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!