The Fi Primer: Single-ended Amplifiers
6L6-GC Single-Ended Ultra Linear Amplifier
10 Watts at 3% HD; OPT XFMR=2.5Kohms - 3Kohms
This is a great little amplifier that breaks all the rules. Not only is it single-ended, but its also a beam tube and incorporates the dreaded devil's negative feedback... both in the screen circuit and around the transformer! Still, I'd wager that many serious listeners would love this amp and it puts out twice the power of a single 2A3. The trick here is in the output transformer. You've got to track down a tranny with a screen grid tap; i.e. 43 - 50% of the primary stack. The Tango U-808 has this as do some of the Audio Note transformers. You can also have Michael La Fevre of Magnequest make you a DS-025 with a screen tap (order it as DS-025UL) which should make this amp a little giant killer. An EL-34 can substitute for the 6L6-GC for those who just can't seem to live with the idea of a "guitar amp tube" for an output device. But, trust me on this one, this amp is sweet!
2A3 Sinzle-Ended Loftin-White / Shunt Regulated Driver
3.5 Watts at 2.5% HD; OPT XFMR=2.5Kohms - 3.5Kohms
That obnoxiously wordy circuit description does no justice to the elegant simplicity of this little triode amp. This direct coupled (coupling capacitor-less) two stage circuit was designed as a laboratory amplifier in the 1930's by, you perhaps guessed it, the team of Loftin and White. A level of performance was reached with this design which stood for many years after its initial appearance. Output transformer quality and transducer accuracy were the only important limiting factors in its fidelity and usefulness as a measuring tool. The "totem pole" driver stage is not 30's vintage, however, but actually somewhat later (there's some controversy as to when the shunt regulated push-pull stage first appeared). The combination of the two is, in any case, an interesting topology and one can learn some high octane tricks from this arrangement. The two important things to study in this circuit are 1) how the bias for the output tube is provided by the voltage divider created between,the driver stage load (the upper half of the 6SL7), the driver stage (the lower 6SL7), and the extra-large cathode resistor(the series resistors below the 2A3), and 2) how high gain,, wide bandwidth, and low source impedance (these things are normally somewhat mutually exclusive with "single stage" tube circuits ... ) are achieved with an absolute minimum of parts with the SRPP 'front end. While the SRPP stage remains somewhat unknown locally, in comparison to its "totem pole" cousins "the Cascode" and the "Mu Follower", it is extremely popular in Japan and Europe. It is my opinion that none of these circuits (SRPP, Cascode, Mu-Follower) possess the resolving power of a simple plate loaded triode, and it is important to understand that these circuits are actually two stage topologies (two active devices), but in the name of simplicity and musicality the SRPP stage has a solid place in audio. This amp, by the way, is anything but a nostalgic 30's whim. Connected to an appropriate speaker, such as Dr. Bruce Edgar's horn designs, you can expect to humble, humiliate, jibe and cajole your "high-end" ego counterparts on the audio scene, or, just listen blissfully.
300B Single-Ended
8 Watts at 2.5% HD; OPT XFMR=3Kohms - 3.5Kohms
Okay, let's get serious. You've got nothing left to lose, and there's nothing left in your way, all out fanaticism feels like a wimpy cop out and you just want to build one amplifier and forget about amps forever more??? Dahling, this one's for you.
This amp is the quintessential triode amp. It just does its job, period. It is honest and accurate and sweet and will drive a surprising number of commercial speakers to acceptable levels. If you use the best available parts and materials in constructing it (as well as stash away a few tubes for a rainy day), it will pass down to your children and continue to wow 'em in the next millenium.
It is not necessary to use Western Electric 30OBs. The Chinese and American (Cetron) current production sound wonderful and sell for a third, or less, of the cost of a WE. Westerns are for snobby collectors and don't believe a word to the contrary. I'm so tired of these pack rat types (also known as collectors) preaching the mysteries of non-obtanium and hoarding old things. Let 'em keep it, turn up the music!
A direct coupled driver stage and an all low mu triode array are the specific reasons as to why this design works so well. Good transformers are the key to really outrageous performance. I recommend Michael LaFevre's (Magnequest) FS-030 outputs or Tango XE-60S outputs. Remember, the iron will decide, no antique tranny or scramble wound bobbin is going to do the rest of this design justice. There are a host of mods that can be worked into this project if you apply a bit of imagination. Possible tube substitutions might include the 56 or 76, 6DJ8 and 6CG7, 6L5 and 6AH4. A touch of cathode negative feedback around the ouput transformer secondary and a separate power supply for the voltage amplifier and driver stages for the ambitious tweak. All of these things can expand the theme, but build it just as it is drawn and you will have no complaints.
211/VT-4 Single-ended
18 Watts at 4% HD
Pay close attention now, this is not a project for a beginner or the faint of heart. This is the most challenging circuit included in this collection. The power supply relies on lethal voltages and the circuit is relatively complex in set-up and execution. But, if you have the big tube bug, and possess an obsessive nature, this will get it into your system in a big way. I should not forget to mention that simply desiring an amp like this is a clinical dysfunction. You will have to put a cage between it and your significant other, precious animals and small children. If you are clumsy, spaced-out or unmechanical, just forget about it, buy concert tickets instead. In other words, this amp is not practical and should be considered a nefarious competitor to everything else in your life, got me? Okay sickies, here's the deal...
This design enjoys total cult status in Japan for use with Quad electrostatics, both the ESLs and the ESL-63s. The "Ongaku" is simply a tube and parts mod of this exact circuit. In terms of the "Ongaku", I am not a fan of the 12AY7 (which it uses in front) and instead much favor the 6SN7. However, I'm not either at liberty to complain about the use of transmitting triodes and silver wired trannies! Still, I believe the basic design has a great deal going for it. Each stage is configured differently: an SRPP gain stage, a direct-coupled plate loaded gain stage, a cathode follower and transformer-coupled output stage. This apparently complementary mix of topologies has enduring appeal in Japan where it is the most popular 211 circuit and has been for twenty years. There are many versions but this one (with 6SN7's) is the seminal form. This power supply has the slow turn on trick with the 5AR4 which also is why I included it.
