6V6 P-P Amp From RCA Manual RC-19 - diyAudio
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Old 26th August 2009, 04:47 AM   #1
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Default 6V6 P-P Amp From RCA Manual RC-19

Recently I encountered an interesting 6V6 push-pull amp in an archival issue of Audio Express. The circuit came from RCA tube manual RC-19. You can see it at http//mysite.du.edu/~etuttle/electron/elect35.htm (avoiding nasty copyright bullets..). The really interesting feature is the use of starved 6AU6 pentodes as drivers and what appears to be partial feedback. Looking at the circuit closely, I thought for sure that the 330k plate resistors on the 6AU6s were typos, but apparently this guy has actually constructed the amp per the schematic and had it work for him, as he puts operating voltages in the schematic he posted. I'm temped to try this type of circuit using a pair of 6U8s to replace the 12AU7 and 6AU6s. Comments? Has anyone else built this thing?

Last edited by wrenchone; 26th August 2009 at 04:53 AM. Reason: Make link active
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Old 26th August 2009, 07:18 PM   #2
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Let's try this - http://mysite.du.edu/~etuttle/electron/elect35.htm .
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Old 26th August 2009, 08:14 PM   #3
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I have not tried this amp yet.

Experience does tell me though to change the input tube to a 6N1P, 6922, or something a tad bit smoother than a 12AU7 (which IMO, do far better within a gNFB loop to straighten out irregularities).

Cheers!
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Old 26th August 2009, 09:15 PM   #4
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Well, there's feedback from the 6V6 outputs to the 6AU6 drivers, but no global feedback from output to input. I'm pretty much disregarding the 12AU7 input, as that's a real stardard circuit and of no great interest. The thing that interesting is the starved 6AU6s as drivers. The circuit looks like the results of a lot of "cut and try" bench work from some clever RCA engineer. There is also a big 7027A push-pull amp circuit in the back of my RCA manual that uses partial feedback. That one can be viewed at http://www.angelfire.com/vt/audio/rca50wdia.gif

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Old 27th August 2009, 08:18 AM   #5
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Quote:
Originally Posted by wrenchone View Post
Comments? Has anyone else built this thing?
I haven't built it, and I wouldn't waste my time. That design is hideous in more ways than one. Based on the figures given, the plate current through the 6AU6s is 336uA. (WTH is it with these people wanting to run small signal pents at ridiculously low plate currents anyway?) You've already got onehelluva problem right there: it's no where near enough current to charge up the Cgk + Cstray + Cmiller of the 6V6s, and that means a slew rate problem at the high frequencies. And let's not even discuss what happens when that inevitable fast rising transient hits the 6V6 control grids hard enough to drive them positive. Where's that grid current going to come from? They'll clip like transistors.

Why bother to use one of the better phase splitters, only to reintroduce phase-to-phase imbalance with two independent voltage amps that may not have the same gain? A differential second stage, whether with pents or triodes would preserve phase-to-phase balance.

35db(v) of NFB is just way too much. If you're gonna do that, just hop on over to the Solid State forum and build a transistor amp: cheaper, easier to build, non-potentially-lethal voltages to deal with, you don't need to make all those big holes in the chassis, no heater power worries. You won't be able to tell the difference anyway.

I question why 6V6s even need local NFB. This is already one of your better audio finals. All they require is a bit of gNFB to take the edge off, and get your speeks under control. If you have a VT design that needs more than about 12db(v) of gNFB, then your design is EFFFFFFFFFFFFFFFFed UP! Better to go back to the drawing board, rather than rely on gNFB to sweep your mistakes under the rug.

I don't believe the idgit that drew that schemo knew what he was doing.
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Last edited by Miles Prower; 27th August 2009 at 08:24 AM.
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Old 27th August 2009, 08:33 AM   #6
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Hi Miles,

Pentodes in pentode mode are quite forgiving with low Ia... they sound good with lower noise. Of course there are exceptions... EF184, EF86, etc. sound quite nice burning at 9mA

Cheers!
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Old 27th August 2009, 02:16 PM   #7
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Someone in this forum did build it, maybe a search for PP 6V6 will help, maybe not.

I don't like the schematic but who knows?
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Old 27th August 2009, 06:23 PM   #8
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I know there are a few people out there who are big fans of this circuit. If you are worried about drive capabilities, just rescale it for higher idle current on the 6AU6s, reduce the local feedback slightly, and add a global loop. I've been tempted to build a similar amp for a long time.
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Old 27th August 2009, 09:47 PM   #9
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Originally Posted by jon_010101 View Post
I know there are a few people out there who are big fans of this circuit.
There are also fans of Shakti Stones, C-37 goop, $11,000 Volume Controls, and other assorted audiophooleries.

Quote:
If you are worried about drive capabilities, just rescale it for higher idle current on the 6AU6s, reduce the local feedback slightly, and add a global loop. I've been tempted to build a similar amp for a long time.
Even better: find a better design. There are lots of really good designs to be found right here.
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Old 27th August 2009, 10:03 PM   #10
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Originally Posted by Geek View Post
Pentodes in pentode mode are quite forgiving with low Ia... they sound good with lower noise. Of course there are exceptions... EF184, EF86, etc. sound quite nice burning at 9mA
How do you even deal with something like that? The plate characteristics for the 6AU6 don't go that low. This loadline certainly doesn't look too bad, and it has the current reserve to slap around whatever grids you connect as the next stage, so no slew rate problems either.
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File Type: png 6AU6-Loadline.png (84.0 KB, 394 views)
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