power tube replacement?

W

warlordbob

New member
hey what do i need to know about byasing(i donno how you spell it) power tubes? I really need to change the tubes in my Marshall JCM2000 and i wanna learn how to do it myself. Oh, what kinda tubes would be good... they're EL84's...... I 've decided on Electro harmonix 12AX7's. Yeah, i don't wanna screw anything up. would it be better to get someone to do it for me... i hear it's real simple.
thanks
Lee
 
Re: power tube replacement?

I thought that amp had EL34s, not EL 84s.

If you're not good with electronics, get a tech to do the biasing. You can literally kill yourself if you don't know how to do it right or if you accidentally slip up. You can do a search on the web. There is plenty of info if you still want to go ahead.
 
Re: power tube replacement?

Take it to a pro until you can take a class, or have someone qualify you to do it. As mentioned, the voltage inside an amp can kill you if you touch the wrong thing. Spend the $25-35 bucks a tech will charge you and live to play annother day.

As for tubes....the JCM2000 combo's have EL84's. I think JJ is a good choice for those. Go to www.eurotubes.com and look around.
 
Re: power tube replacement?

what i would really like is if someone would take the time to show me..... and yeah i think your right, they are 34's.... i was just too lazy to look, and i haven't investigated it in a while.
i do ok with guitar electronix and homemade pedals and stuff.... but this is the first time im retubing an amp.
 
Re: power tube replacement?

warlordbob said:
hey what do i need to know about byasing(i donno how you spell it) power tubes? I really need to change the tubes in my Marshall JCM2000 and i wanna learn how to do it myself. Oh, what kinda tubes would be good... they're EL84's...... I 've decided on Electro harmonix 12AX7's. Yeah, i don't wanna screw anything up. would it be better to get someone to do it for me... i hear it's real simple.
thanks
Lee
Click to expand...
12ax7 are pre-amp tubes just so you know. If you want to change those you can do it easily on your own. Just pull out and plug in a different one.
 
Re: power tube replacement?

yeah i knew that about the preamp tubes. It's the power tubes i'm worried about. I have a friend i haven't talked to in a while that builds tube amps and stuff. Looks like he's gonna help me with this one.
 
Re: power tube replacement?

I think that there's external bias points/minipots on the back panel of the DSL.
That means you don't even need to remove the chassis from the headcab.
All you need is a volt meter to attach to the points on the back.

What you need to find out is what voltage readings you're supposed to see on your volt meter, because I'm not sure what the specs are. Just pop in a set of JJ or Svetlana EL-34's, and see what it reads. Find out from Marshall or Korg what the values should be set at. Maybe Bob at eurotubes knows?
 
Re: power tube replacement?

The physical process of just replacing the power tubes yourself is perfectly safe. Just pop them in and out like the preamp tubes.

BUT...the process of rebiasing (which you have to do any time you replace the power tubes, and periodically anyway, whether you're replacing tubes or not) is EXTREMELY dangerous if you don't know what you're doing, because it has to be done while the amp is running.

One of many safety tips: never have both hands in the amp because if you do get a shock, the current will go in one hand and then out the other -- meaning the current will pass through your heart, which is very likely to be fatal. :bigeyes:

Electricians call this the "One Hand In Your Pocket At All Times" rule.
 
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