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In Reply to: RE: Starting to lose me.....I think I understand...... posted by dogwan on December 01, 2009 at 20:54:32
"Found this design that I think could work. What do you think?"
That's an "interesting" hodge/podge. A paraphase splitter driving a differential amp. Unfortunately, for good cause, paraphase splitters are out of favor. So, forget that design too.
It seems that you want a well proven "road map". OK, I've got a real good 1 for you. It's the Harman-Kardon Citation V. "Steal" the Cit. 5's small signal circuitry and use "fixed" bias triode wired "finals" into the O/P trafos currently in situ. SS rectify the B+ and use the "5" VAC winding to energize the heater of the 12BY7 voltage amplifier, in a "6" V. setup. The 6CG7 shown in the schematic is 100% electrically equivalent to the 6SN7 now in place. ;>) AA sponsor Jim McShane is THE expert on H/K tubed Cit. units. Send him an EMail about the project. Undoubtedly, Jim will have parts you can use at a fair price.
Eli D.
Follow Ups:
Hey Eli,
Hope I'm not testing your patience too much.
You're right I am leaning towards a proven design I can follow easily. I like the Cit.V idea as well as the 6AN8 idea from the Dynaco mkIII.
So while I was out for a run in the park today I was thinking about this project. With so many options on the table I realized I should list out my goals and prioritze from there.
1. Use existing PT's and chassis, structural modification if necessary.
2. Go UL and upgrade OPT's.
3. Achieve 25-30w output.
4. Cathode bias for simplicity, Or if fixed bias add toroidal xfmr. as per your suggestion. (More power would be nice but I like the more lush tube sound)
5. If possible retain 1 tube for driver/phase splitter (6AN8?) to stay within capacity of PT filament circuit. If needed could add helper small helper xfmr., maybe combine with toroidal for C- rail if fixed bias.
6. Stay within 6L6 family for output stage. 6L6GC, KT66, KT77 ?
7. No preference for SS or tube rectification. Tubes are more romantic, SS could free up some mA's if needed.
I don't want to run this thread on forever but I did find this out there on the internet. It was on a blog site, most of it was in Japanese. To my uneducated eyes it looks similar to the Dynaco driver stage. Of course I would need to make sure it is a viable design. But I'm intrigued.
-Dogwan
Stay away from the KT77 and KT66. The '77 is like an EL34, while the '66 is electrically similar to the 6L6, it draws more heater current.
If you are going to go UL, buy Edcor CXPP100-MS-6.6K trafos. The extra magnetic headroom is insurance against core saturation from the GNFB loop. Also, you keep the option of Class "AB2" operation open, down the road. Big "iron" never hurts.
I don't know where you got 290 Ω for the cathode resistor value. Remember, upwards of 50 bias Volts are needed. 50 V. and an "idle" of 50 mA. works out to 1 KOhm. Can you say heat ? Dude, go combination bias. Build the C- supply. Put a shared 100 Ω/330 μF. network under the cathodes. That's a single grid voltage adjustment and very stable. 10 V. across the 100 Ω resistor = 50 mA. "idle" per tube. :> D
SS rectify the B+ with what I call a "cockeyed" bridge. Construct series wired pairs of UF4007s. Parallel each pair with a 10 nF. high WVDC snubber. Connect the cathode ends of the assemblies to the ends of the rectifier winding. Connect the assemblies' anode ends to ground. Connect the anode of a 1200 PIV Schottky diode to the CT of the rectifier winding. Connect the Schottky's cathode to a CL150 inrush current limiter. Connect the CL150 to the PSU. That setup is QUIET and has a slightly softened start. Use 525 WVDC twistlok (FP) caps. in the PSU filter. Never ignore start up surge, regardless of rectification style.
Finally, install some protection for the screen grids of the "finals". Put 100 Ω resistors in the lines leading to the UL taps.
Eli D.
"I don't know where you got 290 Ω for the cathode resistor value"
- That wasn't my doing. Thanks for the catch. Like I said I mined this from someone else (copied a JPG.) while looking for PP 6L6GC ideas. I would definitely verify all values and layout before building.
"buy Edcor CXPP100-MS-6.6K trafos"
-Do you mean the CXPP60-MS-6.6K? Wouldn't that be more than adequate? Oh wait, as I type this I realized why you recommend.
So I'm liking this 6AN8/6L6GC UL PP direction. I'll start saving up for the the trafos. and acquiring some hardware. Also I'll also draw up the schem's a little better and start verifying values.
I like some of your ideas. Won't be starting right away. Have another project (vintage SS, aaargh!) that I've been putting off. I will re-post and ping you when I get closer to launch.
-Dogwan
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