AB763 Deluxe Reverb with 6G16 Tremolo/Peavey Pacer Conversion

[Platefire]

Here is the schematic that I'm building even though the amp is not completed
and hasn't been fired up yet---some components may be subject to change once tweaking begins. If you see any problems please let me know, I do want the schematic to be correct. Platefire


EDIT NOTE: All corrected versions of schematic have been deleted up to final version. Please refer to last posted version for the latest corrected version. It is a process, har!

[sluckey]

Plate, here are a few things I see. I'll start with the little niggles and work up to the show stopper.  :)

1. V3B cathode resistor should be 1.5K. The original DR AB763 uses an 820Ω, but the resistor is shared with another tube (V3A cathode in your circuit).

2. Add a 0.1µF and a 47K to the plate of V3A (Refer to the schematic I linked to). The 47K simulates the load that the original roach trem circuit puts on the signal path. Without it the amp will have a LOT MORE GAIN than an AB763. It will still work and you may like it, but it ain't AB763.

3. There are several little errors in the area where the trem circuit joins into the bias circuit. The trem needs to be cap coupled into the bias circuit, the 47µF cap needs to be on the right lug (not wiper) of the INT pot, and the bias needs a voltage divider resistor to ground (Refer to the schematic I linked to).

The linked schematic below was drawn from Doug's AB763 layout. I drew it a while back to help me visualize his circuit. I prefer looking at a schematic rather than a layout when pondering how a circuit works. This should be exactly what you're after, except the B+ node resistors are not the typical DR 10K values. Just chop off the Normal channel at the top of page 1.

If you like the looks of this schematic, I can easily modify it exactly to your specs. Or you can use it as a model to develop a PC Express version. In fact, I'd be surprised if Jeff doesn't already have a PC Express version in this forum.

Hope this helps...

http://home.comcast.net/~seluckey/Hoffman_AB763.pdf

[Platefire]

sluckey

Thanks for the review! I have copied your schematics and post so I can go through it item by item and mark it on my schematic. When I get through going over it, I will come back with any questioins I have. Right now I'm just sitting aroung the house waiting to see what the Hurricane will do. This will help me keep my mind occupied, Har! Thanks, Platefire

I still don't see what you ment by the "Show Stopper" ?

[sluckey]

It's not a whole amp show stopper. But the trem won't work as drawn. The 47µF cap on the wiper of the INT pot will shunt the lo freq trem signal to ground. Also, the bias as drawn will put a high negative voltage on the grids of the output tubes, causing them to run very cold, maybe even starved. Making the circuit look like Doug's will fix that.

BTW, I'm just sitting over in Mobile watching the feeder bands pass by and watching the hungry news media pump up the storm damage around Mobile. Hope you guys don't get too much damage.

[sluckey]

YOW! I just saw another biggie. I see you're feeding the bias supply with 330VAC. Your 470Ω/1W bias range resistor needs to be more like 100K to 180K. Look at a Princeton Reverb circuit.

[Platefire]

Wooha!!! My schematic was really out of whack! Does this looks better?

BTW-Thanks for the offer of modifing your schematic to my specs but it really helps me to draw the circuit out to get a better understanding--just like you said helps you. I intended on doing this all along but just now getting around to it. I figured it would be a good idea to draw it out before I did the final soldering and wiring so I could flush out most of the mistakes now, instead of after fire up.
  On the 10K node resistors, I'm still trying to decide reather to go with the original 10K or something less for more voltage. I might try the 10K's first and maybe experment with lesser values like you show in your schematic. I've got a new set of JJ's 6V6's, so I know they will take more than the old vintage ones. 

Thanks, Platefire

[sluckey]

Getting close now. But you show the wiper of the INT pot labeled as "E". I know that connects to the "BIAS" point for the output tubes. But you also have a point "E" on the SPEED pot. That can't be right.

Your reverb pot will work as drawn. However, I would connect the .003 cap to the high side of the pot and connect the 470K to the wiper of the pot. That's how Leo and Doug did it.

Other than those little points, the amp should work at first power up if you wire it as drawn. Just don't connect the 2 "E" points together.


EDIT:  FireMedic and I were communicating about changing a AB763 to cathode biased. I have drawn this schematic to illustrate what I personally would try.   with respect, Tubenit

[Platefire]

Corrected. As we say here in Louisiana, Thanks a Bunch! Platefire

FYI-I edited the schematic again 02 Sept 08. We are talking about the authinic AB763 circuit---so we also left the 220K resistor off but I do have on my layout drawing also mounted in my board(like Doug's) except he had one for each channel (two) and I only have one channel--so I used one.
Also on V3 a & b---my actural pin out is backwards from the original where stage #1 is reverb recovery and stage #2 is preamp prior to PI. So I want it shown exactly like I'm putting it together. Again if you see a problem let me know.  :) 

Ok Modified it to put 220K bias resistors in front of 1500 grid stoppers like original.
Also changed .02 cap to .003 on V3b plate coupling cap. 03 Sept 08.

Wow! I just checked my inventory and I do have some old NOS .003 polypropylene caps/600V. Good to go!

EDIT:  I added to Platefires schematic some tremolo suggestions that came out of the ARCHIVES.  With respect, Tubenit

EDIT #2: Thanks Tubenit, Looks good! I changed the 100 pF bright cap to 120pF on schematic (like AB763) and also changed it in my amp.

EDIT #3   IF someone wanted a low wattage AB763, then using cathode biased 330R/10w .......... and different rectifiers  (GZ34, 5V4 & 5Y3GT)
               and a VVR .................. one could run 6L6's, 6V6's  & 6K6's for only 8watts of power.

*  Sluckey drew up a Visio layout for a Princeton Reverb preamp into a Deluxe Reverb LTPI and power amp without vibrato.  I included a
   schematic here to go with that given it has strong similarities to a DR.   The thread is here:  http://el34world.com/Forum/index.php?
   topic=21261.0