6N7S's PPP Class A

[sorenj07]

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

[sorenj07]

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.

[jon_010101]

Hello Soren,
Are we looking at different data? The data sheet I see makes it out to be like a 6N8S with more gain. If so, the 5687s might make better output tubes (and could be driven by a 6N7S diff amp). Mu=35 and Rp=11k would be pretty useful. And 10W total plate dissipation is serious business. See page 3 (link).

[sorenj07]

From what I understand, staying in purely class A with no grid current, the tube behaves like a 6N8S with more gain as you say, but the difference with the 6N7S is that it can be driven positively at much higher currents. Take a look at the plate curves on the bottom left of the second page of the PDF you linked me...

[andyjevans]

I have to sound unenthusiatic here - I've always hated the 6N7. If you are thinking of PPP then there are much better sounding tubes like the 12b4 and in particular the 1626. Even 6AH4 and 6S4 are nice. All pretty cheap.

[sorenj07]

Originally posted by andyjevans
I have to sound unenthusiatic here - I've always hated the 6N7. If you are thinking of PPP then there are much better sounding tubes like the 12b4 and in particular the 1626. Even 6AH4 and 6S4 are nice. All pretty cheap.

my question to you is in what context you've used the 6N7. have you used it with negative bias in its traditional position or have you also tried using it as an output tube? what was the rest of the circuit like? kleep in mind that the 6N7S is Russian-manufacture and might well sound very different from the 6N7.

some other things: none of the tubes you mentioned has an amount of power equivalent to that of one 6N7S' in parallel. the only reason it's "PPP" is that each bottle has two triodes inside - i'd rather not eat up real estate with 4 separate output tubes per channel.

[kevinkr]

Same power levels are attainable with a pair of 6CK4 in conventional PP, and linearity is pretty good. (Regarding the objection to four output tubes.)

IMHO Soviet era clones of some Western tubes in my limited experience generally sound somewhat inferior to their Western counterparts, in particular I am referring the various clones of 6SN7, 6SL7 and 6N7.

I haven't used the 6N7 in any recent designs and in the past used them mainly for phase splitting where I ultimately came to the conclusion that other types sounded better.

I have some 6N7S but so far haven't tried anything with them. The experiment you propose would probably be interesting, and if not gratifying other solutions are just a rebuild away.

(Disclaimer: This is only my opinion, is by no means definitive, and also this comment should not be construed as applying to current production. YMMV)

[sorenj07]

Thanks for the advice! Those 6CK4's, in retrospect, would probably have been the way to go, but they are a little pricier and a little scarcer. I see them being used with an OPT of 8K a-a so hopefully if these 6N7S' take the same impedance, I could try switching them in if I find quads or octets or whatever.

Now, for the rest of this prospective amp. I have a couple 6AY3 TV damper diodes I could use as rectifiers, they should be able to handle the current drawn by this amp. I still haven't decided on a splitter/driver setup but as PakProtector suggests over at AudioKarma, an interstage transformer might help with feeding grid current. I've never worked in the A2 range so this should be a learning experience Am I right in guessing that if I want to bias at +15V, say, I could use a regulated supply working off of a doubler-rectified 6.3V heater winding? Morgan Jones, IIRC, has a nice schematic for a pot-adjustable regulated bias supply, which I could hopefully translate for positive-bias use..

[cerrem]

AFter reviewing the plate curves and data..... It looks pretty straight forward..
The curves are for "SINGLE" triode.....and each tube contains 2 triodes...
So I choose to operate the tube plates at 250V .....
Bias it to 0 volts for Class AB2 .... and your quiescent is safe at 15mA which is 3.75W plate dissipation......

You would use 8K plate load for a Push-Pull pair, being a single tube...This would be 10 Watts...
If you use 2 tubes, essentially, P-P-P, you would then parallel the two plates of each tube...then you would load it with a 4K plate load and obtain 20 Watts....

In reality you would want the 250V on the plate at "full output power" .... So this is AB amp, and you most likely will not have a regulated supply... So aim for 280V to 300V at quescent, depending on the supply impedance ....this way when it droops it will be roughly at 250V when fully driven....

For the transformer..... The plate resistance per tube roughly averages 10K ....so you have it internally paralled, this brings it to 5K equivent, then you are in P-P so your source is in series so your back to 10K for your source resistance... This means to get good frequency response from a 4K plate load... (-3dB points at 3Hz and 60kHz) then you would need 136 Henries of inductance and your leakage should be no greater than 37mH ......
These figures are easy to achieve and you should be able to excee them...

Chris

[sorenj07]

Thanks for all the calculation legwork! My only concern is that 10W of audio power coming from a single tube in PP - it's the power output that I'd seen listed for one tube in full class B. I'm shooting for pure class A from a PPP arrangement, and was predicting around 10W from that. If one tube in class B gives 10W, then two tubes in parallel should give 20W - shouldn't class AB be somewhere in between those figures? This is pure armchair logic, which I probably won't have to figure out until later.

For now, though, have I understood the following right?
"an OPT with impedance of 5K will work with PPP 6N7S'"

Does anyone happen to know of any schematics with positive bias off-hand? I'd like to research the implementation. If I get access to a scanner I'll try to sketch something out, but being on a mac I'm having problems figuring out how to run PSUDII and TubeCAD, at least until Leopard comes out.