Mesa 400 bias mod pots

The center of this pic shows the adjustable bias my tech instaled in my 400+. I'll send him an email and see if I can get some info or hopefully get him to post in your thread.

 

Hi Anderbass,

Would it be at all possible to get the bass 400+ bias mods from you ?
I just got a new set off 5881's from watford valves ( Ia - 60mA, 4.6mA/V), but the 6L6GC's that are currently in the amp are Boogie STR427's ( 38 AC YEL code on tube), I've tested all of 'em on my mates AVO CT160, and found that the actual Anode current (Ia) for this set of tubes are between 38 - 40mA, and Transconductance (gm) is around 4.2-4.4 mA/V.
I'm not going to fit the Watford tubes, until I can get adjustable bias. otherwise they will be glowing red hot within seconds......

Much appreciated if you could help out bro

Kindest Regards,
Steve
[email protected]

 

I dont know any details about the parts or procedure my tech (timv) used on mine but he's a pretty helpful guy and would probably pass along some good advice your amp tech could use. He's a TalkBass member, you'll find his email link here on his profile page: http://www.talkbass.com/forum/member.php?u=5609

 

Hi Anderbass,

I've tried contacting timv, but on his profile, there's no email addy !
I'm also looking for the schematic for the amp - i've tried reading the one in the other thread, but my eyes are not that good !!
have you a copy, that I could get hold of ?

kindest regards,
Steve

 

The 5881 is NOT a 6L6GC. It is a 6L6WGB and as such has a lower operating voltage and less anode dissipation. Guitar players like them because the overdrive earlier, due to running out of dissipation sooner. With 5881s fitted you will get less output and shorter tube life.

Paul

 

On timv's profile page: http://www.talkbass.com/forum/member.php?u=5609 you'll find a link titled "Email: Send a message via email to timv". Just left-click this email link, type your message and hit send.

They do seem kinda small... I dont have any 400+ schematics but in case your not already, just left-click on any of these Attached Thumbnail 400+ schematics (http://www.talkbass.com/forum/showthread.php?t=387202&page=10) to show a larger size.

(edit)
The older 400 (non +) schematic is supposed to be very close besides the power section. Here a link to that: http://www.tubefreak.com/schema.htm#bass400

 

My tech put in Cermet (?) bias pots, apparently that is a common mod, -mine was also pulling about 38mA out of a new 12 of the 5881s.
The 5881s that kid is talking about is not the 'real' 5881 (old, tungsol type, smaller than a 6L6, brown base...) but a Sovtek 5881 (6L6WXT), which is a military grade 6L6GC equivalent, VERY tough tube, excellent as a 6L6GC alternative. Very good sounding tubes.
I doubt anybody would want to pay for a dozen matched real 5881s, if you could find a matched set, it would probably be over $800 for a set of those?