• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

400+ ????

#21
rbonner said:
I wonder what would happen if we put some sort of switching circuit in the SCREEN supply to drop the screen voltage on the final tubes. This would also lower the gain, but of the final not the preamp... I dont believe it would cause distortion either. Could peal 150 Watts right off the top.
Click to expand...

Considering the Bass 400+ is a roughly 250 watt amp, the issue is not one of too much power, but too much gain. Subbing the 12AU7 takes the circuit which is running at super high gain, and lowers the drive stage gain and gives it a larger current capacity to drive the power section. The net result is more headroom and useful volume without any significant tonal changes. IMO, what changes there are in tone are improvements.

In regards to screen dumping, that is an extremely BAD idea. Unless you like oscillations and severe biasing issues, screen voltage should NOT be messed with.
 
#23
Thanks for all the replies.
Let me start buy saying, I never thought I would get this many answers from my question. Im will be trying the Master and Gain controls first and if that doesnt work for me then i'll do the 12au7 mod this weekend. It sounds like the safest way to go. Ive been 10 years waiting for the deal I got on this head and do not want to, for lack of a better word, start hacking on a head that is in new condition. PsychoBassGuy seems to be the one everybody refers back to and Im glad he's willing to share his knowledge along with everyone else on this site.:hyper:
By the way, Is there a way to date this head short of calling Mesa? You know, without dinner and a movie!:D
 
#24
TONYTHOMAS1956 said:
Thanks for all the replies.
Let me start buy saying, I never thought I would get this many answers from my question. Im will be trying the Master and Gain controls first and if that doesnt work for me then i'll do the 12au7 mod this weekend. It sounds like the safest way to go. Ive been 10 years waiting for the deal I got on this head and do not want to, for lack of a better word, start hacking on a head that is in new condition. PsychoBassGuy seems to be the one everybody refers back to and Im glad he's willing to share his knowledge along with everyone else on this site.:hyper:
By the way, Is there a way to date this head short of calling Mesa? You know, without dinner and a movie!:D
Click to expand...

Mine had a date sticker on one of the transformers...
 
#25
Big +1 on using a 2X10 cab - still gets you the tone that you like about the the 400+ but at much lower volume. Another big "hell yeah" for playing with the master volume.

Used to have a 400+ and sold it when I stopped gigging regularly - got a chance to play through my old rig just last week and man was it sweet!!!

Enjoy the tube-y goodness!

Edward
 
#26
TONYTHOMAS1956 said:
Ive been 10 years waiting for the deal I got on this head and do not want to, for lack of a better word, start hacking on a head that is in new condition.

Is there a way to date this head short of calling Mesa?
Click to expand...

If you were referring to my 400+ amp mod as "hacking", a much better term to use would be a Cathode lift mod... ;)
I realise you didn't mean any offence, but the term hacking is kinda strong being as I preformed this fairly simple procedure
to be fully reversible and could be completely removed in an hours time leaving no evidence it was ever instaled.

TONYTHOMAS1956 said:
Is there a way to date this head short of calling Mesa?
Click to expand...

Your amp tech will probably find handwritten dates of assembly on serveral large blue resistors easily viewed once the amps
bottom panel is removed.
 
#27
anderbass said:
If you were referring to my 400+ amp mod as "hacking", a much better term to use would be a Cathode lift mod... ;)
I realise you didn't mean any offence, but the term hacking is kinda strong being as I preformed this fairly simple procedure
to be fully reversible and could be completely removed in an hours time leaving no evidence it was ever instaled.



I did not mean any offence, Like I said "For lack of a better word".
I dont want to ruffle the feathers of anyone on this site, To much valuable information.
My apologies.
Click to expand...
 
#29
PBG, what are you using for the source of information rearding reducing screen voltage and oscillation? I'm talking pealing 50-100 volts not removing the screen voltage. This reduces the gain of the stage. I have seen oscillation problem with going the other way, too much stage gain in my experience. Just curious. BOB
 
#30
It's not that Im against any mods but I tend to be extra carefull with things like this.
I had palyed this very amp a few years back when I subbed for the bassist that replaced me in the band I was with. I "TRIED TO RETIRE" but as everybody knows, that does not happen.:bassist: He had a BITE that Im trying to reproduce and Im not having any luck yet. It was to long ago for me to try to remember his set up. But i do remember it had a profound ballsy tone. Im thinking he must have had a pod or somekind of modeling unit. Any ideas?
 
#34
A modelling device would only serve to degrade the awesome sound quality the 400+ has on its own. I used to use a Sansamp on occasion to make my 400+ sound less "tubey" and more "scooped" when I needed a more modern (less pleasant) sound.

I did almost always use a DBX 163X or 163A compressor with it. Just made it sound a little bit better.
 
#35
Well, Last night i changed the channel, master setup and it made a lot of difference with the only thing im not happy with was the upper freqs being low. Still playing with the comp/limiter(Berhringer autocom) thinking that may have something to do with it. The overall tone was there tho. Everybody said this head is picky on the controls. I also made a cab change as well. I have a bassman 70 cab loaded with Jbl e-130's that Ive used for years. Im hoping i havent made too many changes at once.
 
#36
what Chef said. Since you've got 2 preamps to pick from, you can choose tour goodness. Ch 1 is cleaner, Ch 2 gets dirtier. You can play as clean as you want, or get some drive going at almost any volume. I've used a 400+ for a couple of years and I think you'll be happy with the tubey goodness. If you really want that hot tube sound, you'll have to get a SVT and make everybody's eardrums bleed.:hyper:
Another thing to try is to lose the comp. You should be able to get as much high end as you want. The amp is pretty picky though. What sounds good with my jazz sounds average with my P.
 
#37
rbonner said:
PBG, what are you using for the source of information rearding reducing screen voltage and oscillation? I'm talking pealing 50-100 volts not removing the screen voltage. This reduces the gain of the stage. I have seen oscillation problem with going the other way, too much stage gain in my experience. Just curious. BOB
Click to expand...

When you mess with screen voltage, it affects the overall transfer curve of the tube, especially without a corresponding drop in plate voltage. Rather than linear reproduction, the curve will get odd peaks at various points in its frequency resoonse. If these are severe enough, they will cause the tube to feed back on itself and oscillate. What you're proposing is to put a class A/B output stage much closer to class B than it should be, basically a redlined engine driven by low gear, not a lower RPM rate.
 

Latest posts

Top Bottom
Back