Back-story: Ok, so a few months back I got rear-ended by a rather large pickup truck on my way to band practice. My Marshall AVT50 was in the trunk and it didn't quite survive. I ended up picking up a Fender Champ 25SE used for a decent price. It's my first tube amp (kinda).

So what I want to know - The Marshall AVT50 has a ECC83 preamp tube that's apparently 'specially voiced'. The Fender Champ (although it SAYS it has a solidstate preamp on every website) has a 12AX7 tube in the back.

I understand these are pretty much the same tube but I was curious if the 'specially voiced' thing was a gimmick or if maybe it would actually make my amp sound different. Would it be ok to switch these out? And does voltage have anything to do with the tubes? The Marshall is a 50w and the Fender is a 25w

Also, I'm not 100% sure what pooped out on the AVT50. It still works out of the headphone jack (making me think the preamp section still works) but doesn't work any other way so if I were to take a guess, I'd say the speaker was damaged upon impact. I popped it open and the connections are still good but I was thinking it could be the connection from the speaker to the back of the amp.

So seeing as the Marshall has a Celestion speaker, and the Fender has a Fender speaker, I was wondering if I could throw the Celestion in the Fender? The thing I'm afraid of is that my Fender has an 8 Ohm speaker in it and the Celestion is a 4 Ohm speaker.

I did a little research and most people say it would be ok except my amp might run a little hot. I'm playing at decently loud volumes usually and I don't want to do any real damage to my amp.

If it helps: Here are specs for the Marshall, and Here are specs for the Fender.

Hope you guys can help me out :-) Thanks!
the 12ax7 in your champ could be a phase inverter tube. i.e. in the power amp to split the phase on the way to the power tubes.

i highly doubt the 12ax7 in the avt is "specially voiced". More likely "specially voiced" to be as cheap as possible (or to sound like ass, in which case, mission accomplished ).

I'd strongly suspect you can just swap them around and no harm will be done. Don't quote me on that, though (don't want to be sued just in case for some arcane reason it doesn't work... I don't see why it wouldn't work, though, you don't normally use special 12ax7s for the preamp versus phase inverter).

regarding the speaker, some mismatches are safer than others (but i can never remember which way round). i probably wouldn't bother swapping the speaker if they're different impedances.
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Specially voiced probably refers to the actual circuit design, not the tube itself.

ECC83 is the British designation for a 12AX7. They are the same tube but there are various different makes so the tone and actual specs can vary.

With tube amps you can get away with using a lower-impedance speaker. Solid-state amps are just the opposite. Either way, the amp won't run at full efficiency so the output level won't be as loud. You should really use a speaker with the same impedance as the amp was designed for.
Last edited by Invader Jim at Apr 28, 2014,
Thanks for the info guys!

I guess I'll trade out the tube and see what happens and just save up my money for a Celestion if I decide I want one down the road for it.
Also just because it's a Celestion doesn't necessarily mean it's a good model, you shouldn't swap the speakers. I would just leave your Fender alone.

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