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Old 9th October 2007, 05:18 AM   #1
rman is offline rman  Canada
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Default yet another 6em7 amp

Hi.

I just thought I would share what came up with using the cheap
but cool 6EM7 dissimilar dual triode. When I first got interested
in tube amps about a year ago I built a copy of Gary Kaufman's
Direct coupled amp. It turned out really well so I wanted to try
coming up with my own design. I read John Broskie's articles on
Tubecad journal about complementary inverse distortion and
something he calls The accordion output stage where two tube
in series on each side of the output transformer. I came up with
the attached cicuit and it sounds really good. Actually it sounds
the same as Gary's amp but has more power, and uses basicly the same power supply only with higher voltage. It is nessasary to use a separate heater supply for the fist and second tube because of wildly different cathode voltages. I used blackgate bypass capacitors, orangedrop coupling cap's and Edcor output transformers.
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Old 9th October 2007, 11:00 AM   #2
mus is offline mus  Canada
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Very interesting circuit! Did you get the output power double or even more?
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Old 10th October 2007, 01:26 AM   #3
rman is offline rman  Canada
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Hi mus.

I dont' think I got quite twice the power. If I recall correctly
the maximum peak to peak voltage into an 8 ohm resistor
went from about 9 to 12 volts. I don't remember offhand
the formula but I think it worked out to about 1.2 watts for
Gary's design and 2.2 for mine.

Both amps could make more power with more grid bias on the outputs. But looking at the plate curves, distortion is greatly reduced by keeping it down around 20 volts just like gary's
Direct coupled amp.
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Old 10th October 2007, 06:27 AM   #4
mus is offline mus  Canada
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Thanks! I'm thinking to use this output topology on 6AS7G some day.
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Old 16th October 2007, 03:04 AM   #5
rman is offline rman  Canada
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Hi mus.

I just looked up the 6as7g looks interesting, good power, low rp.
maybe -40v grid 2.5k load? Best of all 300volts heater to cathode
positive or negative So you can use one bottle per chanell.
I like it, hope you build it. What driver?

Cheers.
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Old 17th October 2007, 10:01 AM   #6
mus is offline mus  Canada
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Hi rman,

For the driver I would consider the single stage 5842 or 6S45P with CCS plate loaded at 10~20mA and capacitors couple to 6AS7G. I think 2~2.5K OPT would do. But I don't have time to start this project yet. I have to build my speaker system first (Fostex FE127e on Fonken design).
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Old 17th October 2007, 02:26 PM   #7
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Just remember that 6AS7's mu is less than 2. You will need the cathode of the driver tube to swing a good deal more to drive them than to do the same with the "deaf" side of the 6EM7 (mu=5.4)

Gastón
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