![]() ![]() Another option would be to try an EF37A pentode in place of the 6SL7 but, in either case, I’d need a reason to build another one as I have far more solid state Hi Fi amps than I need. Hindsight is a wonderful thing and, if I had my time again, I’d probably just use 12AX7s and E元4s – the amplifier looks great and sounds perfectly good – but it is a tad quiet, even for a small single-ended amplifier. ![]() Instead, I followed the input buffers with a paralleled 6SL7. I could have done the same using 6SL7s but the 6SL7 doesn’t have as much gain and KT88s are harder to drive than E元4s. The Byrith design uses a single 12AX7 for both the input buffer and the gain stage, and this makes it easy to base each channel around a single device. With a stereo configuration, it made sense to use one 6SN7 to buffer both channels. Im finishing up a simple 3 stage mic pre based on 2 6sl7 triode stages with a 6sn7 doing a cathode follower on the output for an unbalanced output, and im expiermenting with output impedance and trying to figure out what i should have on the output. As mentioned earlier, I’d also decided to use up some of my spare octal triodes so the cathode follower input buffer was based on the 6SN7, which has good drive capabilities but not much gain. The amp was intended for stereo use, so rather than build two monoblocks, I decided to put both channels on a single chassis. I have not attempted to measure the performance of the amp – it sounds good to my untutored ears – as this was a) a present for my brother, and b) an opportunity to experiment with something new. I should stress that, while I have departed slightly from the Byrith design, these aren’t suggested as improvements and the components I used (particularly the output transformer) were significantly inferior to limit costs. Problem is, the cathode follower circuit can The circuit posted is a common. Through careful analysis of the components he was using, Byrith achieved excellent performance from a very simple topology (a cathode follower as an input buffer followed by a single triode gain stage driving the single-ended output stage). In particular, the local negative feedback is achieved by referencing the cathode biasing circuit to the secondary of the output transformer (i.e. If nothing else, Byrith’s discussion of the challenges of the output transformer is illuminating, but he also presents a detailed, practical analysis of techniques for minimising distortion through the use of both local and global negative feedback. The circuit is derived from the work of Claus Byrith who has described the design thinking behind a novel single-ended design based around a specific Lundahl output transformer. With a view to keeping things particularly vintage, I decided to target the earlier octal double triodes (6SN7 and 6SL7) driving single-ended KT88s (which morphed into 6550s when I discovered my used KT88s were duds). Having created my first guitar amp, I decided to have a crack at building a Hi Fi valve amp for my brother. While I am very openly not a member of the golden-eared brigade, I obviously understand the need for clean amplification and the benefit of decent speakers for listening rather than playing.
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