Mark III C+ Mod

[AtomicMassUnit]

From what I can tell by reading endless treads on the subject, the C+ mod for the Mark III is a change to the lead circuit. Is it just a matter of A/B'ing the schematics and changing the III's lead circuit to be the same as the IIC+?

Also, when biasing, when I shunt the meter from the OT center tap and pin 3 of each tube I get 22ma for the EL34s and 19ma for the 6L6s. Is that a reading per tube or is it like a Twin Reverb where that's the reading for that pair? Just trying to get the bias set right, hoping it will get rid of some of the sizzle.

 

[AtomicMassUnit]

Ok guys, I was able to talk to Mike B. over at Mesa and he gave me the scoop. It changes the whole preamp to IIC+ specs. It's still not the same amp of course, but very close. I decided for now I'm not going to do the mod, considering I haven't ever heard a Mark II to know if I'd like that better. What I did do is check up on a couple things inside the amp and it already has the 'reverb mod' since it's a green stripe model, and it already has the 2.7k grid resistors. I installed an adjustable bias and biased it up properly, and there was a definite improvement in the tone overall. I'm liking it better. I think it's now down to understanding the controls on the amp and proper tube selection. Who knows, down the road I might end up C+'ing this, but for now the only thing I might want to do is wire the EL34s to triode mode.

 

[boogietone]

How did you install the adjustable bias? I have a MKIII and it has a bias test point but no adjustment. Also, when you check the bias at that test point, you hook the red lead to the test point and the black lead to ground. Is that ground just the chassis or is there a specific ground somewhere your supposed to use?
Thanks!

 

[Russ]

Just out of curiosity, is Mike B still doing the C+ mod to the IIIs? If so, how much was he going to charge you for the mod not including shipping. I had thought about getting a III just for that purpose.

 

[AtomicMassUnit]

here's how I did the adjustable bias. If you look at the bottom corner of the board near the power tubes there is a spot on the board labeled R117, and it looks like there was a resistor there that was clipped out of the circuit. You install a 50k or so trim pot across that. But, I strongly recommend you put the pot in series with a resistor (I used 10k) so that you can't turn the pot down to 0 and short to ground, because that will definitely mess something up, probably blow a cap in the bias supply. I ended up using a 100k trim pot because that's all I could find, and it worked perfectly. I don't know about any bias test points for this amp, I use the shunt method on these, set my meter to current and read between the B+ line of the output transformer and pin 3 of the power tube, whichever one you want to read the bias on. That gives the idle current for the tube in milliamps.

Russ, Mike B. is still doing the mod. I'm a mesa boogie dealer, so I'm not sure if the price he quoted was his normal price or a dealer price kinda thing. I'm going to wait until I actually hear a IIC+ to decide whether or not to do this mod.

 

[joelap]

Anyone know exactly what the Mark III -> IIC+ entails? I'm pretty handy with a soldering iron and wouldn't mind getting my hands dirty on my own (soon to be my own) Mark III. If anyone has the details, could you post or email me? joelap at Gmail dot com

Thanks

Of course, if I do get a copy of the mods and do it myself, I'll record before and after clips.

 

[AtomicMassUnit]

From what I understand you can just look at the schematic for both amps and copy the IIC+ schematic. The most legible schems I found were in the Tube Amp Book by the way.

 

[Paul Secondino]

here's how I did the adjustable bias. If you look at the bottom corner of the board near the power tubes there is a spot on the board labeled R117, and it looks like there was a resistor there that was clipped out of the circuit. You install a 50k or so trim pot across that. But, I strongly recommend you put the pot in series with a resistor (I used 10k) so that you can't turn the pot down to 0 and short to ground, because that will definitely mess something up, probably blow a cap in the bias supply. I ended up using a 100k trim pot because that's all I could find, and it worked perfectly. I don't know about any bias test points for this amp, I use the shunt method on these, set my meter to current and read between the B+ line of the output transformer and pin 3 of the power tube, whichever one you want to read the bias on. That gives the idle current for the tube in milliamps.

Russ, Mike B. is still doing the mod. I'm a mesa boogie dealer, so I'm not sure if the price he quoted was his normal price or a dealer price kinda thing. I'm going to wait until I actually hear a IIC+ to decide whether or not to do this mod.

Thanks for passing on the info. My stock Mark 3 was pulling the same readings as yours. 19 mA is way too cold for a 6l6 and the El34's in the outer sockets sounded brittle at 20/21mA

Any recommendations on who supplies good quality parts for the trim pot and such ?

 

[disassembled]

alliedelec.com, mouser.com, or even radio shack will suffice

The bias trim pot is not a critical piece as to where you need the best. You just need something that will do the job.

 

[boogietone]

Thanks for all the adjustable bias info! I think I'll do it when I get back from my trip.

 

[boogietone]

AtomicMassUnit, this maybe a really stupid question but I want to make sure before I do this. When installing the 10k resistor and the 50k trim pot in series across R117, do I have to worry about polarity at all with the resistor and trim pot? I guess my question really is, is it possible to put the trim pot in backwards or the resistor wired in series to the wrong leg of the trim pot? Hope that makes sense and I didn't word it wrong. Thanks!

 

[AtomicMassUnit]

Resistors don't have a polarity. The 10k is just there so you can't turn down the pot to 0, which would short to ground. With the 10k resistor in series when you turn the pot all the way down you get 10k as a minumum.