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Is the output traffo good enough to make a GOOD El Cheapo?
I have picked up a pair of these in my travels over the years. Power supply looks to be in the neighborhood for B+; will probaly be just about right with higher mains w/ an added LC. And I have nearly everything else in the junk box, save chokes, some tubes, and the CCS.
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Looks like a fun project that should yield some decent results. Ultimate destination is a bedroom system for my teen daughter. A few more details of what I am thinking of:
- keep the units physically intact as monoblocks w/ the faceplates, and build the pair into a cabinet; daughter is into the 'vintage' look
- CCS in a project box in the cabinet; will I be taking a chance of hum issues w/ this?
- keeping it 6V6 based - less hassle, not worried over a few $$
- per schematic, B+ rail is exactly 355V; BUT I expect it to come in high w/ higher mains; adding a C354 choke should get me back down to 355 +/-
- will drive a pair of BD pipes - TQWP, uses two Rat Shack 40-1197's); bought a bunch when they closed 'em out; wire the drivers in series, and I even have a 16ohm load
Any tube in the 6V6 family can be used in an "El Cheapo", with zero parts value changes. ;> ) Current production "reissue" TungSol 6V6s stand up very well compared to NOS.
If you have a C354 use it. Otherwise, a less expensive Triad C-24X gets my nod for the choke.
Eli D.
you can do the 'El Cheapo" straight into the circuit you have...
The output trannies are excellent by the way, and the Ampex/JBL 8" fullrange speakers in those units are some of the best every manufactured...they need a subwoofer, so that is something you could add...a solid state buffer (or tube buffer..line level) to add to the unit.
anyway good luck,
you have the most excellent basis for a really good circuit...I second the Tung Sol 6v6 Reissues...they are great..
-3db
360 volts on AC side. Plenty enough.
The Mind has No Firewall~ U.S. Army War College.
If the power trafo RMS value is 360 V., a choke I/P filter is in order. The "ideal" B+ rail voltage in an "El Cheapo" is 355 V. The 'T7 triode sounds very good with 200 to 220 V. on the plate and an I B of 3 mA. The 50 KOhm load resistor drops 150 V. and under 2 V. get dropped in the decoupling network
Eli D.
Please excuse Eli ................... going slightly off topic.......I'm perusing Sams photofact book of Schematics and there is this schematic from Brociner company putting 350 volts on 6AQ5 plates and 345 volts on the screens !? We both know by the book it should run around 250/275 volts .........I understand artistic license and all that but .............this is waaaaay over recommendation. Not all bottles are as tough if one was using a 'fair rated curve' but this implies they are far tougher than I otherwise thought...............after all, 6V6 from today would go thermal *maybe* running 345 in triode. Perhaps ALL valves were tougher in the mid 50's?
The Mind has No Firewall~ U.S. Army War College.
"Is the output traffo good enough to make a GOOD El Cheapo?"
I would think so. The stuff Ampex used is better than that found in guitar amps and we initially called for Fender "Deluxe" replacement "iron". Even that "bottom of the barrel" O/P trafo gave OK results. The better the "iron" the better the finished amp. The signal circuit topology is valid.
Between triode mode "finals" and a 68 nF. cap. in the I/P high pass position, I can't see how you will go wrong. Employing 68 nF. for high pass sets F 3 at 23.4 Hz. The lowest note a double bass produces is 31 Hz. ;> )
BTW, if you have an o'scope, you can get phase compensation "perfection". If not, just connect a 120 pF. mica or NPO ceramic cap. from the "hot" side of the O/P trafo secondary to ground. Inductive wirewound resistors as anode loads in the splitter/driver "peak" HF gain. Natural roll off in the O/P trafo brings things into alignment. The small cap. shorts the NFB loop out above 80 KHz., which keeps the circuit from slew limiting.
Eli D.
I would not strip the 620s as I think they're worth more than the parts... I'd sell them which would more than finance a DIY project...
You are right Tom...............but I did {from Ampex 692}. 10,000 primary into 8 ohm was ideal for 6AQ5 build and I had no other iron that fit the bill.
The Mind has No Firewall~ U.S. Army War College.
z
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