Hi-Fi Stereo Amplifiers
“Epona” (6EM7 Single-Ended Triode “Spud,” 2017)
I built this amp for my brother. The 6EM7 is a dual triode that was used in televisions as a vertical sweep tube (not technically a compactron because it has eight pins), but it has a following among enthusiasts online as an audio tube. It’s not hard to see why: it has a gain stage that is something like half a 12AT7 and a power stage that’s something like a 2a3 packaged into one bottle. This allows designers to build an entire stereo amp with only one tube per channel – a so-called “spud” amp. I am a big fan of this tube and have about 20 in my private store. Unfortunately, they aren’t manufactured any more, but there are always a few on ebay, although not always at reasonable prices.
This design is not particularly original but it sounds great and is relatively inexpensive to build. It has the benefit of requiring a low B+ voltage, and the wiring is simple because there are only two tubes. I use red LEDs to bias the signal triode, and bleed off some current from the power tube to keep a healthy 10mA running through the LED, so voltage should not vary according to current through the signal tube. Because there are capacitors bypassing the power cathode resistor, there’s no need to worry about fluctuating current through the LED even when current through the power tube is swinging.
Sonically, I’d say this amp is about 90% as good as my 45 amp. It has beautiful detail and realism, especially with small acoustic ensembles and vocals. Bass is actually very respectable too. Soundstaging is excellent.
I have a sort-of-joking design kicking around that uses the 6EM7 in a configuration similar to this, but in an extremely complicated circuit that has a 6DJ8 to form a mu-follower with the signal triode, and a tube regulator supplying the B+ that has a 300B as the series-pass element with a pentode gain stage and with a gas voltage-regulator tube. It’s not something I’ll probably ever build, but I like the idea.
“El Pendejo” (EL84 Triode Push-Pull, Pure Class A, 12AU7 driver, 2016)
I build this amp for a friend, and as sort of a design challenge using common tubes. My friend’s music tastes focus on electric blues and classic rock, so I thought that push-pull was the way to go for tight bass and a little more power than the single-ended triode designs I usually favor (based on loadlines, this amp should produce about 6W). The basic idea (a triode gain stage direct-coupled to a concertina) is similar to historical EL84 amps, and Morgan Jones’ “Bevois Valley,” but I wanted to run the EL84s in triode mode and no global negative feedback, for more tube sweetness at the tradeoff of less power. For coupling capacitors, I used surplus Soviet military K40Y-9 paper-in-oil that I got on ebay.
Sonically, this amp sounds about like I thought it would (although I didn’t listen to it extensively before sending it off). It has more of a warm “tubey” sound to it, and has plenty of kick in the bass, as well as noticeably more power than my usual 2W amp. Midrange is very sweet and liquid. In my brief tests, the amp didn’t have quite the same presence or detail as the single-ended amps, however.
This experience with push-pull triodes makes me hopeful for a design I’ve had my eye on for a while: the 6B4G push-pull amplifier that was used by Albert Einstein!
45 Triode Single-Ended, Direct-Coupled with 6SN7 Driver (2012)
This is the first amp I designed, and one of the first I ever built (apologies for the photos taken with the flip phone). I still use it today, with the original 45 tubes I bought on ebay. It’s inspired by the “Loftin-White Amplifier with Choke Load” described by John Broskie. My schematic doesn’t show the power supply, but it’s a basic 350-0-350 Antek transformer with diode rectifiers, and a pi filter using a 10H choke. The output transformers are inexpensive Edcor ones that I had from a previous project.
I like the fact that there is no coupling capacitor and that it uses the 6SN7, which of course is a very low-distortion tube. The drawbacks: very high B+ voltage, lots of heat dissipation from the cathode resistor, and low power. The 6SN7 steals current from cathode of the power tube, so the current demand is reduced for the circuit overall. In addition, the choke allows voltage to swing up above the supply voltage, and also acts as a constant current source, which means this is pretty much the lowest-distortion driver you could ask for. While the output resistance of the 6SN7 isn’t particularly low, it is plenty to drive the low input capacitance of the 45, and the choke has very high inductance, so this shouldn’t be a problem. As with all my hi-fi amps, I used a wooden silverware chest as the chassis.
I wouldn’t build this amp to give to someone else because of the high B+ and heat dissipation, but I think this is the best-sounding amp I’ve built (or even heard, although I haven’t heard that many tubes amps). Everything I read about single-ended triode designs and the 45 in particular is here: detail, clarity, and ghostly realism. Although two watts (or as I like to think of it: 2000 milliwatts) per channel doesn’t sound like much, this amp has plenty of power for me, and my speakers are relatively inefficient towers. Granted, I do live in an apartment.
There really is something magical about the 45.