Attached is the schematic for a Chinese 6J1 based valve pre-amplifier kit (the schematic is freely available in the public domain). This kit is widely available from various sellers on eBay and elsewhere for about £12 including postage. It's a fairly simple design that uses a single 6J1 pentode per channel wired in triode mode. Only one channel is shown on the schematic. The power source is 12 AC which is quadrupled using two conventional voltage doubler circuits to provide power rails of -28V DC and +28V DC. This gives an anode (plate) voltage of around +56V. I can understand most of this circuit because it is pretty conventional, except the bit circled around TR1,TR2 and TR3,TR4. These components are attached to the output of the quite conventional voltage doubler circuitry. Could anyone explain what benefit they provide and how they do it? TR1 and TR3 look like they are configured as "emitter followers" but TR2 and TR4 are a complete mystery. Thanks for your help!
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