Learned something interestimg about Mesa 3 channel Rectifiers.....

[Gone_Shootin]

Yesterday I got a hair up my ass, put everything else aside and finished modding my Marshall JVM 410H. While I was doing the OD2 Rectifier Mod, I was re-reading parts of the thread about how to do the mod over on JVMforum.com. When I got to the part about the resistor and cap soldered in series that gets jumped across the front board, Casey_Butt (the OP of the thread and Guru in all things in JVM) mentioned that this was to mimic the false Presence control on the Third Channel of the Three Channel Dual Rectifier. And the reason Mesa has a false prescience circuit is because on the third channel, the amp has no negative feedback on the poweramp, thus making any presence circuit inoperable.

Assuming this is also the case on the three channel Triple Rec, it explains why the Presence knob on channel 3 acts so differently from a normal presence control, and also explains why that channel has such a different feel.

I've been wanting to throw some EL-34s in my Recto, now I really want to. mg:

 

[DankStar]

Re: Learned something interestimg about Mesa 3 channel Rectifiers.....

People are modding their jvm's?

 

[Van Noord]

Re: Learned something interestimg about Mesa 3 channel Rectifiers.....

People are modding their jvm's?

All day, every day. Some people just gotta tinker. Even when it's not needed.

 

[Gone_Shootin]

Re: Learned something interestimg about Mesa 3 channel Rectifiers.....

Yep, lots of people. I did the Plexi One Wire, Plexi Cap, and the OD1 2204 mods, which are either just traditional Marshall mods adapted to the JVM, or bring it more in line with classic Marshalls on certain channels. The Blackface mod I did is just to make the clean channel sound more like a Fender, and of course there's the OD2 Rectifier mod I also did.

Nothing wrong with the amp in stock form, but if something can be improved with a few dollars worth if caps, resistors and pots, why not?

ETA: Forgot...I also did the Negative Feedback Control mod, which allows the user to adjust the headroom in the power amp. With the resistor/pot combo I used, I'll be able to go from a setting that has more headroom than an old school Plexi to a nearly complete lack of headroom like an old school Vox.

 

[PFDarkside]

Re: Learned something interestimg about Mesa 3 channel Rectifiers.....

I remember something about this from 8+ years ago. I was tinkering with my Road King recently and I definitely found a difference in channel 3 vs 4 (I believe similar to 2 vs 3 on Recto) especially in the gain/presence portion. I had forgotten about this difference (and hadnt pulled the manual out).

 

[Soulcrusher_X]

Re: Learned something interestimg about Mesa 3 channel Rectifiers.....

The 3 Channel Rectos and the Red channel on the 2 Channel Rectos have the presence control wired as a treble shunt directly after the tone stack. The Green Channel on the 3 Channel may be the only channel that has a presence control in the NFB network. The reason why CH 3 has such an aggressive presence control is the value of the potentiometer and cap.

I can't remember off the top of my head, but the Green and Orange channel have a 5k or 10k control, and the Red channel has a 25k control, so it keeps more high end in the signal. On my old 3 Channel, about 12-1 o'clock on the Orange channel presence was about the same as 9 o'clock presence on the Red channel.

 

[serkan80]

Re: Learned something interestimg about Mesa 3 channel Rectifiers.....

@Gone_Shootin. This is an interesting read. However, in my opinion channel-3 has been designed to work that way. From the first post I understand that it has no negative feedback but that is just to ensure that aggressive open-loop behavior in this case. I might be wrong but this is what comes to mind. As the last post stated, you just use the offset to change in manually but you can't really expect it to follow the output. I hope I'm right but in case I'm wrong, please do forgive me and correct me. We're all sons of Adam and to error is human.

 

[Gone_Shootin]

Re: Learned something interestimg about Mesa 3 channel Rectifiers.....

Interesting, I didn't know channel 2 was that way too.

I did end up getting a set of Svetlana (New Sensor) EL34s for it a couple weeks ago. They gave it a nice growl and really tightened up the amp. It's not as tight as my Splawn, but still much better than it was with 6l6s.

 

[some_dude]

Re: Learned something interestimg about Mesa 3 channel Rectifiers.....

Channel 2 is optimized for Raw/Vintage while channel 3 is optimized for Modern. It's the reason the presence pots are different.