• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

help with "bruce heran" el84 amp S2


2010-11-14 11:49 pm
My first project is finished Heran EL84 PP (Oddwatt S2) amp. I'm only getting a half wave amplification on the output. I tried switching the output leads as suggested with no change. Looking at the schematic the upper EL84 is conducting while the lower one is not. Pin #2 on the upper tube has the input from the 5751 @16vac to ground, pin #2 on the lower tube has no voltage. All other voltage measurements appear to be the same. I tried alternate tubes, no change. Any assistance would be appreciated. I'm very new to this hobby.

Regards, Richard
OK - from your description it seems that one side (bottom) is not working.
The odd watt design drives this side from the cathode.
1) That the pin 2 (grid 1) is definitely tied to signal ground (0V) via the resistor.
2) That you have high voltage to the anode of that tube.
3) That you have high voltage to the screen (pin 9) of that tube (from the Ultralinear tap).
4) That the cathode of that tube is sitting at approximately the same DC voltage as the upper tube cathode.
I think you will find one of the above is not correct and that will point you in the right direction for fixing the amp.
CAUTION: If the bottom tube is not conducting current then the top one will conduct twice its normal current to compensate - that could cause over heating of that to tube.

Also check the balance pot in the cathodes is not open circuit at one end. Checking that the 2 cathodes are sitting at close to the same DC voltage (with no signal applied) will check this.

Just some quick notes - hope it helps.