push-pull 300B power amplifier

  
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Most of the devices used in amplifier output stages are intrinsically seriously non-linear; the worst of all are transistors (both bipolar and FET) which are almost always used inside a global feedback loop for this very reason. Even tetrode and pentode valves have pretty non-linear gain with voltage and current, which is the reason that they are again normally
push-pull 300B power amplifier - schematic

used with negative feedback, and this is the case with the EL84s in my Croft. The natural choice, therefore, is triode valves, and in particular directly-heated triodes such as the 300B, 211 and 845, for the output devices. A potentially significant consequence of feedback-free amplifiers is that the output impedance can be several ohms so, to avoid the frequency response being unduly affected by the varying impedance of the loudspeakers, careful matching of the power valves, output transformers and speakers is vital. Despite these potential disadvantages, the few amplifiers I have heard that have not used global negative feedback have had a naturalness and directness to their sound, as well as a very convincing representation of instrumental timbre, and my experience seems to agree with the consensus of others "out there". Martin Colloms has some interesting things to say about negative feedback, as does Kevin Kennedy. Besides, my preamp and crossover use no global feedback, so it made sense to find a power amplifier to match. I wanted an output power of 20-30W and a gain of 20-26dB, as I knew that my Croft parallel EL84 amps fit in well with my system, particularly with my 90dB/1m/1W speakers. This ruled out single-ended



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