6CY7 Single-Ended Hi-Fi Stereo Amplifier

6CY7 Single-Ended Hi-Fi Stereo Amplifier

A Exercise in Reuse

This amplifier came about due to the rebuild of a headphone amplifier which I was no longer using. While looking at what to do with the old amplifier, I realized that the power transformer used for the headphone amp could be reused for a small stereo amp. The power transformer is an older Hammond 269EX with a 190-0-190 @ 65mA high voltage winding and a 6.3v @ 2.5A filament winding. As it turned out, this is exactly what was needed to build a circuit using the 6CY7 tube which I had prototyped a few years before using that very transformer.

Photograph 1: Single-Ended 6CY7 Hi-Fi Stereo Amplifier

Electrical Design - 6CY7 Stereo Amplifier

This amplifier is based on a type of tube not usually considered for audio amplifiers. 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).

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.

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.

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:

f -3dB =1/2π⋅C c ⋅R eq

where: f -3dB is the low frequency roll off in Hz,

C c is the value of the coupling capacitor in Farads,

and R eq 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.

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.

Power Supply Design - 6CY7 Stereo Amplifier

The power supply is a typical split rail design with separate filtering for left and right channels. This helps provide good channel separation and a nice wide sound stage.

Figure 5: Power Supply Schematic - 6CY7 Single-Ended Triode (SET) Stereo Amplifier

Please note that the capacitor immediately following the 6CA4/EZ81 vacuum tube rectifier must never exceed 50µF.

For more information about audio amplifier power supplies, see my notes regarding power supply design for vacuum tube amplifiers.

Warning: This vacuum tube amplifier project uses high voltages. Contact with voltage potentials of this magnitude can cause serious injury or possibly be fatal. If you do not know how to build high voltage vacuum tube projects or you are not comfortable with projects that use these voltage levels, it is strongly recommended that you do not build this vacuum tube amplifier. Follow your governing electrical codes for all wiring and connections.

The Build - 6CY7 Stereo Amplifier

The build for this design needed to proceed along a little bit different lines as the chassis was already built. The new design needed to make use of the current wood chassis layout as this was not going to change. A new top plate was in order because of the different number of tubes, the headphone jack went away and was replaced by a power indicator, and a new rear plate was made to accommodate the speaker binding posts. I also decided that I wanted a nice inductor up top to match the power transformer, so I ordered a Hammond 193H for the primary filter choke. I would be reusing the power transformer, IEC connector, fuse and holder, power switch, tube sockets, input jacks and volume control, 2H filter chokes, and the handles on the top plate.

This level of redesign meant that I only needed two new pieces of metal. These are shown in the picture below, just before painting.

Photograph 2: Metal Chassis Parts - 6CY7 Stereo Amplifier

The only real challenge to this design was to decide how to mount the audio output transformers. I decided to use Edcor open frame XSE10-8-5K transformers and I needed to mount them both inside the chassis. This turned out to be not too difficult. After this decision was made, it was time to start wiring. I decided that I would do as much wiring as possible with the top plate removed from the chassis. This way there is plenty of room to work and seeing things is not a problem. Here is an in progress picture of the top plate wiring.

Photograph 3: Top plate wiring in progress

Here the individual components for each channel are mounted to the sockets and the primary portion of the power supply and filter are mostly wired. Wiring in this fashion is significantly easier than doing it with the plate installed on the chassis. Here is an inside shot of the final amp all wired up.

Photograph 4: Inside Wiring - 6CY7 Stereo Amplifier

The two filter caps on the upper left are the individual 100μF capacitors for each B+ channel. They are suspended in mid air over (or under) the 2H chokes.

Testing the 6CY7 Stereo Amplifier

So this amp went through the typical test regiment I give most of my new builds. Here it is all hooked up and in progress. The signal generator is on the left, the dual channel AC voltmeter for checking response is in the middle, and the oscilloscope for checking waveforms is on the right. The unit right to the right of the amp is a dummy speaker load.

Photograph 5: Testing - 6CY7 Stereo Amplifier

At full output the 6CY7 amplifier produces a clean 1.3 Watts output per channel into 8Ω. The Edcor XSE audio output transformers are rated from 70Hz to 18kHz. This amplifier did a little better than this having a -3dB bandwidth from about 46Hz to about 30kHz. Here is a plot of the frequency response measured into an 8Ω load.

Figure 6: Measured Frequency Response - 6CY7 Single-Ended Triode (SET) Stereo Amplifier

Impressions - 6CY7 Stereo Amplifier

I still can't get over how good and how loud this amp sounds. It handles everything from classical, to blues, to choral music equally well. And the sound stage is awesome. I will never go back to building common power supply filters for both channels ever again. I've had this amp plugged into my system on and off for the last couple of weeks and I am absolutely blown away by the quality of the sound coming from this little tube unit. I don't know what it is, but these little triodes are just the best sounding tubes I have ever heard.

Photograph 6: 6CY7 Single-Ended Triode (SET) Stereo Amplifier

This is quickly becoming my favorite amp. I currently have a set of NOS Sylvania tubes in it, but I have also used a set of old used GE tubes which sound just as good. This is really a pretty inexpensive build. I highly recommend that anyone looking for a little iPod or computer amplifier give this one a try. You will NOT be disappointed!

Comments and questions about this DIY amplifier build are welcome in the 6CY7 Stereo Amp thread.

About the Author

Matt Renaud is a career Electrical Engineer with over 25 years experience in advanced radio frequency system design. He has a passion for tube audio and teaching tube based circuit design to others through participation in tube audio forums and his own website, www.CascadeTubes.com. The majority of his designs are fully original and may be found on his website and on the DIY forums. Here are a some DIY audio projects and articles contributed by Matt: