AB763 nornal channel mod

[joesatch]

is there a trainwreck mod for the normal channel or some other gain mod? I dont mind adding a triode if there is a good mod that can get me a good crunch with dynamics and controllable feedback

[sluckey]

Look at some trainwreck schematics. The express basically just adds one more triode.

[SILVERGUN]

If you'd like to try something a little stupid you can just run a clip on jumper from the wiper of the volume pot to the grid as pictured in red below. Keep the Volume low and ease it in.
Yes, it's stupid because you wind up with two tone stacks, but, you'll never hear how it sounds if you don't do it.
I tried it once and the signal needs some attenuation to make it "right", and I just never got back to it.
Sometimes dumb stuff makes you learn things.

[joesatch]

i did the TW mod for the normal channel. added a triode. it seems to work good but of course i have to crank it to get any break up as the TW circuit has no master volume. Now i have the last .1uf coupling cap (from the cold clipper triode) feeding the mixing resistors. not sure if this is correct but not hearing any issues. I may have to add a master vol to this amp

[joesatch]

Ok well i have an issue with the tonestack. Only the Treble pot works. the middle and bass pots dont do anything. Ideas? What's weird is the middle pot has no input on the diagram. This tonestack is wired differently than what i'm used to

[tdvt]

I've got a normal channel mod temped in an amp I'm wrapping up. Still fooling with the mod, but now waiting for a few parts.

I am essentially running the normal channel signal, post-tonestack, into the reverb channel mixing stage, for an added gain stage.

Waiting on some pots & also parts for an ABY box, as the channels can now be jumpered together.

[dwinstonwood]

Take a look at my AA864/Trainwreck Express preamp. It has an MV. I've built two and starting on a third.
This one shares more with the Express than the AA864 Bassman.

[tdvt]

Take a look at my AA864/Trainwreck Express preamp. It has an MV. I've built two and starting on a third.
This one shares more with the Express than the AA864 Bassman.

What is connected to node "E" at the MV, on your first schematic?

[dwinstonwood]

tdvt, that section is actually from the single-ended version I'm working on. "E" is the insertion point for sluckey's Trem-O-Nator.

Also, in my push-pull versions the NFB connects the same way as it does in AB763's and the Express, not like it does here.

[tdvt]

tdvt, that section is actually from the single-ended version I'm working on. "E" is the insertion point for sluckey's Trem-O-Nator.

Also, in my push-pull versions the NFB connects the same way as it does in AB763's and the Express, not like it does here.

Thanks for the clarification

Regarding the MV design, how did you arrive at the 220K/500KA series configuration? Did you use the same setup in your first two?

 

[dwinstonwood]

tdvt, the first two, a 40ish watt 6L6 version and a 15 watt 6V6 version, both ended up keeping the 220k-220k divider that's in the AA864 and added the 500k pot right after:

https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_aa864_schematic.pdf

sluckey spent a lot of time guiding me through the first 6L6 build. It's really his work. Here's a link to it that he kindly hosted:

https://sluckeyamps.com/misc/AA864_Musings.pdf

Now I'm thinking that I meant to include the second 220k in the third single-ended version, and just forgot to add it.  I'll add it in since the MV's work really well in those first two versions.

joesatch, I hope I haven't derailed your thread. Reading your first post, I thought maybe my solution might be useful or helpful.

[tdvt]

Thanks for the info.

Hopefully Joesatch will also find it helpful, as we seem to be chasing pretty much the same thing.

Also good to know you are happy with yours, as a point of reference.

[joesatch]

all is good. the tonestack works. i swapped speakers. gotta crank this thing it's a beast

[dwinstonwood]

Either way: with the voltage divider (two resistors) before the MV pot, or the pot acting like the lower half of the divider, I see it as what the Valve Wizard describes as an "attenuating high-pass filter." In the first instance the second 220k resistor is in parallel with the pot; so, I guess it's "effectively" decreasing the value of the pot.