This story documents the design and construction of a pair of single-ended Class A monophonic tube audio amplifiers, including some discussion about the design decisions that were made along the way.
I built these amplifiers the traditional way. They contain no solid state devices of any kind. They have no transistors, FETS, integrated circuits, silicon diodes, microprocessors, LEDs or any other solid state components. There are no printed circuit boards either — all of the wiring is point-to-point and soldering was done by hand.
I chose AA334 for the model number because the output tube is a 300B and the driver tube is an EL34. The AA334 is not a kit. The circuit is my own design, and I built these amplifiers using hand tools.
The output tube is a 300B directly heated triode, which means the heater filament itself is the cathode.
Currently, I am using a matched pair of new production Electro Harmonix Gold Grid 300Bs that were made in Russia. Power output with the EH Gold Grids is 6.5W RMS at 1% THD+N, 8W RMS maximum. At 1W RMS the THD+N measures 0.24%. The AA334s use no global negative feedback.
The amplifiers are capable of driving a pair of Klipsch Cornwall II speakers to very satisfactory levels. The Cornwall IIs are really dynamic with a rated sensitivity of 101dB for 1 Watt at 1 meter, and with low distortion.
The 300B is a very linear triode but it requires a grid signal of about 150Vpp to drive it to full output. Because of this high drive requirement I decided to use a triode-connected EL34/6CA7 as the driver tube. The EL34/6CA7 tube was designed for audio and has very musical characteristics, especially when configured as a triode. It is a power tube so it is capable of driving the 300B with ease. I am using JJ EL34s from the Slovak Republic, but I plan to test other makes as well.
The preamp tubes are NOS military grade Sylvania 6SL7s made in the U.S.A. 6SL7s have a refined, transparent sound. The input stage configuration is a modified symmetrical totem pole, or SRPP. The stage is DC coupled, which eliminates at least one capacitor from the signal path.
The AA334 uses vacuum tube rectification. Our rectifier of choice is the 5U4GB tube. The more shapely 5U4G can be used instead, but in bench tests the GB provided about 5V more B+ voltage than the G.
An Amperite 6NO45 delay relay provides power-on delay. When AC power is first applied to the amp, all of the tube filaments except the rectifier receive power. About 45 seconds after power-up, when the tubes have had a chance to warm up, the delay relay closes the rectifier filament circuit and high voltage is applied to all the circuits.
The output transformers are hand wound Silk S-325F from SAC Thailand. They are rated for up to 20 Watts output power and are properly gapped for single ended Class A use.
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