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In Reply to: RE: old pic... posted by Eli Duttman on November 29, 2009 at 18:45:20
Here ya go, again this is an old pic before I started work on it.
Follow Ups:
That O/P trafo does not look particularly hefty. I strongly suspect the tubes installed in the "beast" now are 6L6GCs as the "finals" and a 5U4GB as the rectifier. My "educated guess" is that OEM was (respectively) 6L6s (metal envelope/19 W. max. plate dissipation) and a 5U4G. There's no problem with the later variants being in situ, as they will "loaf".
IIRC, you previously indicated that O/P trafo replacement was a possibility. Edcor's model CXPP60-MS-6.6K seems suitable. The ultra-linear (UL) taps are a blessing. Full pentode mode requires a lot of NFB of 1 kind or another to obtain a satisfactory damping factor. UL mode requires less help from NFB to be acceptable in the damping factor dept.
Eli D.
If I want to add another driver stage and beef up the OPT I think I would pretty much have to scrap the chassis. At which point the only thing worth salvaging (aside from the I/O parts I've added) would be the power tranny.
Might be a fun project. Maybe go with a different output tube at that point.
The learning experience is why I picked these amps up in the 1st place. A scratch build would definitely challenge me.
-Dogwan
Those chassis have a sweet appearance. Remove the can cap & use seperate caps under the chassis. A better audio transformer should be able to fit in the new space. Edcor has some interesting products and many of the audio transformers are only 3.2" wide. Smallest Edcor high quality full bandwidth PP transformer is rated 25-watts and superior to the existing audio transformer.
Before you start a scratch build project, do something constructive with the Wurlitzer monoblocks.
Let's leverage the 30 W plate dissipation max. of the 6L6GC compared to the 19 W. max. of a metal envelope 6L6, while allowing for lack of heft in the OEM O/P trafo. SS rectify the B+, to increase the rail voltage. Triode wire each 6L6GC by connecting g2 to the plate via a 100 Ω Carbon film resistor. A small toroidal trafo will be installed to energize a bias rail (C-) for the "finals". "Fixed" bias will allow for the greatest possible plate to cathode differential. Power O/P will be less than that of the OEM setup, but strain on the O/P "iron" is lowered and loop NFB requirements are reduced.
It will not be the end of the earth if a Mullard circuit's LTP is a 6SN7, as gm is still reasonably high and R P is reasonably low. LTP performance will be enhanced, if the tail resistor is replaced by a 10M45S CCS. Mount a 7 pin mini socket for a 6AB4 voltage amplifier. 200 to 220 V. on the plate and I B = 3 mA. is an excellent set of operating conditions for the voltage amplifier. The 5 VAC winding will be used to energize the 6AB4's heater. Hopefully, regulation effects will bring the actual voltage within 6.3 VAC +/- 5%. It's not inconceivable that a small series resistance will be needed to prevent an overvoltage condition. Remember, a 5U4's filament draws 3 A., while a 6AB4's heater draws 150 mA.
Eli D.
Think one of the multi-cap cans gave out. I had the bottom open so I could double check the power tranny spec's. Heard a crinkling popping sound then the power fuse blew. Replaced the fuse then turned on for just a 1/2 sec and got blue flames in the 5U4GB.
Nice thing about mono-blocks is you can compare readings. Was worried it was the PT so I compared resistance readings across all windings between the 2 PT's and they matched up. But when I compared C8's I couldn't get any resistance or capacitance readings from the suspect one.
Anyway I did find the spec's for the PT online: 380,0,380 @ 150ma, 5v @ 3a, and 6.3v @ 3a.
I was trying to see if these would support a Dynaco MKIII or IV clone. If I move just one of the tube sockets I should be able to squeeze the Edcor OPT in on top.
Take a look at the attached schem. Do you think this could work on the chassis with the existing PT's and those Edcore's?
Thanks for all your help,
-Dogwan
"Take a look at the attached schem. Do you think this could work on the chassis with the existing PT's and those Edcore's?"
No go! :> ( 2X EL34s draw all 3 A. the 6.3 V. winding can supply. 2X 6L6GCs draw 1.8 A. from the filament winding.
3 A. is enough to power 2X 6L6GCs (1.8 A.), an ECC99 (0.8 A.), and a 12AT7 section (0.15 A.). :> ) The total is 2.75 A.
Look at the RCA 6L6GC data sheet. Notice the triode plate curves show upwards of 50 V. as the grid bias for rational "idle" currents. More than ever, "fixed" bias is (IMO) the obvious way to go. SS rectifying the B+ will yield a rail of approx. 490 V. A 50 mA. "idle" current seems sensible, as anode dissipation will be under the 30 W. max. allowed.
While it is possible to derive the bias (C-) rail from the 5 VAC winding, I would not do so. A (SIC) 16 stage voltage multiplier occupies lots of space and is not inexpensive. Just tie the 5 VAC wires off. An AnTek (www.antekinc.com) model AN-0124 toroidal trafo costs $10. Connect the 2X 24 V. secondaries in series and Greinacher ("full wave") voltage double the 48 VAC with 2X 100 PIV Schottky diodes to get the negative bias rail. Notice that you will have the option to run the "finals" at a small "idle" current, should you so choose.
Eli D.
But I would need to see it laid out in a schematic.
Re: the Mullard EL34 circuit I forgot to mention that I knew I would have to add a small cheapo 6.3v "helper" transfo. for the heaters, probably a 4a just for the EL34's.
Found this design that I think could work. What do you think?
-Dogwan
"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.
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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