AN/TW Wonderland Mixing Resistors

[W5FH]

Have researched extensively, have started a build -
Does anyone have knowledge on the best values for the mixing resistors R-27 and R-42? I realize I may need to experiment with them but I have seen various combinations going different directions gain-wise and am a little confused. I plan to leave the dry signal voltage divider R-21 at 1 meg and R-22 at 68K, this seemed to be the consensus amongst most builders.
I am building a 50 watt version using Hammond 290-EX power transformer, 8 H choke, 2x6L6GC outputs with an O'Netics A-431 output transformer 4.3K to 4,8,16 ohm. In the 100 watt version of Wonderland amp the NFB resistor R-60 is 12K off of 8 ohm tap. Can someone suggest for me the suitable R-60 value for a 50 watt version?
Any other desirable revisions welcomed. I am really excited about this build and am putting some of my best higher-end parts in it.
Byron W5FH

[cdemike]

To scale negative feedback proportionally to output section power, you'd be able to use Ohm's law and the standard power equation, P=IV. Using Ohm's law, we can determine I=V/R and substitute this in for current in the power equation, giving P=(V^2)/R. So an amp with 100w will produce sqrt(2) times more negative feedback signal than a 50w amp with the same value NFB resistor. In reverse, you can determine your new NFB resistor value by dividing 12k by sqrt(2), which would suggest ~8.49k. Closest standard values would be 8k2 or 9k1, so it's dealer's choice from there.

[Helmholtz]

The relevant NFB measure/degree is the ratio of power amp gain with NFB divided by gain without NFB.
This is called the feedback fraction.
The power tube voltage gain is gm times load impedance.
Paralleling power tubes doubles effective gm, but when Raa is halved the voltage gain is the same as with a single tube per side.
In other words, everything else being the same, a 100W version using 4 power tubes will have the same gain as the 50W version with 2 power tubes when using appropriate OT primary impedances.
So for the same amount of NFB, the same value NFB resistors should be used.

[pdf64]

... everything else being the same, a 100W version using 4 power tubes will have the same gain as the 50W version with 2 power tubes when using appropriate OT primary impedances. ..

See https://www.marshallforum.com/threads/c ... st-2445671

[Helmholtz]

You got me.
I actually forgot to consider the different voltage step down ratios.
So the NFB series resistor should be lowered for the 50W version.
Thanks for correction.

[pdf64]

So reduce R60 to about 8k2, 12k x 0.71, for a 50W version

[W5FH]

Thank you very much for the explanation and help here.