3 Band Vacuum Tube Tone Control V2

3 band tube tone control V2

     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.

Three Band Vacuum Tube Tone Control

        Being my ears are not what they used to be, I built a three band vacuum tube tone control. This circuit was designed by Max over at Fun with Tubes.  (A 3 Band tone control that you could love) . It is a unity gain three band control using two 12AX7’s and one 12AU7. I removed the Glassware attenuator I had originally installed in the EL34 amp and installed it in the tone control.

# band Tone Control
Tone control Schematic

Schematic as Max has designed it. you can read a description here , scroll down the page to find it. he has recently revised the schematic.

3 band tone top inside
3 band tone top inside 2

The board is single sided cut out on a CNC.

KT77 SE Stereo Amplifier

     KT77 single ended stereo project. I have to say, it sounds far better than I expected for a low power amp. I would have never believed 4 watts of tube power could sound so nice without hearing it myself. No flaws in this single ended amp. I am impressed with the amount of clean bass and clarity of the high frequencies. Being that I have never heard a single ended stereo amplifier ( well that I can remember in the last 50 years) it had been on my mind for awhile. I ended up using couple of new release Gold Lion KT77’s, Audiocap Theta coupling capacitors, Transcendar output transformers, and a vintage Olson electronics power transformer.

Chart KT77

     When figuring out some of the final values for a SE amplifier it helps to use simulation software to get in the ballpark. Using Glassware Audio’s Vintage SE Amp Cad software I was able to see the results of using KT77’s in this amp. It is a handy piece of software. You can enter in values including transformer specs to see what the scenario results would be. Trancendar’s web site listed detailed specs on the transformer which helped a lot.

KT77 stereo schematic

     KT77 amplifier schematic.  The coupling capacitors in this project are Audiocap Thets’s. I usually am happy using Mallory 150P’s, but I thought I’d try the Audiocaps. As I usually do,  the pre-amp tubes have regulated DC on the filaments.  The amp is dead quiet, nice. With only a 6.3 volt tap on the power transformer I would usually have used a voltage doubler circuit with a regulator and the pre-amp tubes in series. But, the power transformer I used was a new old stock made for Olson Electronics, remember them?  The secondary voltages were a little higher I’m guessing because it was made for 110 – 115 volts. So there was enough headroom to uses a full wave bridge and a regulator to push out a solid 6.2 volts DC for the pre-amp tubes.  The output tube filaments are running at 6.6 volts which is not as high as the 6.9v limit on the power tubes.

KT77 amplifier

     I find myself listening to Triode mode most of the time, though Ultra-linear and Pentode mode sound nice to. The tubes you see here are EL34’s which I used before the KT77’s arrived. 

KT77 chassis
KT77 chassis 2

     The chassis is a standard Bud 12×12 aluminum chassis with a dark metallic paint.  Drilling and punching the chassis is the least fun part for me. The output transformers are Transcendar  TT-336 OT.

KT77 inside chassis
KT77 se2 amp rear

Inside view of the chassis and rear view of the chassis with the usual hardware.

6L6 Push Pull amplifier

6L6 Stereo Front

      I wanted to build a good sounding 6L6 amplifier in the 20 watt range with minimal components. I believe less is more to attain good sound. With only 3 coupling capacitors involved per channel it delivers a solid 17 watts per channel at 1.5V input. Using Sovtek 6L6WXT output tubes, NOS Tung-Sol JAN 6SL7WGT preamp tubes, Edcor transformers, and regulated DC on the 6SL7 filaments.

6l6 Stereo Amplifier

     Here is the amplifier schematic.  It is based on an old AcroSound circuit. The PC boards are the same boards that I used in my 6V6 amp and were modified with a tag board posts to accommodate the AcroSound circuit. The original AcroSound circuit was a tetrode configuration which I changed to an ultraliner configuration. The original description describes a circuit that with a quality output transformer and a stable feedback arrangement permits a clean output with excellent distortion and transient characteristics. The original designed load line required a AcroSound TO-280 9K output transformer. I substituted an Edcor 8.6K which worked nicely.

6L6 chassis 1

     The chassis is a standard Hammond aluminum chassis.  After gathering your components you can layout the chassis by leaving the protective peel off plastic on. I use a fine tip permanent marker to layout the holes.  You can mark your holes for drilling with a spring loaded center punch.  I make the larger holes with a vintage radio punch set which does a nice job on the transformer wiring holes and tube socket holes.

6L6 inside chassis

 Inside view of the chassis. The power supply sits in the middle.

6L6 chassis 3
6L6 chassis side
6L6 Chassis Rear

EL34 vacuum tube amplifier

      El34 Push Pull Stereo Amplifier. I used Edcor CXPP-25-8-5K transformers, Hammond 159T choke, TAD EL34B STR Premium tubes, two 12AU7’s and one 12AX7 while implementing fixed bias, a Rubli high voltage turn on delay board, and regulated DC for the pre-amp tube filaments.

El34 Stereo Amplifier
El34 Schematic

     Here is the amplifier schematic.  For each channel, half of a 12AX7 provides signal for a 12AU7 long tailed pair which provides drive for the EL34’s.  You can use your favorite schematic, customize an existing circuit as I often do, or design your own.

El34 Power Supply

     Power Supply.  Sometimes I use off the shelf parts and PC boards when building.  The power supply is a Glassware Audio PS-3 board with a few component changes. A Rubli high voltage delay board turns on the HV after the filaments warm up.  I had got the Rubli board to experiment with, This amp can have over 525v available until the tubes start conducting. It just makes sense that it will put less stress on the tubes on warm up. In a fixed bias amplifier with a high voltage delay, you can be sure that proper bias voltage is up before HV is applied to the tubes.

El34 chassis

      The chassis is a standard Hammond aluminum chassis.  After gathering your components you can layout the chassis by leaving the protective peel off plastic on. I use a fine tip permanent marker to layout the holes.  You can mark your holes for drilling with a spring loaded center punch.  I make the larger holes with a vintage radio punch set which does a nice job on the transformer wiring holes and tube socket holes.

El34 board top
El34 board bottom

     Sometimes I use PC boards. But you can use point to point wiring just as easy if you choose, or have a PC board made from your design. You also can purchase one of the pre-made boards as those such Pete Millett or others.

El34 inside chassis

      Bottom of the chassis.  You can see the Mallory 150p coupling caps, I like them, how they sound. PS-3 board is to the right. The Rubli board is mounted to the right out of sight to the right. Bias test points are located on the chassis at the rear of the PC board.

El34 chassis top 1

     Top of the chassis before the PC board was installed. You can see the holes for the bias adjustments near each EL34 socket, and the bias test points between the output transformers. Power transformer, power supply choke, and a separate filament transformer for the regulated DC pre-amp filament supply.

Ivan the cat inspector

And of course Ivan the cat inspecting my work!

Ivan napping after inspection

Satisfied with my work, Ivan takes a long nap!

Sherwood S-5000 II Stereo


S-5000 II stereo amplifier

     This is a great Sherwood S-5000 II amplifier which uses 7868’s for the outputs. Beware if you find an amplifier that uses these tubes, you need to replace the sockets unless you find vintage tubes. The new 7868 tubes have much thicker pins and you will damage your expensive tubes trying to insert them into those older sockets designed for thinner pins.


S-5000 II stereo amplifier top

S-5000 II stereo amplifier tubes

S-5000 II stereo amplifier top 2