This amp is near perfect and there's not much discussion necessary to explain why. It's a differential design with no blocking caps and few parts and no feedback and no tricks. A constant current diode completes the recipe. The few Class A watts it makes are flawless. The output tube is a bit unusual in that it is cheap and no one uses them, but hey, we ain't slaves to fashion are we? With efficient speakers you will not ever need an amp like the 211 monster previously mentioned. This is my own personal idea of the best way to make a great amp. Top Shelf! (it sure don't look as cool as a 211 amp though)
6B4-G Dual Diff Driver Push-Pull
15 Watts at 5% HD Class AB1 O'PT XFMR: 3.5K - 7K Primary
This amp has all of the virtues of the previous circuit plus power to drive many more kinds of speakers. As you may know, any of the 2A3 type family (2A3, 6A3, 6A5, 6B4-G) and type 45 or WE275, can be substituted in the output stage provided that tube socket, filament voltage and bias voltage differences are taken into account. These are available in most tube manuals. 6B4-Gs seemed like a good choice because they use an octal socket, have a 6.3 volt filament, and there are quite a few NOS still around. There are, however,chinese 2A3's available for very reasonable prices. (And Sovtek is now producing excellent single-plate 6B4Gs - dave) Also, if you've got the patience to track down the rare tubes (and the money!), good 45s in this amp could give those geeks down at the audio club apaplexy (not to mention the uncontrollable urge to toss their own equipment from the nearest high place). Within the power band, all of these triodes will slay your basic (Mullard type or Williamson type) ultra-linear or pentode tube amp. Marantz 9 fans or Mac 275 lovers better run and hide.
807 Push-Pull Ultralinear
20 Watts at 0.1% HD Class A Tetrode OPT XFMR: 5K -7K
This is an interesting amplifier that uses a clever combination of subtle tricks to get tremendous performance from a simple design. The mix of positive and negative feedback with screen grid feedback, the cross-coupling of the transformer secondaries with the cathodes of the output stage higher current/lower voltage operating point insure wideband linear amplification, low phase shift, and low output impedance. 807s are a strangely overlooked output tube. Why, I really don't know. So many crappy amps have been made with 7868's,8417's,6LF6's,KT90's,KT-99's, and KT-100's etc.; i.e. really weird tubes (Barkhausen plates anyone?) The 807 on the other hand has no quirks other than its simplicity and reliability. It has class A operation good to the low mhz range and a plate dissipation of 25 watts. The tubes are plentiful and inexpensive. What more can one ask of a beam tube? This amp will hold its own against any "high end" pentode amplifier regardless of cost.
845 Push-Pull
30 Watts at 0.8% HD Class A OPT XFMR: 5K - 7K (50 watts)
Once again, don't even think about building this one unless you've got a script for Prozac. Family and pets do not combine well with this device. Got it covered? The combination of 845s and a transmitting type power supply make for a very serious amp option. If you use electrostatics or Strathern-type ribbons (moderately efficient) this circuit will drive them to acceptable levels. You must insure that the insulation of the output tranny will withstand the 1000 volt B+. The Tango XE-60 5P is a perfect candidate. Otherwise, consult your transformer supplier as to what is appropriate. The place to find the right construction techniques for this amplifier is the ARRL Radio Handbook on transmitter construction. Follow this advice to the letter and you will not only enjoy one of the most wonderful amplifiers, but also live to listen another day! These big tubes do allow one to get both the high power of pentodes with the sophistication of triodes, but at a certain cost. Expect to spend alot of money and care building and maintaining this sort of thing. The chinese 845s are available for a very reasonable price. They also sound great. The components you choose must allow for the stress of high voltage DC and AC. Caddock and Mills manfacture wonderful resistors for this purpose. The capacitors will also be heavily stressed so get ready to spend your money on the good stuff.
