Introduction:
After the success and sale of my single-ended 1625 amplifier, for my next amplifier build I decided to return to the venerable triode. However, I wanted a bit more power than your average single-ended 2A3 amplifier, but still didn't want to use the pricey 300B. With that in mind, I looked at several push-pull designs using the 6B4G or 2A3. Once again I ran into expensive tubes, even for Russian or Chinese versions, and pricey interstage iron. However, after reading several threads on diyaudio.com, I came across Thomas Mayer's blog describing the use of the 6CB5A - a TV vertical deflector tube - in single-ended mode to make 7Ws. He touted it as a budget alternative to the 300B, and at ~$5 to $6 a pop on Ebay, it certainly would cut down on the output tube cost.
Circuit:
Once I had the signal schematic in hand, I went and designed my own power supply circuit around a Edcor power transformer. Note that the 6CB5A uses a mighty 2.5As of current at 6.3V. This required an additional filament transformer for the 6N7 driver tubes, and, in order to reduce costs, I stuck with a 5V rectifier tube since the filament tap was available instead of the recommended TV damper diode which would require an additional transformer..
Parts:
Once again, this is a budget build, so Mouser and Allied Electronics were used heavily. This means electrolytics used in series to get a high enough voltage rating, Cornell-Dublier metallized polypropolene coupling capacitors, plastic speaker binding posts, and plenty of KOA resistors. Output transformers are 3.5K/6-ohm units from Edcor. The power transformer was also sourced from them. For the top plate I used Front Panel Express. Wood chassis is from Valab, an Asian Ebay seller.
Construction:
This was an exercise in point-to-point wiring, which can be frustrating if one is not experienced. A single star ground is used near the RCA input jacks. Wiring is all plated solid-core. Due to the simplified power supply and lack of regulation, this unit was much easier to finish than the 1625 amplifier, my last project.
Listening:
After a quick voltage and current check, I hooked this amplifier up to a pair of Pioneer BS-21 speaker that I use for test purposes. At 84dB efficiency, these aren't exactly a great match for single-ended amplifiers, but at least I can tell if music is being made. The sound, via my VPI table and Quicksilver preamplifier, was very smooth without any noticeable hum or other noise. With my medium-output moving coil cartridge, I had to turn the volume control way up to get any decent playback level. Of course such a low gain amplifier will prove to be useful for much more efficient speakers than the test Pioneers units.
After everything checked out, it was time to listen to the 6CB5A amplifier with the KEF iQ30s. Once again my big UREI 813As, which would be a much better match, are in storage so I had to make due with what is on hand.
This amplifier is smooth, coherent and almost touches the better 300B amplifiers I have built. The soundstaging in uncluttered with a deep and immersive soundfield - all traits of a triode amplifier. The bass was also surprisingly well-controlled, lacking that fat underdamped sound that plagues some tube amplifiers. Treble extension was smooth with a nice shimmer and swirl to high hats. Detail was also very good, providing plenty of definition - eg, recordings sounded different and weren't congealed into a "it all sounds the same" blob.
Switching to the less efficient B&W 805s and I noticed the amplifier would run out of steam on big peaks. Obviously not a good match, but at lower listening levels it was quite the pleasurable listening experience.
Conclusion:
Single-ended amplifiers - even using triodes - require careful speaker matching. Many speakers are too inefficient and have wild impedance curves that require more power and low output impedances. However, the 6CB5A coupled to the Edcor iron did a decent job driving any of the speakers I have on hand. Those into rock or heavy orchestra would benefit with more power or horn speakers, so keep that in mind if you're interested in building something like these amplifiers.
