i have a tweed usa pro jr. i think its time for some new tubes...what do you all reccommend?
(2-el84, 2-12ax7) brand wise? im not by any means an amp tech, so i dont know any of this stuff....do i need to do any biasing?

If you are changing power tubes, it is in your best interest to rebias, for the best sound.

You can swap preamp tubes in and out as you please without rebiasing the amp, that's why they are your tone shapers, for the most part.

im just a player. i dont know dick about what tubes are good, and what arent. what would you recommend in terms of brand? ive been told that as long as you use a matched set of power tubes, you dont need to re-bias. also, if i did change the power tubes without re-biasing, that wouldnt ruin the amp right? it just wouldnt sound its best?

Here's a question, why are you retubing, first of all?

It's not going to ruin the amp if you don't rebias. BUT, we all want the best sound, right? Rebiasing your amp will make it sound much better (maybe not better, but it will keep it from sounding worse), and will be much healthier.

Well, by matched set, they probably mean matched with one another, now, if those aren't exactly the same as the one's that you have in your amp now, then you would still need to rebias. It's just the norm for when you change power tubes, kind of a rule of thumb thing.

Are you looking for new production tubes or NOS tubes.

well, the tubes that are in it now are kinda noisy...microphonic...and from what i hear, NOS tubes produce a more disireable sound? i hear all this talk of mullards, ect...i also hear alot of good things about JJ tubes...by the way, i appreciate all your help..since i dont know much about this....thanks again...

Isn't the Pro Jr. fixed bias?

For EL84's, I'd recommend JJ/Teslas. Current production, full clear sound. I use them in my Peavey C30 and they sound great.

For 12AX7's, you may want to play around with NOS (ie, out of production brands) tubes, as they are definitely better than current production.

The first preamp tube sets the basic tone, so that's what you want to pay the most attention to. A Mullard would be great in this amp, as it's overdrive sound is one of the best. RCA's are very chimey and clear if you don't plan on turning up the volume to overdrive the amp, although it sounds nice overdriven as well.

The second preamp tube is the phase inverter. If you're looking for a bit more headroom out of the amp, I'd suggest a 12AT7 instead of a 12AX7. Less gain, but would really clean up the sound. Since this amp uses both sides of the triode to invert the wave, you're better off getting a balanced tube.

Some links to good tube vendors:

www.dougstubes.com

www.kcanostubes.com

Both are very helpful in answering any questions you have about tubes. Good luck!

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It's not going to ruin the amp if you don't rebias. BUT, we all want the best sound, right?

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I highly disagree with this statement. "It may not ruin the amp" would be truer. That amp is fixed biased not cathode biased, and tubes can vary greatly. I've changed tubes in amps before where I had the old tubes biased at 35-38ma and the new tubes were reading over 80ma. That could destoy the tube fast and possibly hurt the amp in the process. That being said Fenders are usually biased kind of cold at the factory (which is another good reason to rebias as it will probably sound better set a little hotter) so you may be ok, but it's a risk.

Matched tubes will not help with this either. They are matched to each other, not the old tubes you are replacing. Some companies rate their tubes for bias also (I think Groove Tubes and Mesa Boogie) they claim you can replace tubes without biasing as long as you use the same rating. Seems to me I heard you may be able to get tubes from Fender that are supposed to biase correctly in their amps with no adjustments, but you'd have to check on that. Of course this assumes nothing has drifted in your amp. I always rebias.

Unfortuntly that amp doesn't have a bias pot, you have to change a resistor to get the proper bias. I'd have a tech replace the resistor with an adjustable pot for easy rebias.

I think you should at least check the bias whether you are getting tubes that are matched to your old ones or not. It is just a good routine to get into. It is even good to check the bias after the tubes have been used for a little while. I know this is not practical for some but just my 2cents. Kinda like keeping your car tuned up.

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Originally posted by Blues Lyne:

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It's not going to ruin the amp if you don't rebias. BUT, we all want the best sound, right?

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I highly disagree with this statement. "It may not ruin the amp"

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Yes, good call on that one. I should have said it that way Bluea Lyne.

i just knew this topic would get way over my head! well, i dont know...ill probably deal with it for now, and take it to an amp tech when i get some extra $$...haha "extra $$"...thats funny...

You do not need to rebias the amp. You could use "un-matched" power tubes if you want, and you're not going to kill the amp (certainly not a PJ). Now, having said that, it is a good idea to use matched power tubes and getting the amp re-biased won't hurt a it a smidge (your wallet on the other hand...) It's unlikely you'll hear any quantitative differences from the power tubes unless you drive the amp to breakup, and then you might be able to hear a difference. You'll get a far more significant tonal shift with the preamp tubes, and in a PJ, that really means the first tube mainly. The phase inverter is not as sensitive to tube brands, though there will be a difference between tube types (i.e. 12AX7 vs 12AT7.)

A '59 bassman, on the other hand is very tube sensitive, especially with the phase inverter. There, a high quality 12AT7 will significantly tighten up the bass and give sparkle to the sound. And also rectifiers will all have an impact. I haven't found power tube to have any appreciable impact on the clean sound, though.

In all, YMMV, and watch out for tube snake oil.

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Originally posted by Blues Lyne:
Unfortunately that amp doesn't have a bias pot, you have to change a resistor to get the proper bias. I'd have a tech replace the resistor with an adjustable pot for easy rebias.

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That seems to be the general consensus with the PJ. Bob from Eurotubes recommended same. When my tech diagnosed & re-tubed my Pro Jr. with JJ's, he replaced the bias resistor, but a pot would be cooler.

Generally, amps biased toward the cooler side sound boring. Those biased hotter sound great, but can munch tubes before their time.