Bogner XTC Blue gain stages

[SacredGroove]

I'm conflicted reading different info about the XTC blue/red channels. Blue channel is 3 stages? Red channel is 4 stages? On the schematic I have there are 5 (dirty) tube stages. On my Randall module, I have 4 stages and the first is actually the second stage on the schematic. I want to incorporate the blue channel into it, but I'm not sure which should be the first stage.

Any help with this issue?
Thanks!

EDIT: The tube board isn't really visible, but I can list what the pins are and how its connected to the main board. Also, the pic of the bottom is inverted to make it easier to compare it to the top side.

[Chuck H]

What are you doing? modding your Randall? Building a clone? Do you have schematics for us to reference so that we can see what your asking?

[SacredGroove]

Yes, I am modding my Randall module. You may have answered my question already, but here is the schem I was going by.
http://www.blueguitar.org/new/schem/bogner/xtc_od.gif

[defaced]

Cathode followers that drive the tone stack aren't counted in the "gain stage" count because they're not adding gain in the traditional sense, they're buffering. Both schematics have 4 gain stages and a cathode follower, the hand drawn one also has an option of three gain stages, can't see anything past that. Just as importantly as the number of gain stages is the architecture of the Randall. If they don't line up well, you're going to have fun modding it to the XTC circuit.

[SacredGroove]

Cathode followers that drive the tone stack aren't counted in the "gain stage" count because they're not adding gain in the traditional sense, they're buffering. Both schematics have 4 gain stages and a cathode follower, the hand drawn one also has an option of three gain stages, can't see anything past that. Just as importantly as the number of gain stages is the architecture of the Randall. If they don't line up well, you're going to have fun modding it to the XTC circuit.

Ok, so the stage that has the "structure" switch is the 2nd stage? The CF is the stage right before the tone stack? On my module, v1a is v1b on the schematic. If I'm understading you correctly, I am missing an actual gain stage?

[defaced]

Yes, the structure switch is in the cathode of the second gain stage. Yes, the cathode follower is immediately before the tone stack. Notice that the signal for the tone stack is taken from the cathode, that's what makes it a cathode follower. I have no clue what you're missing because I don't know what is in the Randall module. Schematic? Pictures of both sides of the board with/without tubes? What module are you planning on modding?

[SacredGroove]

Yea, I have pics of the pcb. I'll post later tonight after class. Thanks for your help!

[SacredGroove]

Updated OP with more pics.

[defaced]

Ok. Couple of different scenairos on this one. First, Randall could have only made the three stage + CF (assuming it's CF driven) channel. Second, they could have botched the 4 stage design and made the TS plate driven. Third, they could have given you the 4 stage design and used a MOSFET or other semiconductor to drive the TS.

You'll have fun on this site. Grail Tone - Randall MTS Series Module Modifications
Too bad they didn't post schematics, but it looks like all of the semiconductors on the board are for things other than driving the tone stack, so it looks like options 1 and 2 are still standing. The musicians friend blurb on the module says it's the "red" channel - whatever that means. There is however an entire forum on that site dedicated to the MTS. I bet they have some info worth looking at.

[SacredGroove]

Thanks a lot for your ideas and time. I REALLY appreciate it.

I'm actually a regular over at the Grailtone MTS forum, but when it comes to modifying modules everybody shuts up. A few guys have really taken off with their modding and have done well for themselves, but everything is such a secret now. I can appreciate and respect their work, for sure, but it's like they have the forum by the balls when someone inquires about trying to mod their own module.

I have another question: Say I wanted to switch a gain stage in/out. Would that just be a matter of connecting/disconnecting the components (plate, grid, cathode)? I'll use the XTC as an example. How does the signal of the Blue channel (3 stages) get to the TS without the 4th stage being on?

Scott

[GainFreak]

Hi,

I can draw module's PCB from your pics for couple of hours. I'll also need couple of pics of the tube board. Then we can trace the schematic and compare it to those floating around. It's going to be much easier to mod it this way. This is the XTC schematic I have.

[SacredGroove]

Hi,

I can draw module's PCB from your pics for couple of hours. I'll also need couple of pics of the tube board. Then we can trace the schematic and compare it to those floating around. It's going to be much easier to mod it this way. This is the XTC schematic I have.

Thanks a lot! Any help would be most appreciated. I have the module traced out for the most part, but there are a few places I'm scratching my head. Pics of the tube board may be difficult, but I can try. I've traced it out and can relay it to you. I was also adding labels to another pic of where the main board and tube board meet. The frustrating part about this module is that it is the only one like it and most of the other modules are more recognizable, universal, or versatile, if that makes any sense...LOL.

FWIW, I tried using the values on the schematic and I got no sound at all. The parts I changed were all Rp, Rk, and Ck bypass.

Thanks
Scott

[GainFreak]

This is a rough sketch of what I can see from the pictures. It looks like the schematic pretty much matches the one that I posted except for the first coupling cap and Gain pot which should be 1M.
I guess all modules have a common first stage somewhere in the amp. On this PCB are stages 2-5, with Boost and Structure on.
If you see any mistakes let me know.

[SacredGroove]

Of course! V1 in the module is actually V2. The preamp chassis has a common input tube stage that I didn't even consider. That reminder helps out A LOT!

What did you do that drawing with? That could be really handy for me.

If I wanted to switch out stage 5 can I just tie both cathodes of stage 4 and 5 to their own side of a switch and then tie the switch to where the CF meets the tone stack?

Great Help! Thanks a lot!

[GainFreak]

What did you do that drawing with? That could be really handy for me.

The software I'm using is Sprint Layout 5.0. From all PCB software I've tried so far this one is the easiest to work with and the most intuitive. One of the features I like is you can import a BMP picture, scale it and draw the PCB from the picture. That's what I did using the pictures you posted. It can also export all files necessary for PCB production.

If I wanted to switch out stage 5 can I just tie both cathodes of stage 4 and 5 to their own side of a switch and then tie the switch to where the CF meets the tone stack?

I didn't quite get that. Stage 5 is the CF. You want that out of the circuit (switchable)?
You can't connect TS to the stage 4 cathode. It should go to the plate via coupling cap but then CF shouldn't be left with the grid floating " in the air".