#1
Last night I put a new preamp tube (12ax7/JJ EEC83) into my VOX VT30 Valvetronics amp. There wasn't any problem with the old tube as far as I can tell but after a few years of constant use, I just thought it was a good idea to change it. It seems to me that the amp got noticeably louder. I am not much of an electronics guy so my question is: can changing that tube (which seemed to be OK) actually increase the output to the amp section or does a tube in the preamp actually lose output as it gets older, or am I just imagining that it's louder. Hope that makes sense. Thanks for any comments.
Last edited by Rickholly74 at Sep 19, 2014,
#2
Yes, new tubes are generally louder than old ones. But also, was your old tube a 12AX7 as well? Because if it was something like a 12AT7, then an AX7 would definitely be louder and maybe harmful to your amp if it wasn't designed for it. You're probably fine.

The other thing it could be is that the new tube might have more midrange than the old one, which can give you the perception of it being louder.
#3
Thanks. The original tube was a 12ax7. The tube I replaced it with was a JJ EEC83 from Tube Depot. I understand they are interchangeable. I hope so.
#4
Yeah, they are. You said it seems to be noticeably louder, have you tried to put the old one back to confirm? Usually all 12ax7 have a gain factor of about 100 I think...
#5
^Not if they're old and worn out they don't.

Getting a bit more output is common if your preamp tubes are tired. The Valvetronix uses the tube a bit differently than most all-tube amps would use a 12AX7. It's integrated more with the power section than the preamp voicing, so it makes sense that you might see some volume increase if the tube that was in there was worn out.
#6
Thanks again. I thought about putting the old tube back in but VOX didn't make it easy. You have to diassemble the whole amp to get to the tube. I have another backup VOX VT30 that I will A/B it to side by side but that amp has gotten a lot less use than the one I changed the tube in so the comparrison may not be fair. Roc mentioned the unusual way that VOX uses the 12AX7 tube in this amp and it's something I never understood from their description.

" The VOX Valve Reactor circuit is the key factor that separates the Valvetronix amplifiers from other modeling amps. The 100% analog original power amp circuit actually has the same power amp circuit as a full-tube amp, using a 12AX7 miniature triode to reproduce a distinctive tube amp sound unattainable by digital technology. In other words, it replicates aspects that have an important influence on the sound of a tube amp, such as the power amp's class A/AB operation and the interaction of the power stage with the speaker. You get not only the response of the original amp's power amp circuitry, but also the power amp's distortion as well as the nuances and subjective sound pressure of a full-tube amp."

What???
#7
It's marketing gibberish, it's not supposed to make sense. It's just supposed to sound technical.

The actual topology goes like this:
Digital modeling preamp feeds into the tube, which then goes into the solid state power section. It would be like putting a tube preamp in the effects loop of a solid state amp. It's not an integral part of the preamp or the power amp, but the idea is to "warm up" the signal with a tube at the last possible minute before it gets amplified to its full volume and sent to the speakers.

Vox doesn't like that explanation because it sounds like the tube is just there to do a tiny bit of tone shaping but mostly as a marketing ploy. So instead of just telling you what's happening, they give you that bit of nonsense and bury the actual function of the tube deep in the manual.
#8
Great explanation. Thanks. That VOX explanation made less sense than Dr. Emmett Brown explaining his flux capacitor.