ODDWATT - ECC802S SRPP / EL84 PUSH-PULL TUBE AMP
Bruce Heran 
Page 1: Introduction, Schematics
Page 2: Tube Amp Construction, Measurements, Listening
INTRODUCTION
This project is a bit of a departure for me. I really didn't need another amplifier as I have lost count of how many are around the house, perhaps as many as 15 or 20. This project was an exercise in what would happen if I combined some interesting concepts. I first saw this type of output circuit on diyparadise and if mine looks a lot like that one, it is because it is similar. So why did I call it Oddwatt? Well at first glance it looks like it shouldn't work or at best work poorly. To my surprise, it not only works, it works well!
Photograph 1 - ECC802S SRPP / EL84 Push-Pull Tube Amplifier
CCS OUTPUT STAGE
The following is a simplified description of the output stage. Tubes conduct current in response to changes in the voltage between the grid and cathode. In order to establish a basic flow of current through a tube, a voltage is applied called bias. For power amplifier tubes cathode bias and fixed bias are the most common. Cathode bias is accomplished by raising the cathode voltage above ground and keeping the grid at approximately ground. For example, a tube requires -10V of bias to conduct 50mA, then the cathode can be raised to +10V with the grid at 0V. This is done with a resistor in the cathode circuit. Following Ohm's Law, 10 (volts) = 0.05 (amps) X R (ohms). So R is 200 ohms. In fixed bias circuits, the cathode is frequently at ground potential and the grid is negative. This is accomplished usually by feeding a negative voltage to the grid through a relatively high value resistor. Both cathode and fixed bias is used in high quality amps. In the Oddwatt, I use a constant current source (CCS) in the cathode circuit. A CCS regulates the current flow very closely. It allows the voltage to vary while keeping the current constant. With this in mind, here is how the Oddwatt works. With both cathodes essentially tied together, each sees the same cathode voltage. The CCS allows the cathode voltage to vary and maintains the current constant. So with the one grid at ground potential (through the 47 ohm resistor) and no signal on the other grid, both tubes will conduct an equal amount of current. If you apply a signal to the driven tube, the difference between the cathode voltage and grid will change. As a result the tube will either conduct more or less current following the signal voltage. If it conducts more the CCS will raise the voltage on the cathodes and thus increase the bias on the second tube. If the first tube has a negative signal and conducts less current, the CCS will reduce the cathode voltage and the second tube will conduct more. It is a sort of see-saw arrangement. The only name I could find for this type of circuit was "common cathode impedance, self balancing inverter" (CCISBI). It is an old design that I suspect fell into disfavor as quality performance requires a closely regulated current through the tubes. This would have been difficult and expensive in the early days of hi-fi. It will also only work for class A amps. It is possible to implement the circuit with a fixed cathode resistor, but the results are probably going to be unsatisfactory. Additionally, the cathodes can not be bypassed as it will upset the signal balance. There are however advantages. There is only one coupling capacitor. The drive requirements from the previous stage are half those of more common designs. There is at least one fewer tube section needed. The circuit according to the literature retains the low harmonic distortion levels of other more common push pull designs. I could not follow the math so I will have to accept it on faith.
Figure 1 - ECC802S SRPP / EL84 Push-Pull Tube Amplifier Schematic
EL84 PP DESIGN CONCEPTS
Since this amplifier project was an experiment, I wanted the design and not the components to influence the sound quality. Thus, quality parts are used throughout. EDCOR transformers, Ruby electrolytic capacitors, Auricap coupling capacitors, matched tubes ... I suspect that good results can also be achieved with less expensive parts. Another feature I wanted to try was an SRPP driver stage.
Tube Amp Features:
- DC heater supply by a 12 volt SMPS.
- Ultra linear operation of the output stage with Sovtek EL84 tubes.
- SRPP driver stage with ECC802S tubes.
- Provision for tube balancing.
- Constant current sources for the output stages using LM317 Regulators.
- Relay control of the B+.
- Minimum component count in the amplifier stages.
TUBE HEATER POWER SUPPLY
The heaters are supplied with a regulated DC supply provided by an off the shelf switch mode power supply (SMPS). The SMPS is rated at 12VDC / 3A. The output is quite clean and even though I used tube heater snubbers, I probably could have omitted them. I did not see a significant amount of trash or noise on the 12VDC output. Each channel was wired for 6V heaters. The center point was returned to the main system ground. This made one set of heaters positive to the ground and one negative. I could not identify any adverse results of the arrangement. The SMPS also provides power to the indicator lamps and B+ switching relay.
HIGH VOLTAGE POWER SUPPLY
The B+ supply is rather straight forward. Two fast recovery rectifiers working off a center tapped 180-0-180V, 250mA EDCOR transformer. Filter capacitors are Ruby electrolytic bypassed by Solen polypropylene. An extra pair of electrolytic capacitors are used for the SRPP plate voltage connection. This arrangement cleaned up a small amount of ripple from the B+ supply. I tried series resistors (1k) in front of the filters and it didn't seem to make any difference, so I removed them. The B+ to the outputs measured 240V and 200V at the SRPP plates.
Figure 2 - EL84 Push-Pull Tube Amplifier Power Supply Schematic
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