Help Required! Single Channel AB763, No Trem With Some Mods

[sgbrewing]

Hi guys, hope you've had a good Christmas and New Years!

Built my first amp during the first lockdown, a JTM45 with a few mods and it sounds really good! Now I'm looking to build a single channel AB763 in a Princeton chassis with a 12 inch speaker (most likely the Eminence GA-SC64). I've read through loads of forums and pieced a few layouts together.

I'm looking for a great clean sound at around 2-3 on the volume and then slowly getting some smooth gain as its turned up. I will use the clean sound with a zen drive for the D type tones and with the amp cranked up a bit for some smooth blues lead.

I haven't learnt to draw schematics yet, that will be for the next build :)

I would really appreciate it if someone with a lot better knowledge of these things than myself could check out my layout and see if its all ok before I start building 

I have another question also:
   What changes can I make to clean up the PI? I will be using the PPIMV a lot at home, so some changes to clean up the PI would be very useful.

I have biased the first gain stage very slightly hot for some added gain and the following 2 stages slightly cool for more chance of some smooth distortion.

I don't want to stray too far from the blackface sound.

Thanks a lot for you help,
  Shawn.

[sluckey]

Looks good. I'd leave the PI as is.

[2deaf]

Dual ganged pot is grounded allowing you to short out the bias voltage and burn up your power tubes.

[sluckey]

Yikes! Can't believe I missed that.   

[sgbrewing]

Dual ganged pot is grounded allowing you to short out the bias voltage and burn up your power tubes.

Good spot thanks, that should fix it!

[sgbrewing]

If after trying the amp and I find the PI is driving too hard with the PPIMV turned down, what can I do to increase the headroom of the PI?

[sluckey]

Get rid of that PR power rail and use an AB763 power rail with choke, 1K, and 4.7K dropping resistors. This will increase B+ voltage to the PI for more headroom.

[brewdude]

I don’t really know, but...
What is the advantage to a “cleaned up” PI if you are using a PPIMV?

[sgbrewing]

Get rid of that PR power rail and use an AB763 power rail with choke, 1K, and 4.7K dropping resistors. This will increase B+ voltage to the PI for more headroom.

Cheers, like this?
Thanks a lot for your help!

[sgbrewing]

I don’t really know, but...
What is the advantage to a “cleaned up” PI if you are using a PPIMV?

As this master gets turned down it drives the PI, don't want it driven too much as I want the pre amp drive to be more in balance at low volumes (have a 5 month old baby)

Cheers!

[Bieworm]

I don’t really know, but...
What is the advantage to a “cleaned up” PI if you are using a PPIMV?

As this master gets turned down it drives the PI, don't want it driven too much as I want the pre amp drive to be more in balance at low volumes (have a 5 month old baby)

Cheers!

I think you need to go the vvr route for  overdrive on low volume. Everything else you do is low volume and clean or high volume and overdrive. Or build a 1W amp...which is still loud for a 5 month old baby in the house.
Headphone output? Engineers should start building headphones with celestion g12h experience 😃😃😃

[brewdude]

Would a pre PI MV be a better choice?

[Bieworm]

Would a pre PI MV be a better choice?

Not really, it tames the signal going to the PI.  Nothing dramatic, but the first thing it does is remove overdrive...
The princeton design is pretty clean in nature. The overdrive on "low volume" is still signal pushing the PI.  When the PI gets pushed the amp is already very loud..
I have buit the 18W tremolo TMB. That's the only amp i know with decent overdrive on low volume. Its designed that way. But I have to settle with an amp that's always kinda overdriven... very nice amp though
https://www.dropbox.com/s/k4rlwdg0c64a92z/TMB%20tremolo%202.2k%20CR%20on%20pin%203%20V2.m4a?dl=0

[sgbrewing]

Would a pre PI MV be a better choice?

Not really, it tames the signal going to the PI.  Nothing dramatic, but the first thing it does is remove overdrive...
The princeton design is pretty clean in nature. The overdrive on "low volume" is still signal pushing the PI.  When the PI gets pushed the amp is already very loud..
I have buit the 18W tremolo TMB. That's the only amp i know with decent overdrive on low volume. Its designed that way. But I have to settle with an amp that's always kinda overdriven... very nice amp though
https://www.dropbox.com/s/k4rlwdg0c64a92z/TMB%20tremolo%202.2k%20CR%20on%20pin%203%20V2.m4a?dl=0

Sounds good! I'm using a jtm45 with the Rich Mod II PPIMV with the PEC 500k dual pot, sounds very good at bedroom volumes, check this out:

Will try the same master on this single channel AB763. If it still pushes the PI too much I will change to PI values to silverface still with the 12AT7 valve, should clean it up a hair.

