6N7S's PPP Class A

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6N7S's PPP Class A

Postby sorenj07 » Wed Aug 29, 2007 8:03 pm

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I just picked up four of the russian 6N7S tubes with the aspiration of puttng together some type of class A (AB-leaning-heavily-A at least) push-pull amp. I chose 6N7S's because they are readily available and they put out a decent amount of power. I was a bit surprised to see that they've been traditionally used for pure class B, but I hope they have some real hi-fi potential.

If each bottle has a 12W max dissipation, I'd dissipate around 20W per pair, hoping for around 10W of nice audio. Is this feasible? I don't have a printer so I haven't done any load line drawing, but let me make sure: with the PPP setup, the load impedance that EACH triode sees is the same as the anode to anode impedance of the OPT right? Any ideas about what OPT primary impedances might be best for such a project?

I'd have to look at some load-lines to see how much swing is needed to drive the tubes but I have a few 5687's kicking around that hopefully could be up to the task. Maybe an LTP feeding a regular grounded-cathode stage? a LTP'ed SRPP?

If such an amp isn't practical I'll just save these to use as somewhat beefy drivers for another future amp. Planning ahead, I am :)
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Postby Gingertube » Wed Aug 29, 2007 9:45 pm

I see no reason why you could not do a very nice amp with those tubes in the output.

In Push Pull Class A each tube of the push pull sees 1/2 of the Raa output impedance.
When you transistion to Class B each tube sees 1/4 of Raa.

IF you parallel 2 output tubes on each side, do the load lines / calcs etc as if you had just one tube each side of:
twice the anode dissipation rating
twice the gm
half the rp
same mu.

Driving 2 tubes in triode mode each side will need a grunty driver as the Miller capacitance will be substantial.

Something like this but with 2 6N7 each side inplace of the triode straped EL34 would probably do the job. Change to fixed bias if preferred.
http://www.vacuumstate.com/schematics/pp-1c_s.gif

Cheers,
Ian
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Postby erichayes » Wed Aug 29, 2007 10:18 pm

Hi Soren,

Can you clue me in on where to get one of these critters? I'd like to get one and run it against an Amurrican 6N7 on the 570 to see how similar they are to each other. The spec's for the 6N7S are a little lacking in detail, but appear to be similar to the 6N7's.

I used to think about a PPP 6N7 amp when I was a kid, primarily because my Dutch Uncle mentor dumped a bunch of them on me. I'd be interested in seeing my ideas brought to fruition, so let me know how I can assist.
Eric in the Jefferson State
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Postby sorenj07 » Wed Aug 29, 2007 10:28 pm

just hop on eBay and search "6N7S" and you'll see them a-plenty. In actually looking at the plate curves (something I recommend doing before buying tubes :) ) I saw that this tube probably wants to be biased positive for class A2 operation. I'm looking at around +15-20V bias. what kind of driver do you think could provide all the grid current? maybe some kind of 5687 in a long-tailed SRPP? those are beefy little tubes.

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Postby sorenj07 » Thu Aug 30, 2007 7:55 am

any chance of seeing the detailed CCS schematic?
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D'Oh!

Postby EWBrown » Fri Sep 07, 2007 9:18 am

I must be in one of my slightly dyslexic states, I keep seeing "6SN7" insead of "6N7S" (or in Russian Cyrillic, 6H7C )

Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_11 :o

And, more than once I've gotten overly excited when I find a cheap 6AC7 at a swapmeet or hamfest... D' Oh!..... Good thing I always look inside the box first, Yellow_Light_Colorz_PDT_07

/ed B in NH
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