with EL34's go with some Mullard Derivitave.

itsevolutionbaby

Pacomc79 wrote:

http://www.tubedepot.com


http://www.thetubestore.com

5881's Eh? I think I might be tempted to rebias to EL34's that's just me If you like it, don't F with them.

how difficult, (expensive) is it to rebias?

Pacomc79

itsevolutionbaby wrote:

how difficult, (expensive) is it to rebias?

It depends on the amplifier, I believe the bias can be adjusted on your amp so it's not too terribly expensive I have to actually change out capacitors on mine to change the bias point, perhaps someone more experienced with paying bench fees can tell you more. You might just ask a local amp tech, you can probably find one at http://www.tedweber.com just go to the find an amp tech link. Probably in the 60 dollar range, although you might be able to adjust it yourself it's not terribly difficult. I think JofZ and I had a discussion on it one day. You just need a few tools.

http://www.tubedepot.com/whisbipo.html

itsevolutionbaby

BTW- Is there a reason that you would suggest changing the tubes over from 5881? What exactly should I expect to change in the tone?

All Those Yesterdays

Two things, everyones opinion on rebiasing amps differ.

I lean the way of getting it rebias'd each time. I had lots of tube problems when I first switched over and didn't go that route. I've rebias'd everytime sense and everything finally is clear. Each tube is slightly different. My other opinion with that is its a used amp that you've had for a while. Who knows what the previous owner did with it. My local guy that I used to take my stuff to will go thru the amp and clean out the pots and tighten the sockets etc... when I get a tube job done.

The 2nd thing is, What in the heck was Marshal thinking when going with 5881's, that makes no sense at all! Don't get me wrong I do like those amps, the dual reverb is nice (but I'm not a huge marshall fan).

itsevolutionbaby

What's the difference in the 5881's over the EL34's sound wise? What is the big problem/disadvantage of the 5881's?

Pacomc79

itsevolutionbaby wrote:

What's the difference in the 5881's over the EL34's sound wise? What is the big problem/disadvantage of the 5881's?

There's no advantage/disadvantage. It's a tone character thing. 5881's produce a different character than EL 34's. More headroom, glassy slower to break up more of an American character if you will. EL34's are more of what I would quantify as a "british character" They are wooly and warm with fairly early breakup and less headroom, crunchy and also "chimey" (but not as much chime as it's little brother the EL84)

neither is wrong, if you like the sound with 5881's if you want a slightly warmer woolly tone go with EL34's. Bogner actually builds a 6L6 and a EL34 amp for the same reason and Mesa Boogie created the "Progresive Linkage" concept to get both sounds potentially in the same amp. It's very cool. Check out http://www.mesaboogie.com Don't look at the price, just scope the engineering.

Before you retube, see if it is possible to go play an EL34 equipped DSL into a similar cabinet to see the differences.

enharmonic

Generally speaking, you should rebias every time you change your power tubes. A bias adjustment isn't required for preamp tube swapping.

The GT34;s aren't bad, but I prefer Ruby or SED winged C's.

itsevolutionbaby

thanks for all of the advice guys. Should I look to replace both the preamp and power tubes?

Pacomc79

itsevolutionbaby wrote:

thanks for all of the advice guys. Should I look to replace both the preamp and power tubes?

It will sound best if you do, but the Power Tubes are the most pertinant.