Mesa Power Tubes

[Blind Lemon]

Any body out there know what the specs are on Mesa 6L6's. They claim all these tubes are plug and play in their fixed bias amps. They must sort through a lot of tubes. Got a bass player friend that has a Mesa 400+ that he wants to re-tube with JJs.

TIA

BL

[funkmeblue]

contact eurotubes.com, they know what boogs need

[selloutrr]

you have to convert the mesa to adjustable fixed bias.

[Blind Lemon]

"you have to convert the mesa to adjustable fixed bias."

Never messed with one of these monsters. To be honest I haven't looked at the schematic. Not sure what would be involved.


BL

[212Mavguy]

A visit to the mesa factory website will help explain their philosophy and process of tube selection. Randall Smith has written a few papers that are found online. They sort through a very large quantity of modern production tubes, including their 6l6's on an ongoing basis.

My two channel Mesa amp has been converted in a unique manner to adjustable cathode bias by my tech. Stock and unmodified it runs a quad of el84's biased very hot. Although they call it a class A amp, with the kind of plate voltages it runs that is as much a fallacy as the Vox AC30's class A rating.

It's a Maverick in more ways than the model name. Every single Mesa labeled tube has been replaced by vintage old stock, and the original British made Celestion V30 speakers have been replaced with vintage JBL MI-12's. It has never sounded better. My camera is broken right now so I can't post the pics of the mod nor do I have the equipment to post clips... sorry.

[David Root]

212Mavguy, I don't know about EL84s but you can't beat those NOS Sylvania (Labeled MESA) STR-387 and STR-415 in the 6L6 amps

[selloutrr]

http://cgi.ebay.com/Bias-Mod-Mesa-Boogi ... _0#vi-desc

as an easy solution

http://forum.grailtone.com/viewtopic.php?f=25&t=47733

http://forum.grailtone.com/viewtopic.ph ... r+location

[212Mavguy]

Thanks for the links, very cool. I checked out that kit on ebay and my amp model was not listed as one of the ones it will work with. I'm guessing that the kit works on a different method of bias adjustment than the mod I had done on my amp.

@David Root...

I agree that the Syl's are badass tubes for the 6l6gc family. The same tube can be had in the same brand as a 6BG6ga for a quarter of the cost, get or build a pair of adapters and you have a secret weapon.

I just individually auditioned a pair TAD 6l6wgc's and I was shocked at how good they sounded in my SE Frank-en-champ. Am stocking up on a few pairs of 6l6gc and scored a pair of Siemens el34's for an incoming D-clone HRM amp. Have a tightly matched pair of VOS GE 6l6gc grayplates that sound nice.

[selloutrr]

6BG6ga - If you use this tube make sure it sits upright. otherwise you risk the cap falling off. It can also cause problems if you don't take extra precautions for vibration dampening. The cap is pressure fit and can vibrate on certain frequencies. - other then that great tube at a great price.

400+ was listed scroll down the ebay page.

The unit on ebay is not needed it's nothing more then a Potentiometer with a resistor in series.

My thinking is you should be able to open your amp and locate the bias resistor and swap it for the adjustable pot. look over the mesa boogie forum posts and you should be able to draw a solid conclusion as to which resistor you swap out.

To make it easy you can snip the resistor and use the posts to solder on to or solder the pot over the resistor so you never loose bias in the event of a pot failure. then depending on how you turn the potentiometer you cancel out or add resistance. similar to this attached drawing.

[selloutrr]

From what i've been able to conclude.

Mesa boogie 400+ bass amp uses twelve 6L6 output tubes.

Mesa boogie uses a 22K resistor or close to it (22.1K) as the bias resistor for the 6L6.

look for a resistor color coded (2)Red (2)Red (1K)Orange (10%)Silver or (5%)Gold

[212Mavguy]

@ selloutrr...

Your post is impressive and helpful! Wow!

My tech was worried about heat buildup in the bias adjustment parts, the four power tubes are cathode biased, in parallel, and that means a fair amount of voltage and current to heat things up. So that is why he didn't use a conventional bias pot. Instead, he used a vintage Ohmite rheostat in series with one resistor and also wired in parallel with another resistor, (I can't remember the values) yielding an adjustment within the Rheostat's knob travel having a narrow, workable range. The hottest setting is slightly less than the stock Mesa resistor would do, and minimum has enough plate current to work and still sound great. He mounted it to the back of the chassis, and put a spare knob on, for all the world it looks like a tone knob sticking straight out of the middle of the back back of the chassis, the chassis metal also acts as a bit of a heat sink, it turns like a tone knob, acts a bit like a tone knob with a global effect on the amp's tone as he result of different settings and it works very, very well. It almost looks like the amp came with it stock!

