yet another 6em7 amp

rman · 9th October 2007, 04:18 AM

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.

mus · 9th October 2007, 10:00 AM

Very interesting circuit! Did you get the output power double or even more?

rman · 10th October 2007, 12:26 AM

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.

mus · 10th October 2007, 05:27 AM

Thanks! I'm thinking to use this output topology on 6AS7G some day.

rman · 16th October 2007, 02:04 AM

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.

mus · 17th October 2007, 09:01 AM

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).

ghpicard · 17th October 2007, 01:26 PM

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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9th October 2007, 10:00 AM	#2
mus diyAudio Member Join Date: Mar 2006 Location: bc	Very interesting circuit! Did you get the output power double or even more?

10th October 2007, 12:26 AM	#3
rman diyAudio Member Join Date: Oct 2007	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.

10th October 2007, 05:27 AM	#4
mus diyAudio Member Join Date: Mar 2006 Location: bc	Thanks! I'm thinking to use this output topology on 6AS7G some day.

16th October 2007, 02:04 AM	#5
rman diyAudio Member Join Date: Oct 2007	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.

17th October 2007, 09:01 AM	#6
mus diyAudio Member Join Date: Mar 2006 Location: bc	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).

17th October 2007, 01:26 PM	#7
ghpicard diyAudio Member Join Date: May 2005 Location: Buenos Aires	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