Ron's Audio
                                             Sound for the regular guy


EL34 Stereo Amplifier


    El34 Push Pull Stereo Amplifier. I built this one for myself. Edcor transformers, Hammond choke, TAD EL34B STR Premium's, two 12AU7's and one 12AX7.  Fixed bias, high voltage turn on delay and regulated DC for the pre-amp tube filaments.


    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.


    Power Supply.  I love off the shelf parts an 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 wasn't worried about so called cathode stripping as I don't buy into that idea on a amplifier in this wattage range.  I have read that it is an issue on high powered transmitters and such.  I had got the Rubli board to experiment with to see for myself, so I installed it. 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.  But the choice is yours when building.  Someone had asked me since you don't buy into the idea of cathode stripping, why the high voltage delay board?  Consider bias voltage.  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.


Here is a scope shot taken a t 1000hz just before clipping into an 8 ohm load. 52 wrms - 20.5 vrms and 111 wp-p at 58vp-p output. But that output is not entirely true as it was only one channel driven. I need to get another dummy load.


    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 PP board top
EL34 PP board top
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I like using 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 with ExpressPCB, Robot Room Copper Connection etc. You also can purchase one of the pre-made boards as those such Pete Millett or others.


    Bottom of the board.  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.


   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.


And of course Ivan the cat inspecting my work!

Satisfied with my work, he takes a nap!


Guard cat

Messy bench