#1
hokay. so. i just bought a used jcm 900 and it still had the stock tubes in it- so i bought some brand new el34s because from what i've heard, they will do wonders. ive read you have to bias your amp whenever you change tubes, and ive read about DIY biasing and it was all way over my head. i live in kinda a small town with no music stores (no decent music stores, i should say) so i dont really have anywhere to take it in and have an experienced person do it. so how important is it to bias? is my amp gonna explode? cuz i dont want it to explode. but the jcm 900 and el34s are both common, and used commonly together enough that would i have problems if i didnt bias? and if i need to.. is it as difficult as it seems to do it myself? is it worth driving my amp into the city to have a professional guy do it? or should i attempt the inevitable.
#3
No it won't explode as such but it could damage your amp and/or the tubes. If the bias is set too high it will easily burn through your new tubes in an hour. What tubes were the stock ones? If it had the same type i.e EL34s, you MIGHT get away with not biasing but there's no guarantee it will be ok just because it's the same type. I would get a tech to do it or do it yourself. It's really easy if you have a tube biasing tool but there's other methods (1ohm resistor method for example). For some info you can look here
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#4
The stock ones where likely 5881's (though they could have been EL34's). If they are EL34's you can just change'em like lighbulbs. However, if they are 5881's then you need to do a re-bias. About six months ago I picked up a JCM900 with stock 5881's and performed the rebias for EL34's myself. However, it can KILL YOU if you do not know what you are doing. Touch the wrong capacitator or wire - and good bye...

Bob from Eurotubes talked me through my job with it - I must have sent the guy about 10 emails about it, and must say his bias prode proved invaluable as I would not have performed the rebias without it.

Do you know anyone you can send the amp to? Do you have any electrician friends? It is not hard - just dangerous if you don't know what you are doing.
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#5
I have a jcm 900 with 5881's and I am also interested.

^ how would you compare the sound?
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#6
IMO they sound better with EL34's. If you don't have ANY tools or ANY experience and are desperate AND don't really care about optimal tube usage then you could adjust the "bias pot" simply by the tone desired. The range on the bias pot to stay within' will be anywhere from 65-95% (100% being turned completely clockwise. I don't really recommend this though as it's only for short term jams/emergencies. If you have JJ E34L's you could probably turn it up to 100% without any problems, but don't take my word for it

#7
Sound better sounds like it could be good, but what are the tonal differences? Are the EL34's grittier/dirtier or something else?
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#8
I just got a JCM 900 with 5881's too...is it better to replace them with the EL34s? What is the difference? What's better? Is it a matter of preference or what? Thanks.
#9
Quote by Soneric84
I just got a JCM 900 with 5881's too...is it better to replace them with the EL34s? What is the difference? What's better? Is it a matter of preference or what? Thanks.


i would like too know this too