I have done a bunch of semiconductor-based circuit design. I actually design semiconductors for a living and until I started the design of this Spud, I had never dabbled in vacuum tube technology. As all beginners I figured I’d start big. Ya’ know… At least 50 W per channel, push-pull, etc., but after reading up on tube technology, I decided to start in the kiddie end of the pool. A couple of realizations led to this decision. First off, it generally pays off to familiarize oneself with the pitfalls of simpler circuits before going all in. Secondly, I did the math on how much power I actually needed in order to create a reasonably loud music playback experience and ended up concluding that a couple of watts would be sufficient. Subsequent measurements have revealed that for critical listening, I tend to want to turn the volume down when the peak output power approaches about 1 W even with my 87 dB/W*m efficient speakers. So I settled on designing an amp that would provide a few watts of output power, that would be possible to build with parts available at major electronics component distributors (well, except the tubes), and that didn’t cost the monetary equivalent of my first born child. Soon The Spud was born.
The amplifier design below deals with high voltages — LETHAL voltages. If you are not comfortable or qualified to deal with these potentially deadly voltages, please do not attempt to build this circuit. The design is provided as is with no warranties or service agreements whatsoever. It is provided in the spirit of DIY and may only be reproduced for non-profit purposes. Proceed at your own risk, expense, and responsibility.
A 6LU8 TV horizontal sweep tube is about perfect for a small audio amp. It a Compactron that contains a triode and a pentode in one glass envelope. The triode makes a fine grounded cathode input stage, the pentode a good output stage providing about five watt when operated single-ended in ultra-linear mode. The schematic for one amplifier channel is shown below. The output transformer — an Edcor XSE15-8-5K — hooks up to J1. OUTP and OUTN marks the positive (P) and negative (N) connections to the secondary. I chose to use cathode feedback from the secondary of the OPT as this provided lower distortion. The input to the amp is connected to J2.
The power supply is fairly straight forward. I chose a 5AR4 rectifier tube because of its long warm-up characteristic. This provides a bit of soft-start for the circuit. An N-channel MOSFET forms a ripple filter. All circuit ground signals meet at the star ground of the power supply. The power transformer is an Antek AN-1T250. At the time of writing, it looks like the AN-xxx transformers are being phased out in favor of shielded AS-xxx versions. The AS-1T250 should work just as well. Just ground the shield. The 0.68 ohm resistor, R22, is used to drop the 6.3 V from the transformer to the 5.0 V the 5AR4 requires. A different way of doing this would be to remove a couple of turns on the secondary winding used for the rectifier tube. The LED connected at J6 indicates that the amp is on.
The amp was built on a piece of scrap PCB material that I fabricated using toner transfer. A 100 kOhm ALPS blue pot was used for the volume control.
The chassis is black painted 1.6 mm aluminum on walnut. The wood is treated with a satin finish clear coat. I use a sheet of perforated aluminum for the bottom cover. All metal surfaces are grounded for safety reasons.
Amplitude response at 1 W into 8 ohm resistive load.
The low-frequency response is limited by the bandwidth of the small Edcor OPT’s. But considering that those transformers will set you back a cool $20 each, that’s still incredible bang for the buck. The distortion performance is also rather less than stellar…
As one would expect from a pentode output stage, the distortion is mainly 2nd ard 3rd order harmonic distortion with some of the higher order distortion products present.
Despite the high distortion at LF, the 6LU8 Spud actually sounds quite good. The bass is a bit muddy but the highs and midrange are really quite nice. It’s not precise or sterile as a high global negative feedback semiconductor amp, but it sounds good. As a friend of mine noted, “it’s not precise, but I like the presentation”. Metallic instruments sound metallic – not fizzy. Human voices sound unstrained and natural. The output transformer is the clear limitation of this circuit. Actually, I tried the bigger $80 Edcor CXSE25-8-5K transformers with this amp. That combination really rocks! The bass tightens up considerably and there’s much more detail in the highs. Those transformers also about doubles the cost of the amp…. Tradeoffs, tradeoffs.