Mesa 20/20 Tube Replacement and BIAS mod.

[HarryBoy7]

Hi All, this is my first post with the board so be gentle!

I have a Mesa Recording Preamp --> Mesa 20/20 (deep mod) --> Marshall Vintage Quad. It's a great setup and I get comments about my tone all the time. However, I think the 20/20 is a little sterile sounding when you try to push it into tube breakup. I have read many posts online of users buying a matched quad set of JJ's and 12AX7's from Eurotube and then getting their amp tech to install a pot to adjust the bias accordingly. I have discussed this with my amp tech and he said it sounds like the thing to do, and that the 20/20 runs cold in comparison to other valve amps. I realize that the power tubes may have to be replaced sooner - maybe every two years but hey, they are as cheap as chips.

Has anyone got the schematic and or details on the 20/20 bias mod so I can pass this onto my tech and get the mod done ?

Any help would be greatly appreciated !!

PS: Eurotubes are asking on the order if I want a) Early Breakup b) Max Headroom c) Middle of the road ?

 

[crane]

I think Mesa EL84 amp are the only ones that DON'T run cold !
I'll check mine and post some measures in a few minutes.

EDIT.
Ok, they don't run cold at all !!!
Anode voltage is 382V and cathode current for ONE tube is 48mA.... so anode's dissipation is approximately 18W (12W is the limiting value) !!!

So what you don't like is maybe the brown sound produced by these really hot hot tubes. A bias mod could help by bringing the bias to a more normal value, but I think you'll loose the 20/20 "sound".
There is a 20/20 schematic available on the Triaxis group from Yahoo.

 

[elvis]

I believe this is discontinued, so Mesa will email you a schematic if you call and ask them to.

The overall impression I get, based on what I have read and my own experience, is that JJs are not good tubes.

 

[HarryBoy7]

Thanks for the fast reply.

Crane - to perform the bias mod on the 20/20 - is it the same procedure as the the 50/50 ?
From the 20/20 schematic, there appears to be dual bias controls - so that would mean a bias adustment pot for both channels ?

 

[crane]

Yes, you'll have to do 2 bias mod, one for each channel.
I'm not really sure how to do it, you'll have to ask you tech.