Back in 2017 I built a vacuum tube 3 band tone control based on the design of Max Robinson over at Fun With Tubes. Max has since improved the circuit using Spice. You can read about his design on his Practical Tone Controls page. As with the original circuit the improved circuit uses two 12AU7’s and one 12AX7 which is what I had used in my original tone control. This time I used one 12AU7 and two 12DW7’s. A 12DW7 contains two sections, one section is equivalent to a 12AX7 section, and the other is equivalent to a 12AU7 section.
As always I try to use what I’ve accumulated in the parts drawer. The power supply is standard, a PC board that I had. The transformer is a 220v primary with a 200 -0- 200 secondary, and 6.3v and 8v taps. This worked nice using 120v on the primary delivering 200v across the secondary. Connecting the 8v and 6.3v taps in series provided 8v for the filament regulator when using 120v across the primary which was designed for 220v. The regulator is a bit overkill because it is designed to handle up to 10 amps output but I had the board and parts already so I used them.
I designed the board using Express PCB Plus and had a prototype made without a silk screen. Since it worked out well and I now have the Gerber files I can have more boards made with mask and silk screen. I mounted the board where the controls are not centered vertically because I had the case which was already pre-drilled. I used the 12DW7’s because it made the PC board a little easier to layout.