Push pull from nose to tail...distortion signature

[TonyTecson]

there is another schematic of that wherein feedback from the plate of the output tube was routed to the plate and the cathodes of the 6au6 drivers...

 

[smoking-amp]

there is another schematic of that wherein feedback from the plate of the output tube was routed to the plate and the cathodes of the 6au6 drivers...

Yes, sounds like the RCA 50 Watter in the tube handbook.
That one doesn't have the -crossed- N Fdbks however. It still -is- an interesting design. The Fdbks to the 6AU6 driver cathodes helps to linearize the current generation and makes for Hi-Z plates of the 6AU6s for use in the shunt Schade Fdbks at the driver plates, besides making for N Fdbk at the driver cathodes.

I'm still mystified why they included the shunt Schade parts, since that lowers the loop gain for the 6AU6 cathodes. Maybe just to lower the impedance drive to the output grids (BW). There was some discussion once about sonic differences between Fdbk to the driver cathodes versus shunt Schade. Beats me why there would be any difference, except for loop gain effects and drive impedance. And then why mix them both? Some sonic trade-off optimized maybe, or they just wanted to lower the loop gain so it would be easier/safer for DIY.

 

[gabdx]

10 watt amp is sub-standards, I like the GEC manual, they have parallel designs but for higher than 100Watt amps and they have transformers.

I had a huge collection taken from the Internet of the Audio research and Macintosh.

B.A.T. have also good parallel designs.

I think they are more appropriate for SS where you have complementary devices, degeneration at each stage is not efficient, multiply the stages, and waste precious gain that could be used to better effects with GNF.

Local fb is OK if you have a 0-GNF pre, or phono, not in a power amp.

It is also OK in the last output stage to remove transformer THD, and prepare the amp for great ClassAB sound (it is excellent), stabilize output tubes, CFB and ultralinear.

For a 10 watt amp you can use whatever you like as drivers I don't think it matters at such lower powers.

 

[Wavebourn]

Yes, sounds like the RCA 50 Watter in the tube handbook.
That one doesn't have the -crossed- N Fdbks however. It still -is- an interesting design. The Fdbks to the 6AU6 driver cathodes helps to linearize the current generation and makes for Hi-Z plates of the 6AU6s for use in the shunt Schade Fdbks at the driver plates, besides making for N Fdbk at the driver cathodes.

I'm still mystified why they included the shunt Schade parts, since that lowers the loop gain for the 6AU6 cathodes. Maybe just to lower the impedance drive to the output grids (BW). There was some discussion once about sonic differences between Fdbk to the driver cathodes versus shunt Schade. Beats me why there would be any difference, except for loop gain effects and drive impedance. And then why mix them both? Some sonic trade-off optimized maybe, or they just wanted to lower the loop gain so it would be easier/safer for DIY.

On small signals and 1 KHz sinewave there is practically no difference if you either use one feedback loop, or several nested loops. The difference starts on extremes, both when output tubes or output transformer start saturating, or you hit a speed limit. Nested loops properly dosed control misbehavior of the amp making saturation sound less annoying, when single loop would lead to harsh coughing and screaming.