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In Reply to: RE: best use for some 26 tubes?? posted by vinnie2 on June 28, 2015 at 04:11:03
Use Rod Coleman DC regulators on the filaments for a totally silent background. I also recommend battery grid bias through an input transformer, as shown in the schematic below. Here are a few pics of my 26 Linestage, which with the flick of a switch is also a tube output DAC using the Twisted Pear Buffalo III SE board and the same 26's as the output stage.Picture of the Preamp/DAC internals:
Picture of the DAC Section Closeup with Sonore USB Interface Board:
Picture of the Power Supply Chassis Internals:
Edits: 06/28/15 06/28/15Follow Ups:
I agree with Eli, a beautiful build, but out of my league price wise. You have me interested in trying to find something I could try tht would be a little less work and afew less bucks.
I'll give you a hint for a low cost "300" V. B+ PSU that's vacuum rectified. ;> ) "Full wave" voltage double the O/P of a Triad N-68X isolation trafo with a 117Z6. The N-68X easily has the "stones" to energize the rectifier's heater, along with the B+. Plenty of B+ current for a preamp.
Given today's higher average line voltages, put a 33 Ω resistor in series with each of the 117Z6's heater connections.
BTW, the Triad N-68X has dual primaries, which makes the setup described above "universal".
Eli D.
I am afraid I would need a schem to put together something like that Eli; it's a bit over my head. Thanks for the thought though.
You asked for it. :> ))
Eli D.
Thanks Eli. Most interesting.
If you want to know all about 26 preamps. Looong, but packed with builds ranging from simple and cheap to complex and extravagant. Well worth the read.
Edits: 06/28/15
Looks like I will have to work my way through this one. Thanks!
The internal impedance of the "9" V. lithium battery you used is lower than that in an alkaline "equivalent". I satisfied myself of that fact, with a "Little Rat" phono preamp, years ago. A RAYOVAC 7 cell NiMH battery was even better, but can't be used in the grid circuit. The cathode current in a #26 would provide a quite adequate "trickle" to keep the NiMH pack fully charged. Can Mr. Coleman's filament regulator be set up "floating"? If so, the bias battery could be moved from the grid circuitry to the cathode circuitry.
Just my biases. I prefer to hold the number of caps. in the signal path, including ground legs, to a minimum.
Eli D.
The cathode of the 26 is directly grounded. There is no cap in the grid circuit with this arrangement; the caps in the picture are a very small value film to bypass the battery for RF (optional) and the snubber cap for the transformer secondary to get nice square waves. The input transformer blocks the DC from the preceeding circuit, and also flips the polarity to counteract the flip from the SE 26 and steps down the voltage 2:1 to give a bit more headroom for the autoformer volume controls on the output.
Lots of ways to do it, of course.
OK, the cap. across the bias battery is a RF bypass. I was wondering. The RC network across the I/P trafo suppresses ringing, among other things, and makes perfect sense.
That's a BEAUTIFUL build, but way too costly for my circumstances.
Eli D.
to answer your question. I float them up to 250VDC in my 833C amps, pumping 10A of filament current through them. Very reliable and very quiet.
Eli, you're in NJ too, right? I'm in Branchburg. Maybe you could stop by for a listen some time. Let me know via pm if you're interested.
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