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This amplifier is based on a type of tube not usually considered for audio amplifiers such as this one from TI:http://www.kynix.com/Detail/99290/TPA2012D2%20EVM.html. Instead of using a typical audio preamplifier tube and then an appropriate power stage, this amp uses one of the vertical deflection dual triodes built for televisions; the 6CY7. These tubes were designed for the deflection systems of cathode ray picture tubes used in vacuum tube televisions. Typically the tubes contain one high gain signal stage used as an amplifier or oscillator and one power stage used to drive the CRT deflection coils. This combination is exactly what we need to build a complete amplifier channel from a single vacuum tube (per channel).
Tube Pinout Diagram - 6CY7 Vertical Deflection Double Triode

Figure 1: Tube Pinout Diagram - 6CY7 Vertical Deflection Double Triode
This particular tube has a power section with a peak plate dissipation of 5.5 Watts. This should translate to a little over one Watt per channel. As I said above, I had prototyped this amplifier a few years before while exploring alternate tubes for audio amplifiers. So I went back and revisited the load line designs I had done at that time. Here is the load line for the power section.
6CY7 Plate Load Line Design - Section 2

Figure 2: 6CY7 Plate Load Line Design - Section 2
This indicates a peak power into the output transformer of about 1.6 Watts using a 5kΩ load transformer and a 1kΩ bias resistor. The distortion also looks well controlled. I need about 29V-peak to drive this power stage. From a line level signal, that means that the driver needs a voltage gain of at least 14v/v or ≈23dBv. Given the nature of the 6CY7 section 1 triode, this should not be a problem. The published amplification factor is 68 and we should be able to get at least 2/3rds of that or ≈45 v/v. The driver stage load line for the 6CY7 section 1 triode is shown below.
6CY7 Plate Load Line Design - Section 1

Figure 3: 6CY7 Plate Load Line Design - Section 1
The linestage uses a 100kΩ plate load and is self biased with a 1.5kΩ resistor. This design shows that the driver stage will meet both the gain and voltage swing requirements without a problem. Because of the gain level, I decided to leave this stage unbypassed to control the overall gain.
The interstage coupling capacitor is sized for a low frequency rolloff based on the output impedance of the first stage and the grid resistance of the output stage. The relation used is:
where: f-3dB is the low frequency roll off in Hz,
Cc is the value of the coupling capacitor in Farads,
and Req is the sum of the driver stage output impedance and the power stage grid resistor in ohms.
Using an interstage coupling capacitor of 0.01μF and an equivalent resistance of 1,036,700Ω (1MΩ + 36.7kΩ) gives a low end rolloff of 15.3Hz. This is more than sufficient given the output transformers I've chosen to use. Here is the final schematic for the amplifier.
Schematic - 6CY7 Single-Ended Triode (SET) Stereo Amplifier

Figure 4: Schematic - 6CY7 Single-Ended Triode (SET) Stereo Amplifier
This is a very simple audio amplifier circuit that should be very easy to drive. With the volume control at max, it requires about 1.2v peak (or 0.85v-rms) to drive the amplifier to full power. Most small devices such as iPods, smart phones, and portable CD players will easily drive this 6CY7 amplifier.
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