AB763 mods help

[davohilts]

Hello All,
I’m wanting make some modifications to the classic fender AB763 circuit for a new build I’m doing.
I want to make it cathode biased for the power tubes and am planning on using an existing power transformer that I have. I’ve worked out the bias resistors for each power tube, works out to 820 R per tube to give me 85% plate dissipation.

The attached schematic shows the power section of my amp.
I have 3 questions based on what I have shown on the schematic:
1. Is my triode/pentode switch plan within the realm of reality?
2. Is my post phase inverter master volume correct?
3. I want to add a pot where I can turn off one power tube to make it a variable SE/PP phase splitter, does this look correct? This is the one I’m least confident in.
Thanks for any help!

ab763mod_1.jpg

[Orson Maxwell]

What is the optimal p2p impedance in triode mode? Also, it seems that your master volume won't work all that well with the PP/SE knob. 85% plate dissipation is awfully bold for a PP while still cold for an SE, you won't like the sound once you turn off one of the valves I bet.

[davohilts]

What is the optimal p2p impedance in triode mode? Also, it seems that your master volume won't work all that well with the PP/SE knob. 85% plate dissipation is awfully bold for a PP while still cold for an SE, you won't like the sound once you turn off one of the valves I bet.

I played an amp a few years back that had a pot on the back that turned off one of the power tubes and really liked how it sounded. I've been thinking about building an amp and wanted to see if I could figure out how to accomplish that. I will be using the master volume when i'm in pp mode only. The triode mode will be probably in pp mode also. I'm not sure what the optimal p2p impedance would be as I'm not an EE but a hobbyist.

[Rockwell666]

Hello All,
I’m wanting make some modifications to the classic fender AB763 circuit for a new build I’m doing.
I want to make it cathode biased for the power tubes and am planning on using an existing power transformer that I have. I’ve worked out the bias resistors for each power tube, works out to 820 R per tube to give me 85% plate dissipation.

The attached schematic shows the power section of my amp.
I have 3 questions based on what I have shown on the schematic:
1. Is my triode/pentode switch plan within the realm of reality?
2. Is my post phase inverter master volume correct?
3. I want to add a pot where I can turn off one power tube to make it a variable SE/PP phase splitter, does this look correct? This is the one I’m least confident in.
Thanks for any help!

ab763mod_1.jpg

1. you don't need any of those resistors. - see attachment. wire the screen resistors to pins 4 and 6. (6v6's want 470ohm)
2.the Post phase inverter master volume will not work as you have drawn. there are a few diff kinds to choose from. You will require a "Dual Ganged" pot. This allows both sides of the phase inverter +/- to be adjusted independently. The real challenge is when fixed bias is present at the 220k resistors. in the modified drawing I have attached is what I call a dirty master. the middle "wiper" lug caps are there to keep DC off the pot. This is the easiest post phase inverter to install.
3. Ive never heard of this before so I cannot comment but I don't think it works. try a Variable NFB pot or something simple.

[danman]

I have a Fender 6g3 Deluxe build where I added a switch to ground one of the power tube grids. It actually works pretty well and the tone of the amp did not change very drastically. It does knock a few decibels of the sound in this psuedo single ended mode. It's not an ideal solution for running single ended but it will work. I believe that Mesa patented this idea years ago from an article I read...https://www.ampbooks.com/mobile/amp-tec ... t-7173488/

[Rockwell666]

I have a Fender 6g3 Deluxe build where I added a switch to ground one of the power tube grids. It actually works pretty well and the tone of the amp did not change very drastically. It does knock a few decibels of the sound in this psuedo single ended mode. It's not an ideal solution for running single ended but it will work. I believe that Mesa patented this idea years ago from an article I read...https://www.ampbooks.com/mobile/amp-tec ... t-7173488/

cool! I guess you would have try each side and see what sounds better.

[davohilts]

Hello All,
I’m wanting make some modifications to the classic fender AB763 circuit for a new build I’m doing.
I want to make it cathode biased for the power tubes and am planning on using an existing power transformer that I have. I’ve worked out the bias resistors for each power tube, works out to 820 R per tube to give me 85% plate dissipation.

The attached schematic shows the power section of my amp.
I have 3 questions based on what I have shown on the schematic:
1. Is my triode/pentode switch plan within the realm of reality?
2. Is my post phase inverter master volume correct?
3. I want to add a pot where I can turn off one power tube to make it a variable SE/PP phase splitter, does this look correct? This is the one I’m least confident in.
Thanks for any help!

ab763mod_1.jpg

1. you don't need any of those resistors. - see attachment. wire the screen resistors to pins 4 and 6. (6v6's want 470ohm)
2.the Post phase inverter master volume will not work as you have drawn. there are a few diff kinds to choose from. You will require a "Dual Ganged" pot. This allows both sides of the phase inverter +/- to be adjusted independently. The real challenge is when fixed bias is present at the 220k resistors. in the modified drawing I have attached is what I call a dirty master. the middle "wiper" lug caps are there to keep DC off the pot. This is the easiest post phase inverter to install.
3. Ive never heard of this before so I cannot comment but I don't think it works. try a Variable NFB pot or something simple.

Thanks for the advice!! I appreciate it very much.

[davohilts]

I have a Fender 6g3 Deluxe build where I added a switch to ground one of the power tube grids. It actually works pretty well and the tone of the amp did not change very drastically. It does knock a few decibels of the sound in this psuedo single ended mode. It's not an ideal solution for running single ended but it will work. I believe that Mesa patented this idea years ago from an article I read...https://www.ampbooks.com/mobile/amp-tec ... t-7173488/

cool! I guess you would have try each side and see what sounds better.

I built a 6G3 a few years ago and this is a great idea, thanks!!

[sluckey]

6G3 is a fixed bias amp. I would expect the tube to self destruct if you ground the grid.

[davohilts]

6G3 is a fixed bias amp. I would expect the tube to self destruct if you ground the grid.

yikes!!

[Rockwell666]

6G3 is a fixed bias amp. I would expect the tube to self destruct if you ground the grid.

this was exactly my thinking. Must have been cathode biased.

[danman]

Yes, sorry for not mentioning that part...I did change the circuit over to cathode bias. I built it single channel without the trem and decided to run with cathode bias to lower the output slightly.