The Last Bass Outpost

Change the driver tube from a 12AX7 to a 12AU7 if you find you need more headroom. It will lower the overall gain of the amp a bit, but that trade will give you more useable volume and headroom. I'd also go with 6550's or KT88's, whichever you can find the cheapest. Mesa output sections are designed for lots of distortion and "dirty" power and tend to be a bit muddy but their preamps are very bright and high gain. Any of the big beam tetrodes, 6550's/KT88's, are going to have lower plate impedance which will shift the tonal balance even further towards lows, but in the case of the Mesa amps, it increases low-end extension and rolls off the highs just barely inside the range of human hearing. The 400 was designed around GE 6550A's, which is what all modern 6550's copy. It's a 200 watt amp, but it's good sounding 200 watt amp. I'm using a 400+ right now with a 12AU7 driver and while it will never have the punch of a beefier tube amp, it is a nice flavor tonally that cannot be derived any other way.

Basically, Randall Smith took what he had learned about making guitar amps and translated it directly to bass amps. There's an interview with him in the new Bass Gear magazine about his design philosophy that confirms everything I have been speculating about Mesa's tube bass amp design philosophy for years. They're not my favorite, but they are distinctive.

Thanks for weighing in - I was hoping you would... I'm leaning towards a set of JJ KT88s from eurotubes, and having my amp guy bias them properly - although eurotubes state that their tube sets are a direct match for my amp, and no re-biasing is necessary... Any thoughts on that???

 I was already planning on trying the 12AU7 driver tube swap... Also, agreed that my 400 is quite bright - rolling off some highs will not be a bad thing for me... While the 400 isn't quite as powerful as an SVT-II(my other tube amp choice), it's much lighter - and I tend to prefer the Mesa sound over the Ampeg sound - so far, I'm fairly happy with my purchase...


    - georgestrings

That's a fine looking beast there.

Thanks - I've wanted one for awhile...



     - georgestrings

I got this in yesterday:






    - georgestrings

I'll just chime in here until PBG posts..

Just curious, what kind of upgrade are you after, soundwise?

I have not used a mesa 400 since 1993, and I did not experiment with tube swapping during the time i played it.

Usually, when people upgrade to 6550 or KT88's, it's usually because they want more clean headroom.
If they just want a somewhat different sound, they might experiment with different manufacturer of the same tube.

In many cases, they end up spending a lot of money (KT's are expensive - at least here in Europe) for not quite the
most impressing results. More often than not, a 'stock' amp sounds best with the tubes it was designed for, if it is serviced/set up/biased correctly. I see many cases where owners have worn out 2-3 different sets of output tubes, with the original driver tube still in. So the ouput stage may not work properly with any output tubes.

That said, there is no reason not to experiment, but I'd check that the power supply and output trannies can handle the tube, and of course check that there is enough negative supply available, and that you will not exceed the total current draw the unit was designed for.

Perhaps not the answers you want, but I'm sure someone here have tried this and can post something on how it sounds

I'm definitely looking for more clean headroom, which a change from 6l6s to 6550 or kt88s will provide, according to all accounts I've read thus far... Also, AFAIK the 400 WAS originally designed to run 6550s - so I doubt that'll be a problem... I also know of several people that are running kt88s in their 400s with excellent results - but I believe they had bias pots installed...

At any rate, I appreciate your contribution..


    - georgestrings

I have a Mesa 400 on it's way to me, and am considering tube options - it already has 6l6s in it, but I'm pretty sure I want to put kt88s in it, instead... The preamp tubes will probably be left alone, unless there's an issue with them...

Any thoughts???


   - georgestrings

Would definitely like one of those - congrats, OP...


    - georgestrings

Welcome, and nice Gibsons...


   - georgestrings

Here's a pic of the MIM Squier I mentioned before:



I added the tort PG and bridge cover - it already had the QPs on it when I got it...


     - georgestrings