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mesa 400+ preamp tubes...

punkrockmike

punkrockmike

Mar 16, 2011
239
3
Minneapolis, MN
www.facebook.com
#1
Hey everybody. picked up a 400+ at a local music go round a few weeks ago. sounds amazing! i was thinking about retubing it and keeping the tubes that came in it for back up since they still have some life in em. I opened her up and the power section is filled with JJ 6L6's. the preamp section was loaded with jj 12ax7s. i decided to play around with the preamp tubes and throw in some sovtek 12ax7's. love the way it sounds now. the change was only slight, but you can really hear it when i turn on the od/fuzz. anyways, i was on the eurotubes site, looking at their retube packages, and saw that they come with ecc83s's for the preamp. that got me thinking, will i damage the amp by just having regular ol 12ax7's in v3 and v4? im a fan of the higher gain of the sovteks, (was even thinking about trying some tung sols) and would like to keep em in there if it wont hurt anything. would it improve the sound/function to throw the JJ's in v3/4? ive read that mesa preamps can be fickle. guess im just asking for feedback from people with experience with tube rolling in their 400+'s. thanks in advance! oh and another thing, if i do retube the power section, do i need to bias the amp? i know these were meant to be fixed bias amps, and im not sure if a mod had to be done to allow bias adjustment to put the JJ's in there.
 
#4
The way mesa says it, and I asked them this on the phone a few months ago... If you retube the power section with something other than what Mesa recommends, then it would have to be rebiased, and that requires a mod. So they suggest just use their recommended tube. if you buy it from them, they have already checked that the tube is within spec. If you buy it from someone else, they should confirm it before they ship it to you. A few of the tube suppliers will do that for you.

As for your 12ax7 questions, I have to refer that to someone that knows better than me... I'm still kinda learnin that bit :p
 
#5
There is a mod that can be done to make it an adjustable Fixed Bias amp. No problem with any tube in the preamp that is in the same family 12AX7 ***, ECC83**, EC803**, 12AU7**, 12AT7. Balanced sections are more of a gimmick IMHO. Can't hurt but most likely won't help either as those stages are self bias.
Most larger suppliers can send a set of outputs that match a amps bias needs if you specify when ordering.
 
#7
You have to remove the amp chassis and look underneath for a small added pc board. Most use a 10 turn trimmer pot but a 270 degree pot can be used also. Either way a tech needs to make the bias adjustment anyway. If the mod is not done the bias can still be adjusted, just means changing resistors after the new value is found.
 
#8
punkrockmike said:
how would i be able to tell if that mod has been done to the amp other than bringing it to a tech? im broke right now so i wont be able to do that for another couple weeks.
Click to expand...


Eurotubes sells sets of powertubes they select for use in a 400+. you could use this tube set whether your amp had an adjustable bias mod or not.

I had an adjustable bias mod done to my 400+, my tech added the little board with the pair of blue trim-pots located near the center of this pic:

But If some other amp tech did the mod to your 400+, the adjustment pot(s) could be located somewhere else and use components that look different. So chances are, you'd never be able to tell if your 400+ has been modded or not, but a qualified amp tech that could.

153_5398.jpg
 

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