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Old 20th March 2013, 05:17 PM   #1
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Default What to do with AA-100 output iron

I was yard sailing, last weekend, and I managed to talk an old experimenter out of a set of Outputs for a AA-100 heath-kit. They test good on my ohm meter, and they only cost me ten bucks (price of lunch) I have already cut out some stainless steel blanks to build monoblock amps out of these, but before proceeding, I thought I would ask the experts. Are there any real "Hot Ticket" designs that employ this transformer? It is 6600 ohms primary (some say 7000) and the guy who sold it to me had it marked "6L6 25W" which probably means what we all think it means. it has no ultralinear taps, and has 4, 8, and 16, ohm outputs. I have not started wiring anything yet, and I need to come up with some suitable power transformers but I was throwing a feeler out to see if anyone had had any "great success" with these outputs. Right now i'm thinking of using some of the horizontal deflection tubes I have and trying for a really fat, full sounding 25 watts. Right now I am listening to about 12 watts, and I am excited about the potential performance increase from having obtained some "real" output iron. Mainly excited about the potential bass response increase. Any info on these trafo's would be greatly appreciated. Thanks.
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Old 20th March 2013, 05:51 PM   #2
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Those are nice outputs, made by Stancor IIRC. I have a pair serving in a PP EL34 amp. Triode-connected with the 4 0hm tap grounded and the output taken from 0 and 16 ohm for my 8 ohm speaker; reflecting about half the load. Sounds great to me! I would think you could do well with 6L6, EL34, or even the way ol' Heathkit did it with 7591's.

I was using my pair with 6L6's in "penntoad" but i like the trioded el34 better and havent really noticed a difference in power with my speakers, fostex 8" coaxials which are pretty efficient.

Or pick yourself up a bunch of 6av5's and go sweep tube nuts! Bogen had an amp with PP 6av5s with regulated screens in pentode mode, have a look for the schematic.
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Old 20th March 2013, 06:00 PM   #3
rmyauck is offline rmyauck  Canada
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They do work nice with EL84 PP amps as there is lots of headroom do their their size. I tried that myself.

You could use them with 7591 types with an EL84 PP circuit as they are easy to drive.

Do to the primary impedence it would be best to use fixed bias or the fixed bias EFB mod.

They should work nicely with 6L6 or even EL34 types on fixed bias with Dynaco ST-70 aftermarket PCB's or ccircuits if wired P2P. Just no UL so maybe Triode operation may be worth a try.

If you want to try DHT tubes 2A3 or 45 types would work, but the power would be at and below 15W with fixed bias.

Since you mentioned sweep tubes they will work nicely with Pete Millets "Engineer's Amp" or the Big Red Board

It is popular so hopefully there are still tubes out there for it, but as George of tubelab said you could always skywire in production tubes with some reworking. (you would have to get the screen V. up)

http://www.pmillett.com/dcpp.htm

Posted new P-P power amp design

Last edited by rmyauck; 20th March 2013 at 06:16 PM.
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Old 10th April 2013, 10:08 PM   #4
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Still trying to come up with the best power amp design for these transformers. Sort of leaning towards triode mode EL-34 push pull
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Old 10th April 2013, 10:35 PM   #5
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I'll grind my own axe and suggest an "El Cheapo Grande", with triode wired 7591 "finals". Expect about 15 nice WPC.

The advantage of 7591s over EL34s is ease of drive. Small signal circuitry that works with 6V6s and EL84s works, unchanged, with 7591s. Fisher, Scott, and Sherwood repeatedly exploited that fact.

Current production 7591s are intolerant of the grid to ground resistance limit liberties the "old timers" took. I like combination bias, with each PP pair's cathodes "standing" on a shared 100 Ω/470 μF. RC network and a single bias supply trim pot., in each channel. Employment of 330 KOhms total grid to ground resistance is, clearly, safe and keeps driver gain up. The 100 Ω part provides a very convenient "idle" current set test point. PP pair matching requirements are reasonably relaxed. As always, very tight gm correspondence is necessary. However, minor differences in cathode current are acceptable.
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Old 10th April 2013, 11:11 PM   #6
roline is offline roline  United States
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The 6.6KRaa can be driven by a large group of tubes, 6L6 family and Sweep tube family.
6JM6-6GV5 can be used with the big red PCB by Millet, a great sounding combination. OR you could go 6L6 family route 6P3S, 6P3S-E, 6L6, EL34, 5881, KT66, 6550=C=, KT77, Kt88 route. I would strongly suggest LTP for phase splitter and plate-gate feedback aka Schade on the output tubes. I've done both and prefer the 6GV5 based amp. I cheat and use compactron tubes, triple triodes for the input for LTP designs. .
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