In the 21st century there is much interest in valve audio amplifiers. The author was looking for a Winter project or two and decided to invest in an amplifier kit advertised on ebay. This was fun to build and test and so another was purchased ....
In general the author has evaluated the clean output power, that is at the point where any increase in input causes a rapid increase in distortion. Usually from flattening of the sine wave. Additionally, many amplifiers tested have far less output than the advert claims. One kit does live up to the advert.
FU-32 Single Ended Stereo Amplifier. A simple amplifier using the FU-32 - (832A) in a single ended stereo amplifier. A clean 2 Watts per channel. The author's own 832A push pull amplifier gives five Watts output.
EL34-B Single Ended Stereo Amplifier. This amplifier, again from Douk Audio on ebay, was built for Garry Hicks. The Chinese do not specialise in excessive documentation and the kit was a challenge with pleasing returns. The write-up adds more detail on how to build to encourage others. The output was measured at 5 Watts RMS taking into account that cathode biased amplifiers driven with a sine wave will show an output that is some 10% below the value possible with a transient.
Mini 6P3P Single Ended Stereo Amplifier. The 6P3P (6L6G) like the EL34 was traditionally used in push pull configurations. The current appetite for single ended amplifiers leads to these inefficient designs. Here the ready built design uses a switch mode power supply - no mains hum just the noise of the cooling fan. This was a 12 Volt fan run from the 6.3 Volt heater supply. The output was under four Watts - but a decent amplifier.
6N8P and 6P3P Single Ended Stereo Amplifier. This kit again uses the 6P3P but with a conventional HT supply - complete with valve rectifier. The voltage amplifier uses a double triode cascode circuit. A good build experience with the author adding a strip-board sub assembly for the HT smoothing components. The five Watt output had a better bass response than the mini amplifier. Altogether an excellent amplifier.
Mini 6P14 Push Pull Stereo Amplifier. Another amplifier with a switch mode power supply. Here the LT heater supply is 13 Volts and the fan runs at full speed with a consequent increase in noise. Noticeable with no signal but not when playing music. Advertised as push pull with eight Watts per channel - it is parallel single end with just 1.5 Watts of clean output. Returned to the maker.
6P6P (6V6) Push-Pull Audio Amplifier. The most expensive kit of the group with a claimed output of 12 Watts per channel. Documentation was confined to a circuit diagram. The under chassis photograph was an essential aid to construction. The kit was exemplary with every aspect beautifully made including ready made signal leads. A real joy to build. The transformer box is of necessity large and this does not add to the aesthetic appeal. On test the clean output was just under 12 Watts and the frequency response excellent. A really good amplifier - a must have.
Single-end EL84 Stereo Audio Amp. A mini amplifier with a conventional power supply and a simple circuit. The EL84 screen grids are fed from taps on the output transformers, other SE amplifiers from Douk have had the screen grids fed by resistors and thus lack the local feedback of the UL connection.