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.