Help me insert a "transparent" triode gain stage and tonestack!

[guitylerham]

Hello all,

I'm in the middle of designing my next build based off an Ampeg Reverberocket R-12-R but with some changes.

Currently, the R12R only has a Tone and Volume control right after the first gain stage. I'd like to instead use a James tonestack (my amp chassis has unused holes and I've built plenty of FMV type TB stacks). However, I don't want to change the character of the amp more than I can help it. Knowing that this tonestack has a higher insertion loss than the simple current tone control, I want to add another triode to drive a plate-fed James stack. I've attached a simplified schematic (so that we only see the signal path that I'm interested in, I've edited and removed the tremolo circuit) so you can see gain stages V1a and V2.

My idea is for the new topography to go Input > V1a > Volume > New V1b > James > V2a > V2b...

It appears that V1a is a high powered, big headroom stage, whereas V2a has lower plate voltage and less headroom--is this accurate?

Since I only want to add enough gain to cover my new loss, I'm thinking the new gain stage needs to have the same input characteristics of V2a but with the output characteristics of the V1a. Forgive my lack of proper terminology but does that make sense? I've been pouring over load lines and impedances but my brain hurts a little as I'm only self-taught! Just as the original Volume/Tone interacted with the input grid of V2a, can a new gain stage/James stack be operated in such a way to provide similar behavior? It feels like I'm trying to visualize something that can't exist...

I've thought about a cathode follower off V1a but I've read that it's not people's favorite tone or it changes the tone more than they liked so I'm instead thinking of a plate-driven James stack. That particular stack has a loss of 20db or so at the mid-frequency so I assume that's my goal for makeup gain.

So things to consider are the output impedance of the original V1a, input impedance of V2a, plate supply voltage and amount of headroom and makeup gain that the new stage should provide.

Can anyone lead me to more concepts to learn about in order give me a path to follow? Can't express enough gratitude for the wealth of information available to us here and elsewhere. It's a journey wading through it all for little crumb trails!

[nandrewjackson]

https://el34world.com/Forum/index.php?topic=23360.msg251108#msg251108




This was a preamp stompbox build and it sounded great IMO.


Maybe a good starting point for your james tonestack.

The bass and treble both at 2 O-clock was my preferred setting. 


That was a stompbox in front of an amp, you may need to change values a bit.

[kagliostro]

To me you must use a CF to drive the James, not a Plate Follower, also move the vol pot after the Tone Control

A CF is a low gain (near unit) stage and has a low impedance output, the low impedance will help in driving the tone control with a low loss level

Franco

[sluckey]

Ampeg liked to drive the James TS from the plate, not cathode.


     http://sluckeyamps.com/phoenix/phoenix.pdf

     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_b15n.pdf

[nandrewjackson]

I don't mean to try to hijack this thread but I spent a lot of time tweaking the values on that preamp stomp box and it sounded darn good. It would do the Aerosmith Walk This Way tone pretty spot on LOL. I don't have it anymore I might have to build it again. I would challenge anyone with the parts and the patience to build one it's pretty cool 😎

[guitylerham]

https://el34world.com/Forum/index.php?topic=23360.msg251108#msg251108




This was a preamp stompbox build and it sounded great IMO.


Maybe a good starting point for your james tonestack.

Thank you for the schematic and idea! Everything helps at this point.

[guitylerham]

Ampeg liked to drive the James TS from the plate, not cathode.


     http://sluckeyamps.com/phoenix/phoenix.pdf

     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_b15n.pdf

Right, thanks for those schematics. Haven't seen the Phoenix before, looks way cool!

I know a CF is the simplest way to go about adding this tone stack but I'm also interested in keeping this amp build in the Ampeg family of design choices for better or worse! What's confusing the situation for me is the following gain stage/reverb mixer, I don't want to muck with whatever mojo is happening there with 100v less on the plate! I'm sure this is way less complicated than I'm making it out to be.

[guitylerham]

I don't mean to try to hijack this thread but I spent a lot of time tweaking the values on that preamp stomp box and it sounded darn good. It would do the Aerosmith Walk This Way tone pretty spot on LOL. I don't have it anymore I might have to build it again. I would challenge anyone with the parts and the patience to build one it's pretty cool 😎

I bet it sounds awesome! If I get the urge to build a tube-powered pedal, I'm looking this up!

[tdvt]

I don't mean to try to hijack this thread but I spent a lot of time tweaking the values on that preamp stomp box and it sounded darn good. It would do the Aerosmith Walk This Way tone pretty spot on LOL. I don't have it anymore I might have to build it again. I would challenge anyone with the parts and the patience to build one it's pretty cool 😎

I bet it sounds awesome! If I get the urge to build a tube-powered pedal, I'm looking this up!

Another quick hijack...out of curiosity, what was you rail voltage for the pedal circuit & how did you power it?