JC120 Clean Channel w/ Tubes

[anevilspar7an]

Evening everyone, I am posting here to get some feedback and knowledge from the amp gurus here that deal with tubes. I am laying out a streamlined Mesa Mark IIC++ 2 channel head and instead of the Mesa clean channel I decided I would do a JC120 with tubes instead of JFETs. I know there are general rules when subbing JFETs for tubes, would it be as easy as doing that in reverse?

 

[Dostradamas]

Adding tubes to one of the most celebrated solid state amplifiers sounds like blasphemy for some reason

 

[anevilspar7an]

We do it all the time with tube preampliers, I figured it would be a worthwhile pursuit to see if the opposite would work, especially since the Mesa Mark II-C clean is lackluster. I would gladly bare the mark of heresy if it turns out splendidly.

 

[Blackboarcult]

The most hassle-free way of doing it is simply taking the Fender twin schematic and changing the tone stack values to those of the JC120. But... Which JC? There are at least 6 or 7 iterations of it, sometimes with significant changes. The only thing in common between versions is the huge headroom and complete lack of clipping.

 

[anevilspar7an]

I'll give that Fender schematic a look and compare it to the JC120 schematic I have. The most common schematics that are floating around are traced from revisions 2 and 3 to the best of my knowledge. I do have access to a version 1 original North American release since my lead guitarist uses it for his clean tone, I just don't want to break his rig down to open it up and trace it.

 

[Passinwind]

This version already has tone stack values that may be plug and play with tubes: https://skemayohan.com/2022/05/preamp-tone-control-roland/

I can plug them into my Twin Reverb Spice model and see how it translates if you like.

 

[anevilspar7an]

This version already has tone stack values that may be plug and play with tubes: https://skemayohan.com/2022/05/preamp-tone-control-roland/

I can plug them into my Twin Reverb Spice model and see how it translates if you like.

Hell yeah man, I would appreciate that quite a bit.

 

[Passinwind]

Hell yeah man, I would appreciate that quite a bit.

Do you use LTspice yourself? It’s super useful for this sort of stuff. I won’t be able to get to my PC for a while, but it’ll go very quickly once I can since it’ll mostly be simple cap and resistor value subs. All the JC schematics I just looked at show the claasic Fender three band passive stack topology, so about as easy as it gets. I always assumed it was a Baxandall or James variant, I’m a bit surprised.

 

[anevilspar7an]

Yeah, when I looked it over it was very similar. The Mesa Mark II starts similar but they do something with it that kind of kills the sparkle an excellent clean should have. I do want to learn LTSpice at some point but don't have the time to sit down and play with it at the moment, the only reason I can layout the Mark IIC++ Lead Channel is I'm familiar with EasyEDA and can take printouts to work to check for errors. Where I work doesn't allow electronic devices so I'm on even more of a time crunch.

 

[Passinwind]

Yeah, when I looked it over it was very similar. The Mesa Mark II starts similar but they do something with it that kind of kills the sparkle an excellent clean should have. I do want to learn LTSpice at some point but don't have the time to sit down and play with it at the moment, the only reason I can layout the Mark IIC++ Lead Channel is I'm familiar with EasyEDA and can take printouts to work to check for errors. Where I work doesn't allow electronic devices so I'm on even more of a time crunch.

Mesa tends to add a mid boost stage elsewhere that diminishes the inherent scoop of the Fender stack, which is great for distortion but does change up the Fender clean vibe for sure.

The LTspice users forum has both Fender and Mesa tube amp models in their repository, ready to go. Not a bad place to start when you get to it. I’ll be sharing models for my preamp pedal modules on my Gihub page sooner than later too.