Instructions for adding a bias system for a Mesa 400+

[PMR]

Hi all you great Bass players out there,
I need some instructions on adding a double bias system in a Mesa 400+ Bass amp. I have years of tube electronic knowledge, but a little help is never a bad thing. I have found a picture of one already done, but it's not a real how to or diy. So if any of the electronic wizards out there can help me out I'd really appreciate it very much.
Thanks in advance from a new member, PMR

 

[Foz]

+1 - subscribed

 

[Jim C]

Perhaps the factory could offer up some advice; worth a call

 

[Dave M]

Perhaps the factory could offer up some advice; worth a call

Here's your Mesa factory advice: "Buy/Use our tubes and you don't have to worry about adjusting the bias!" "Read Randall Smith's white paper about bias!"



The whole thing flies against Mesa's marketing, so don't expect any help from them with installing a bias pot.

 

[BassmanPaul]

With all the respect in the world, if you have to ask HOW you should not try to DO. Adding a variable bias to a fixed bias amp is a simple matter but if you do it wrong your amp could be toast shortly thereafter. Get a tech to do this for you. By all means watch and learn if the tech will allow but keep your hands off.

Paul

 

[nsh50a]

Anyone have more info they could give on this matter. I.E. schematics for the adjustment board or where to solder the leads to on the mesa? I'm pretty handy with a soldering iron and multimeter but will admit, electronically i'm a solid state guy with not too much background on tube amps. And yes, I know to discharge the caps before i go poking around. And I know i should let a tech do it but there are no techs that work on tubes amps within 100 miles of me and I figure looking at the few photos of the mod, it'd be simple enough. I know there used to be a company (mansfield guitars) that sold a ready made board but it looks like there website is down.

 

[timv]

To add dual bias adjustments you have to remove a couple resistors from the PCB and install them on the bias board and then jumper back into the circuit.

It isn't very complicated. Just look @ schematics of other dual adjustment amps and then the 400+ and figure it out. If you can't do it from that, honestly, bring it to a tech.

Also, I barely recommend doing dual adjustments anymore because there are so many companies offering matched sets of 12 tubes that it really isn't necessary and technically you are balancing the sides on the DC part with dual pots but not on the signal AC, how much difference that makes, not really sure but the SVT is a perfect example of matched DC and AC with the balance pot....

I recommend to most people who want me to add dual pots just to do the single and buy matched sets of 12 from wherever.

There isn't tons of room around the bias circuit to add the dual pots and make it look really nice either. I am pretty sure the picture you found is from my work and yes it is stable electrically and physically but I wish I could do it a bit cleaner. without making specific bias boards in a small footprint, I couldn't see a cleaner way.

Oh, way to bump a really old thread.

 

[anderbass]

nsh50a,
Just to avoid any confusion, heres a pic of the adjustable bias mod timv installed in my 400+.


pic is from this thread:--> http://www.talkbass.com/forum/f15/i-have-joined-dark-side-well-not-really-387202/

 

[beans-on-toast]

Link Removed is a bias board that you can buy. They provide a Link Removed as well as a details on the Link Removed. I see that they are out of stock but the schematic is useful if you want to build your own.

 

[nsh50a]

So I understand that you are merely replacing the 2 resistors marked 9.7 and 10 under the blue and brown wires coming from the tube sockets (seen in the un-modded photo attached). The purpose for doing this in my case are 2 fold. I have a couple of matched sets (one mesa, one JJ's) but would like to be able to readjust the sides to match if one is drifting more than the other with age (or does that even happen?) and 2; to be able to adjust the bias either hotter or colder to taste. I imagine #1 would be able to occur with just adding a single bias adjustment but would need a dual in order to adjust the overall bias hotter or colder. Without a dual adjustment being able to change the signal AC, is that pretty much a moot point?

 

[timv]

No.... You do not just replace those resistors.
Please put the cover back on your amp and bring it to a tech if this is truly what you want done.
Doing this to make two different brands of tubes work in your amp is exactly a reason not to do it.
Please spend the money and get a set of 12 matched tubes, you're amp will be much happier.
If you already have some matched mesa tubes, you probably could just buy more of them with the same rating and not have to install a bias adjustment @ all.
Mixing tube types will make the balance even more off because they will respond differently than each other.

So I understand that you are merely replacing the 2 resistors marked 9.7 and 10 under the blue and brown wires coming from the tube sockets (seen in the un-modded photo attached). The purpose for doing this in my case are 2 fold. I have a couple of matched sets (one mesa, one JJ's) but would like to be able to readjust the sides to match if one is drifting more than the other with age (or does that even happen?) and 2; to be able to adjust the bias either hotter or colder to taste. I imagine #1 would be able to occur with just adding a single bias adjustment but would need a dual in order to adjust the overall bias hotter or colder. Without a dual adjustment being able to change the signal AC, is that pretty much a moot point?

 

[nsh50a]

I think you mis-understood the reason in the last post. I am not planning on mixing the the jj's and the mesa's (each are a full set of 12 good, matched tubes). I actually have another set of mesa's however they are not the same color (mesa's red/yellow/green system) and that's why i bought my first replacement set a few years ago. The JJ's are 1 year old and purchased from eurotubes matched for the 400+ to see if they were better than the mesa branded. My question is how much is the bias thrown off with tube aging. I.E. once the JJs are in it for a couple of years are they still going to be a close enough match or will some of them drifted more than others and benefit from having an adjustable bias. I also did not mean literally "replace" the resistor with another resistor. I realize that it is replacing a set of resistors from the board with jumpers to the bias board circuit.

 

[timv]

OK, that makes more sense and sorry if I came across rude....

If you have two different sets of 12 matched tubes, I would honestly just do the single bias adjustment.

The tubes may drift a bit as they age but most likely will drift the same and dual pots wouldn't really be a benefit because it is not very likely that you would have exactly 6 drift one way and 6 drift another.

Also, if your amp sounds good to you now and you have tubes matched for the boogie bias range, probably a good idea to not even mess with it, with two sets of full tubes, you will be set for years..