Deluxe Reverb Blackface mods
I have provided a schematic of an AB868 as a representative example of a Silverface DR schematic. This is the model that did not have the "Pull Boost" function. You can look at the AA763 schematic for a complete Blackface circuit of the early pre-CBS models. There was an AB763 version as well that had minor differences (see http://acruhl.freeshell.org/mga/main/aa763_vs_ab763.html for a good comparison). I will suggest a series of "baby steps" to make incremental improvements to your Silverface DR rather than trying to do a total restoration to the AB763 design. You can stop at any point when you feel the tone is to your liking. Keep in mind that there may be minor discrepancies between your particular model and the schematics provided, but the principles still apply. Now break out those soldering irons!
1. Remove those dumb 1000 ~ 1200pF disc caps that go from pin 5 (Grid) of the 6V6's to ground. Initially, just snip one lead on each cap just in case the amp oscillates afterwards and you want to restore them. Test the amp to see if it oscillates. Most likely, it will be fine. If the amp does oscillate, you can either resolder the snipped leads of the originals caps, or preferably replace them with a much lower value like 470pF and see if the oscillation goes away. If the amp is ok, snip out the caps altogether. Removing (or lowering the value of) these caps will improve the high-frequency response of the amp.
2. Change the .01uF cap that goes to pin 2 (Grid) of the 12AT7 Phase Inverter (PI) tube to .001uF like in the AB763 schematic. I actually used a 1500pF Silver Mica Here. Test the amp and see what you think of the sound now.
3. There seems to be a consensus amongst the experts out there that the Utah and Oxford speakers that came in the DR's generally suck. I would try a new Jensen or Weber speaker next. I finally replaced my Utah with a new Jensen P12N cause I got a good deal at my local music store. It sounds better, but it did not knock my socks off as I had expected from all the hype. Maybe a lower wattage Alnico would have been better. Definitely tighter sounding though.
4. The next easy mods are the 330K resistors to the Grids of the 12AT7. Change them to 1M's per the diagram below. Also change the 680 Ohm resistor that goes to the Cathodes of the 12AT7 to a 470 Ohm. At this point, you should have the equivalent output section to an AB763. My amp already had the 470 Ohm resistor.
5. You can compare the diagrams below for a complete change (not including the rectifier tube difference). I didn't see any reason to remove the 1500 Ohm series resistors to the grids of the 6V6's, or to change the 82K plate resistor on the 12AT7, so I wouldn't bother changing them. In other words, I like the AB763 circuit.
6. The next thing I plan to do is replace all the Ceramic disc caps with either Silver Micas or Orange Drops. My particular DR already had good quality Blue Spragues for coupling caps. They used ceramic discs for the three .1's and the .01 in the schematic below.
What you want to end up with is the AB763 Phase Inverter circuit and replace any crappy caps with good ones. I also plan to add precision 1 Ohm resistors from the Cathodes of the 6V6's to ground and bring them out to test jacks so I can adjust the Bias anytime without having to disassemble the amp.
I have also thought about converting from Fixed Bias to Cathode Bias to get more tube compression, but I think I'll play with this setup for a while and decide. Let me know what your experiences have been. Have FUN!!!
Silverface AB868 Output Section:
Blackface AA763 Output Section:
Blackface AB763 Output Section:
