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In Reply to: RE: 26 DHT preamp posted by andy evans on March 14, 2012 at 01:30:22
Andy,
That thread is 150 pages and >1700 posts. Would you be willing to give the Cliff Notes version here? Maybe just the latest schematic and a few words about it.
Follow Ups:
Sure - here's my own schematic. Only hand draw, I'm afraid but hopefully legible. Rod Coleman sells the filament supplies. You can use a Hammond choke on the filament supply instead of the bigger ones I happened to have.
Andy
Just a few comments on the 26 preamp - the Hammonds 156Cs are a tried and tested plate choke option - not the ultimate but very good. However, they are prone to hum and must be kept away from any magnetic fields - in particular the filament supply chokes which should be as much as a metre away in a separate box. I use 3 boxes - filament supply, PSU and preamp. The cathode resistors are wirewound surplus Dale - quality is critical. Must be 50 watts - they get hot. I use 10 watters in a series/parallel arrangement and mount them on the top of the chassis. 26s can sound a little better in starved filament, but this alters all the voltages with filament bias. I have also used glow tubes, and others use Salas shunt regulators - it's all in the DIY thread. There are lots of other details and circuit options - many use Lundahl LL1660/5mA in 4.5:1 stepdown for instance. If you build a 26 preamp I do urge reading the thread.
andy
Andy,
Thanks very much for sharing this design with us. I know that its apparent simplicity is a result of many hours of listening and experimenting: a tremendous amount of work. I have several 26s in my tube collection and eventually I will get around to replacing my 76/6P5/37 front end (I don't use a preamp) with a 26 to see what all the fuss is about.
Thanks again,
Sandy
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