Audio Note 6V6 Kit 4 Clone

     This is a clone of a Audio Note 6v6 Kit 4 stereo amplifier. I have wanted to build this circuit for about 5 years. It is a vast improvement over the Dynaco 6v6 clone that I had built some years ago. The tube complement is two 12AX7, two 6SN7, and four 6V6.

Audio Note Kit 4 Clone Front
Audio Note Kit 4 Schematic

       The circuit is practically identical to the original with a few changes, but of course a different PCB  layout. The ordinal circuit did not use  ultralinear transformers. I used a Hammond 159S choke in the power supply and had to tweak the dropping resistor in the DC filament supply to obtain the proper voltage.

Audio Note Kit 4 Clone Chassis

    The board is of a similar size to the original. Audio Cap PPT Theta’s are use throughout for coupling capacitors. The electrolytic caps directly in the signal path are Elna Audio SILMIC II. Power Supply capacitors are Cornell Dubilier SLP. Cathode resistors on the output tubes are Ohmite Audio Gold series. Unlike the original this board is easily removed if necessary. I used plug in screw terminal block connectors for easy disconnection. Also you will notice the orange/white plug in connectors. Those are standard power connectors used in lighting ballast connections. They are available at your local electrical supply store or Lowes and rated up to 600 volts. I used those on the ultralinear screen taps since the screw terminal block connectors would not work for that situation.

Chassis top Kit 4

      I wanted to use something different other than a standard Hammond or Bud chassis. This one came from China and for some reason it is built like a tank! The top chassis plate is around a 1/4″ thick. It was a real bugger getting the holes in it. I had to use carbide tipped hole saws then finished with a step drill bit. The silver transformer cover houses the Hammond 159S 4hy 225ma power supply choke.  The output transformers are oversized compared to what you would find for a 6V6 scenario weighing in at 4.5 lbs each. The UL taps are wound at 23%. Why? –  David Hafler and Herbert Keroes did extensive research to determine the optimal set-point for ultralinear transformers for various vacuum tubes and according to their research the optimal for a pair of 6V6 is 23%. The vacuum tubes are JJ ECC83S, NOS Sylvaina 6SN7GTB, and interestingly enough I tried some of the Chinese Apex matched 6V6GT’s from Antique Electronic Supply which actually sound nice!

Audioamp.eu autobias board

     Sometime after my original build, I decided to experiment with a fixed autobias board from audioamp.eu. The dealer for these in the USA is Erhard Audio. You can see where I mounted the autobias board. Being that I did not have a bias supply tap on the power transformer, the board I used has a built in power supply which supplies 6.3v to power the board and also the bias B+ voltage. These boards are really nice, set and forget. Once you set the bias it will auto adjust and keep the bias voltage set for the life of the tube. Also there is no need to use matched tubes because the autobias board keeps the tubes perfectly balanced. also there are LED indicators on the board which will indicate if the tube had failed. You can read about them HERE at Erhard audio. That being said, the amplifier is now of a fixed bias amplifier with a higher output and a better bottom end.

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!