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Message: [QUOTE="solkatten, post: 994255, member: 123032"] The Planar V72SE emerged from a discussion in the tread: [URL unfurl="true"]https://groupdiy.com/threads/valve-mic-preamp-design-incoherent-rambling.77747/[/URL] Many thanks to [B]Winston Oboggie [/B]and [B]abbey roads d efer[/B] for sharing great ideas and solid advice (y) I have been fooling around with the planar tube 6s17k-v, a low noise/microphonics and high gain triode, as front end on a mic preamp. The idea in this design is to use a cascade (jfet + triode) to avoid miller capacitance which tend to degenerate the performance of high gain triodes. The cascade has a gain of 50-55dB. With a varible gain between +20 to +50 dB plus a -20 input PAD. I think you got a practical range for the preamp. The preamp share some of its topology and got its feedback derived in the same way as the classical v72a. The initial idea was to have a parafeed choke loaded CF. A more elegant solution emerged by simply letting the dcr of the SE transfomer elevate the cathode. This way we avoid to many poles and parasitic elements of the cap, choke and transformer solution... Its a clean and simple design: gain controll, -20dB input PAD and pole reverse. The preamp is going to be a single channel unit, built with chassi, transfomers and chokes I already have at hand... ...so some compromises is necessary... As always Im going to build on eyelet boards. The power transformer has a 6,3V winding for the heather of the triode strapped E81L tube and 9v winding which is rectified and filter and feed to a Rod Coleman regulator (a gyrator ccs combo) for the 6s17k-v. (Which has a shared cathode and filament connection.) The HT 180VAC is going to be C-L-C-L filtered to obtain good smoothing without dropping to much voltage. The PT has no 48V winding. The build is gonna be from scratch, so I´ll post pictures along the way... The schematics... [/QUOTE]

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