mesa mk IV bias

[mellowamoya]

Hi,
has anyone done a bias mod on the mesa mk IV, if so was it worth it, i have one that i would like to mod to get away from the high price of the mesa "selected" tubes and allow proper biasing of some quality tubes.
thanks
dennis

[The Dude]

Nothing wrong with doing a bias mod if you like, but there's nothing special about Mesa tubes. Any matched set of tubes should work just fine. These amps are normally biased relatively cold so that biasing is "not necessary". In fact, I wouldn't put Mesa tubes in anything, including Mesa.

[stokes]

I've done the bias mod to almost every MK I've had in my shop,if I get one for caps,I do a bias mod for free.The simul-class amps are almost not worth it.The inner and outer sets are so wide apart that you have to settle on a compromise when biasing between the two sets.My whole reason for the mod is to get the tubes hotter and closer to the sweet spot.With the simul class you cant get there.I like the mod in my own CII+,I use nos tubes and right now I've got 7581A's in it and have used 7027's and KT66 over the years.Never liked the Mesa scheme with the graded system.I've found that sets in the same grade can vary by as much as 20ma's,makes a big difference in how the amp responds.Totally goes against their claim that you just use the same grade and all is good.Its all a marketing scam,just to sell their own tubes.They wont honor the warranty if you dont use their tubes or,at one time,GT's.I dont thinkthey approve GT since Fender bought them.Like Dude says,they are set very cold at the factory,so when they "grade" them they can have a wide range of what is safe,means when they test them they have very few rejects.Getting back to modding the IV,if you do it,be mindful that one pair,I believe the outer pair,will be much colder than the inner pair,so dont set for the colder set to be at max,or the other pair will be too hot.

[Leo_Gnardo]

The simul-class amps are almost not worth it.The inner and outer sets are so wide apart that you have to settle on a compromise when biasing between the two sets.My whole reason for the mod is to get the tubes hotter and closer to the sweet spot.With the simul class you cant get there.

I've found the "outer" pair in Simul-class amps have a resistor to ground from their control grid - which pulls down the bias voltage. It's usually 820K or so. You can remove that resistor to get all the output tubes running similar bias currents. At one time Mesa had EL34 in that "outer pair" position - lower bias current was called for - but they started installing 6L6 and didn't change the biasing scheme -

Nothing special about Mesa's color code, and I've found similar spreads in bias current even for tubes marked with the same "color". Time was 20-25 years ago Mesa was more on-the-ball about their matched tube sets. Heck I bought from them and they were a good source. Now not so much.

[stokes]

Theres where the problem lies,Leo.Many people want to use the EL34's in the outer sockets and still be able to bias.In that case it becomes a balancing act and neither set of tubes can be "ideal".Cant have it both ways.What you also have to take into account is the way the OT is set up in these simul amps.The tranny itself is just a tranny that was originally designed with four taps on the primary,inteneded to connect your plates to the inner taps and screens to the outer,so they supply somewhat lower voltage to the screens.Mesa just used this tranny design to feed two different output tubes.IMO the only thing innovative in Mesa amps was their switching matrix and marketing skills.Their whole tube coding scheme was just a way to sell tubes.Yes it is useful to the novice to just plug and play and be safe without biasing,but getting output tubes to perform ideally is a total crapshoot,and rarely if ever achieved.I have seen MKIV's with the EL34's drawing 8ma's.At that bias those tubes should surely exhibit cross-over distortion but when you have them running with a 6L6 in the inner socket at somewhere around 25ma's or so that side of the OT will be okay,notideal and still very cold,but safe.I dont find that spread between a "matched pair" but rather within the entire color code,one pair measuring 30ma's another pair of the same color code at 15-20ma's.I think in the beginning Mesa kept tighter tolerances within the color codes but soon realized that they could have far fewer rejects by loosening things up and still being safe.I dont use current production tubes in any of my many amps,but if I were inclined to go with the Mesa scheme I would just use a $25 pair of Sovteks instead of paying the premium for any Mesa tube,but if Mesa was to find out they would void any warranty that might be in effect.

[mellowamoya]

Bias

I've done the bias mod to almost every MK I've had in my shop,if I get one for caps,I do a bias mod for free.The simul-class amps are almost not worth it.The inner and outer sets are so wide apart that you have to settle on a compromise when biasing between the two sets.My whole reason for the mod is to get the tubes hotter and closer to the sweet spot.With the simul class you cant get there.I like the mod in my own CII+,I use nos tubes and right now I've got 7581A's in it and have used 7027's and KT66 over the years.Never liked the Mesa scheme with the graded system.I've found that sets in the same grade can vary by as much as 20ma's,makes a big difference in how the amp responds.Totally goes against their claim that you just use the same grade and all is good.Its all a marketing scam,just to sell their own tubes.They wont honor the warranty if you dont use their tubes or,at one time,GT's.I dont thinkthey approve GT since Fender bought them.Like Dude says,they are set very cold at the factory,so when they "grade" them they can have a wide range of what is safe,means when they test them they have very few rejects.Getting back to modding the IV,if you do it,be mindful that one pair,I believe the outer pair,will be much colder than the inner pair,so dont set for the colder set to be at max,or the other pair will be too hot.

So I have a matched set 6l6GT to go in,on the mk4 I will have to set up one bias pot for all 4 valves or do I bias in pairs?
Dennis

[stokes]

Just one bias pot for all 4 tubes.The only experience I have with removing the resistors Leo described on the outer pair of tubes was when helping a guy from NJ by email.It was his first attempt at any kind of surgery and contacted me for help.Removing that resistor,which I believe was a 2meg from the grid to ground didnt get it close enough.He then went on to remove the 220k in series with another 220k that feeds the inner pair and is between the bias supply and grid.Stock you have two 220k's feeding the outer pair from the supply.Dont recall exactly what the problem became there,but I believe it had to do with switching to class A.He ended up putting the two resistors back and just compromising on where he set the bias.Although I've done close to 100 bias mods on various Mk's only 2 were simul-class and I did it the way I described,by compromising.Mesa amps are my least favorite of any amp to work on and I cant be bothered to dig into the simuls any further.Like I said earlier,most people with simul class amps want to continue using the EL34's in the outer sockets.I would think if you are using a matched quad of 6L6's it can be done,but having never done it myself except that occasion helping the guy by email,I cant be 100%.

[stokes]

As to biasing in pairs,one MkIV user asked me about doing that and after looking it over I decided not to attempt it as there just isnt enough room in the chassis.

[mellowamoya]

Thanks for all the advice looks like I'll fit one pot and compromise with the bias
What would you consider as hot a bias as possible with the 6l6gt tubes.

[stokes]

In most amps,I like to bias at about 80% to 85% idle dissipation.In Boogie amps this can be problematic using the lead channel.The distortion is generated in the pre amp and if you idle the power tubes too hot the sound gets kind of ratty.60 to 70% should be okay,but go by what your ears tell you.