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In Reply to: RE: Line Stage Preamps for Fi 2A3 monos? posted by Frihed89 on January 27, 2012 at 15:51:45
There is a huge thread on DIY Audio called "26 pre amp" in which several builders including myself have been endlessly refining how to build the perfect 26 preamp. Since the 26 is about the best preamp tube ever made, I would seriously consider a 26 project. You are unlikely to get better at any price. The designs are very simple and can be built by anyone proficient in building with tubes, but require absolutely top quality components and a sophisticated DC supply - there are ready built boards available from Rod Coleman for this. There is a steadily increasing number of us who see this as pretty much the final preamp - it's not clear where to go from here.
The 26 is to preamps what the 300b, 45 and 2A3 are to output tubes.
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.
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.
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.
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.
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