tablebeast
Well-known member
I have wanted one of these to mod for a while, but was never willing to pay what they cost. I recently got one in trade filled with 4x 4722 input transformers and a 15095 output transformer. It is nice, clean, and totally stock. I want to do some simple mods while I restore it. All electrolytic capacitors will be replaced of course. It is actually a 1567A and I like it stock, but I think I can get more headroom and control over it with some tweaks. The mods I have planned include:
1. Install 1/4" switched jacks for Hi-Z DI Input on channels 1-4.
2. Replace ceramic caps in the EQ circuit with poly and silver mica caps.
3. Tweak the values in channels 2, 3, and 4 of the mixer.
a. Channel 1 stays stock, Channel 2 swap the 220K plate resistor for 100K and 50uf bypass cap for 470uf. Add .1uf grid cap to each.
b. Channel 3 change cathode resistor to 1K5 and bypass cap to 25uf. Change .1uf plate cap with .022uf. Add 68K grid resistor.
c. Channel 4 change cathode resistor to 820R and bypass cap to 250uf. Change .1uf plate cap with .047uf. Add 22K grid resistor.
4. Change EQ 12AX7, both sections from grid leak type with grounded cathode to 1K cathode resistor, un-bypassed.
5. Swap channel 5 line level 250K-A control for pre-master volume at end of EQ circuit, move master volume control to front of 6CG7 circuit
6. EQ bypass switch.
7. Add 1K grid stopper resistors to 6CG7 dual inputs.
8. As for replacing the electrolytics, I will go bigger as space permits.
As you can see I don't want to do anything drastic. No need to rethink the wheel. Changing the channel 5 line input pot with the 500K-A master volume pot allows me to move the 500K-A master volume to in front of the 6CG7 section and now I have three gain stage controls for fine tuning gain and distortion while keeping it from clipping! I will get a redrawn schematic up soon. If anyone has a GOOD .jpg scan of the original it would be a big help as the one floaiting around the net in PDF form sucks with some values impossible to read. So, comments, questions, suggestions?
1. Install 1/4" switched jacks for Hi-Z DI Input on channels 1-4.
2. Replace ceramic caps in the EQ circuit with poly and silver mica caps.
3. Tweak the values in channels 2, 3, and 4 of the mixer.
a. Channel 1 stays stock, Channel 2 swap the 220K plate resistor for 100K and 50uf bypass cap for 470uf. Add .1uf grid cap to each.
b. Channel 3 change cathode resistor to 1K5 and bypass cap to 25uf. Change .1uf plate cap with .022uf. Add 68K grid resistor.
c. Channel 4 change cathode resistor to 820R and bypass cap to 250uf. Change .1uf plate cap with .047uf. Add 22K grid resistor.
4. Change EQ 12AX7, both sections from grid leak type with grounded cathode to 1K cathode resistor, un-bypassed.
5. Swap channel 5 line level 250K-A control for pre-master volume at end of EQ circuit, move master volume control to front of 6CG7 circuit
6. EQ bypass switch.
7. Add 1K grid stopper resistors to 6CG7 dual inputs.
8. As for replacing the electrolytics, I will go bigger as space permits.
As you can see I don't want to do anything drastic. No need to rethink the wheel. Changing the channel 5 line input pot with the 500K-A master volume pot allows me to move the 500K-A master volume to in front of the 6CG7 section and now I have three gain stage controls for fine tuning gain and distortion while keeping it from clipping! I will get a redrawn schematic up soon. If anyone has a GOOD .jpg scan of the original it would be a big help as the one floaiting around the net in PDF form sucks with some values impossible to read. So, comments, questions, suggestions?