[ndross475]

This has interested me for a while, I figured it had to have been done before.  A version I found was Rob Robinette's BlackVibe.  He did what I think you are trying to accomplish with 6L6s.  I am trying to figure out what needs to be changed to run 6v6s.

https://robrobinette.com/RR763_BLACKVIBE.htm#AB763_With_Tremolo_Deleted

Note the Trem "load resistor" , could make that switchable for added gain??

Hope this helps.

Nick

[PRR]

https://robrobinette.com/RR763_BLACKVIBE.htm#BLACKVIBE_6V6   ??

[Williamblake]

You could also build -14dB speaker attenuator. That would be two to eight power resistors depending on the design, for example the one with the two switches from rob but with more attenuation.

[sgbrewing]

This has interested me for a while, I figured it had to have been done before.  A version I found was Rob Robinette's BlackVibe.  He did what I think you are trying to accomplish with 6L6s.  I am trying to figure out what needs to be changed to run 6v6s.

https://robrobinette.com/RR763_BLACKVIBE.htm#AB763_With_Tremolo_Deleted

Note the Trem "load resistor" , could make that switchable for added gain??

Hope this helps.

Nick

Yea the idea of a single Channel AB763 at 20W sounds like a perfect amp :D

I guess using deluxe reverb transformers should be fine :) I've used the power dropping resistor values from the super reverb. The 1K supplies more voltage to the PI and should be cleaner upon turning down the master. And I'm guessing the higher pre amp resistor value will cause the pre amp to distort a bit more. I will try these values first and experiment later if it doesn't sound quite right.

Thanks for that idea about the tremolo resistor! I'll build without and crocodile clip it into place to see the difference, as wiring in a switch later will be easy enough.

I will post here as my build progresses, will be a slow build though as I'll buy parts as I have the money for them.

Have an updated layout here, added the 50v bias line from the PT and hence updated the bias resistor.

[ndross475]

The 6v6 plan he has would work if you are ok w/ no reverb or using a Strymon or better yet a reason to build one of the Revibe units floating around on the EL34 project page... The idea I like is keeping the Reverb like his 6L6 plan but building a 20watter... but for me, at my pay grade, it may be easier to throw in a lossless effects loop and use a pedal... I will keep watching... Good luck

[sgbrewing]

The 6v6 plan he has would work if you are ok w/ no reverb or using a Strymon or better yet a reason to build one of the Revibe units floating around on the EL34 project page... The idea I like is keeping the Reverb like his 6L6 plan but building a 20watter... but for me, at my pay grade, it may be easier to throw in a lossless effects loop and use a pedal... I will keep watching... Good luck

Yea thats why I put together this layout with 6v6 and reverb, with an amp like this I would really like some spring reverb :D only 1 extra tube for the reverb.

Will measure voltages when it's finished as I may need to adjust those.
Should be fine though, it's literally just a deluxe reverb but with the normal channel taken away.

[SILVERGUN]

One way to "clean up the PI" is to put less signal into it.
I found sluckey's addition of a 100K pre-PI master volume very useful. This was one of my favorite amps I have ever built. Of course I modded it, but I just couldn't get it to sound bad.
If you're gonna build something like this you should have a look:


Tweed Deluxe Reverb (sluckeyamps.com)


Guess who did it before robrob = sluckeysluckey 

[Mike_J]

Hi guys, hope you've had a good Christmas and New Years!

Built my first amp during the first lockdown, a JTM45 with a few mods and it sounds really good! Now I'm looking to build a single channel AB763 in a Princeton chassis with a 12 inch speaker (most likely the Eminence GA-SC64). I've read through loads of forums and pieced a few layouts together.

I'm looking for a great clean sound at around 2-3 on the volume and then slowly getting some smooth gain as its turned up. I will use the clean sound with a zen drive for the D type tones and with the amp cranked up a bit for some smooth blues lead.

I haven't learnt to draw schematics yet, that will be for the next build :)

I would really appreciate it if someone with a lot better knowledge of these things than myself could check out my layout and see if its all ok before I start building 

I have another question also:
   What changes can I make to clean up the PI? I will be using the PPIMV a lot at home, so some changes to clean up the PI would be very useful.

I have biased the first gain stage very slightly hot for some added gain and the following 2 stages slightly cool for more chance of some smooth distortion.

I don't want to stray too far from the blackface sound.

Thanks a lot for you help,
  Shawn.

If you look at all the schematics for AB763 amps on Doug's site they either have no mid pot with a 6.8K resistor to ground off the left tab of the bass pot looking at it from the back to simulate a mid control at what I presume Leo must have liked or a 10K audio pot for the mid control. At the suggestion of Willabee as I recall I put a 100K pot in the mid control to be able to get the Bakersfield sound (country as far as I can tell.) If the pot is set to about 2.5 then it is around the 6.8K stock value. Gives more versatility to the amp.


Your layout shows a 250K mid pot with a 6.8K resistor to ground off the left tab of the mid pot. Is this the Bakersfield sound on steroids. I imagine there is a purpose for it but am curious to know what the result of doing the mid pot this way is.