Mark IV Bias Mods

[metalmaster]

Does anyone have experince selecting/installing/using an aftermarket mod to overide the Mesa fixed bias setup? Is it even worth messing around with? I recently looked through the Eurotubes site after reading much praise and personal recommendation. Aside from deciding to try the JJ's I also noticed that Bob advises fixed bias amp (incl. Mesa) owners to implement a bias mod to allow the user to run the power section at optimal settings tone and efficiency wise. Has anyone tried this with successful results? Are the Eurotube kits matched to the Mesa factory bias settings for optimal performance or do I need a bias mod to take full advantage of my amp's power section? Please help me.

 

[Monsta-Tone]

I haven't tried it yet, but have been thinking about doing this to my Heartbreaker.

Right now, I'm running JJ 6V6's in it that I got from Bob, but I've had bad luck with it using other brands of Power Tubes.


When I get some time, I'll draw something up and post it, but it might be a few weeks.

 

[cesjr]

No disrespect towards Bob at Eurotubes, but Mesa amps are fixed bias for a reason. As long as you get power tubes within a certain range your amp will sound great. You don't have to worry about re-biasing after every tube change or your amp's bias wandering out of position either.

Having said all that I know people who have done the mod with great success. Is it worth it? I'm sure that's all in someone's opinion, but I feel if it isn't broke don't fix it.

 

[Boogiebabies]

The HB runs -53 Volts to all 4 tubes, so it would be swapping a resitor for a potentiometer. The Simul-Class amps run the inner sockets at -53 and the outer at -39 so you would need two pots to bias the MK IV. Not only would a tech love to do it, but he would love to charge you $ 30.00 every time you want to change tubes. Do I make money doing bias jobs, yes.
Do I make them on Mesa's, no. Knowing the natural current draw of a manufacturers tubes can get you close enough with the stock Mesa bias circuit. I make money on the tubes, but all I have to do is pop them in.
Out of all the Mesa's I own I have yet to convert one to an adjustable bias.
After tinkering with these things for years, and owning many different models I really think Mesa's dont sound well when they are biased hot. They loose that Mesa sound. My range for Mesa's is -22ma to -35ma. Anything over -35 and they get muddy. Be careful of tube vendors though.
They really want to sell tubes. A set of Mesa Grey to White or GT 5-6 has always gotten me to around -25-35ma. Good enough for me. If you want to get real close, a set of #32 EL-34's and # 32-34 SED 6L6's from Triode Electronics will get you to -35ma on all 4 tubes. I have been running two sets of each in 2 of my IIC+'s for over 2 years and none of them have drifted a single ma. Ned is a matching genius or something.


Good luck.

 

[metalmaster]

Thanks to everyone for all the info. I think I'll leave my Mark IV the way it is for now. Can't wait to hear what the JJ's sound like after hearing so many good things.