For instance, in a quieter room I can turn the knob up hotter and the tone is warmed up at lower master volume settings, and at higher volumes I can turn it down for a bit more upper mid and top end bite, as well as having an effect on the harmonic mix directly resulting from where that knob is turned to. Hotter gives up more "octave" harmonics, colder gives up more of the "octave plus a fifth" ones, I don't need to run a bias probe, it's pretty foolproof. I've run the amp for hours straight with no problems.

Again, thanks for your helpful post. I'm learning something new every day.

[selloutrr]

@ selloutrr...


For instance, in a quieter room I can turn the knob up hotter and the tone is warmed up at lower master volume settings, and at higher volumes I can turn it down for a bit more upper mid and top end bite, as well as having an effect on the harmonic mix directly resulting from where that knob is turned to. Hotter gives up more "octave" harmonics, colder gives up more of the "octave plus a fifth" ones, I don't need to run a bias probe, it's pretty foolproof. I've run the amp for hours straight with no problems.

Again, thanks for your helpful post. I'm learning something new every day.

ummmm..... Bias is meant to be adjusted for the optimal tube life (and with in reason tonal voicing) and left along. shifting the bias point can quickly destroy your tubes and possibly the OT.

Due to the heat and voltage concerns i'm not sure your tech actually addressed the correct resistor in your amp.
- my reason being.
The resistor that would have been tapped into or replaced was at most a 1watt resistor probably a 1/2 watt. a 2W potentiometer is double that (4 times a 1/2watt resistor) so it would have no trouble handling the voltage.

heat and voltage should not be an issue as long as you use a quality pot. The main reason for using a quality pot is minimize the chance of failure and going open bias.
I prefer a locking pot so the setting does not drift after the fact.
I'm not a fan of drilling any holes that are not stock as it lowers the value of the amp by a large amount.
it would have been very easy to add it to the PCB because it's a set and forget.
the only need to have the rear access is if you are swapping tubes on a regular basis. i.e. rectifier head switching from EL34 to 6L6.

[212Mavguy]

OK, now we are having some fun!

A final note on the original post thang...Mesa grades their 6l6's with color coding for ranges of current draw. They will void your warranty if their tubes are not used in their amps, unless the amp owner buys tubes from someone approved by Mesa, like Groove tubes, GT uses a numerical scale to basically do the same thing.

Now to reply to selloutrr,

Shifting bias quickly capable of destroying either tubes or OT? What is bias modulated tremolo as found in certain Fender amps?

The Mesa Maverick is cathode biased, all power tube cathodes are in parallell and their voltages are all controlled by just one resistor, no way is it a one watter, I've seen it, I was there at the bench for the work done, it was big, black, and massive, five or ten watts is more like it. It gets hot. So that is why a rheostat was used. That is also why the scheme of one resistor in parallell besides the one in series, for in case of component failure as well as mounting on the chassis back for a slight heat sink effect.

As far as the need for a locknut, no, because there is a good deal of friction within the rheostat, the back of the chassis is more rigid than even the thick PCB's that Mesa uses. It stays put no matter how loud the amp is set, and the Maverick is surprisingly loud if need be. The convenience factor for the selected mounting method is huge compared to the PCB method of mounting, and the PCB method offers no heat sink. I can just reach behind the amp while playing and turn the knob. Certain lead channel gain settings and bias adjustments allow a harmonic bloom for long, sustaining notes a la Dumble. As the amp gets hot while playing I can compensate for plate current drifting if need be.

Because the amp is versatile and sounds exceptional, I have no plans for selling it, and if I did, 2/12 Mavericks are not all that valuable on the used market. So the drilled hole in the chassis reducing value is a moot point, especially if the reason for it improves the funtionality of the amp. Dirty, torn tolex is more of a value reducer IMHO. I got mine a few years ago for 625 bucks plus shipping on fleabay in pristine shape, it still looks that way. I have 400 - 600 bucks worth of rare, vintage tubes in it, especially in the preamp, it's not going anywhere. As far as swapping el84's, can do, I have accumulated matched-in-circuit quads of RFT, JAN Sylvania, GE, JJ, EH, and vintage Mullard besides the 6p14p-ev's in it.

[selloutrr]

found your maverick bias post on teh boogie forum.

http://forum.grailtone.com/search.php?k ... mit=Search

I guess what confused me was... why did you post about a 400+ on the top of the post? I wouldn't have bothered researching hours of pointless boogie bias info.

You can still damage your tubes if the range of your Rheostat exceeds the tubes limits. (if not for you let it be a warning for others)

Side note- Tolex can be cleaned and worst case replaced. holes are forever. I'm 100% good with a player making the amp work for them I've just seen to many regreted mod's pass my bench for restoration and had to pass on buying 100's of cheap vintage amps cause they couldn't be put back to NOS.

[Cliff Schecht]

I used to be a bias tweaker as well. If you know what you are doing it's a pretty effective way to tune your amp to the playing situation. I used to tweak my first Express by at least a few V at band practice to get the amp in its sweet spot. Not a big deal if you know not to rotate that knob all of the way..