GNFB Resistor Value per OT Tap

[ViperDoc]

I'm progressing through my HP Tweed Twin build, and as beautifully as I buried and zip-tied my unused OT secondaries under my now half-wired board, I failed to realize I needed the 4-ohm tap for the NFB source. I'm running this combo at 8-ohms, and I gather the following from Aiken that output voltage doubles with the square of the output impedance:

"It helps if you think of the equation for power: P = V^2/R. If you have 100W into 16 ohms, the voltage is V = sqrt(100*16) = 40V RMS. If you have 100W into 8 ohms, the voltage is V = sqrt(100*8) = 28.28V RMS. If you have 100W into 4 ohms, the voltage is V = sqrt(100*4) = 20V RMS."

Just looking at this ratio, is it accurate to say that, for a switch from 4 ohm to 8 ohm output impedance, if the stock resistor off the 4-ohm tap is 56K, I can calculate the appropriate 8-ohm tap NFB resistor value to allow for the same NFB voltage would be 56K * (28.28/20) = 56K * (1.414) = 79K?

[martin manning]

Yes.

[Stevem]

Just even to play around and get into the ball park use the fact that every secondary tap will have a 33% change in voltage either up or down.

Going from 4 ohms with a max voltage of 15 will go up to 20 volts on the 8 ohm tap.

[thetragichero]

Just looking at this ratio, is it accurate to say that, for a switch from 4 ohm to 8 ohm output impedance, if the stock resistor off the 4-ohm tap is 56K, I can calculate the appropriate 8-ohm tap NFB resistor value to allow for the same NFB voltage would be 56K * (28.28/20) = 56K * (1.414) = 79K?

i would use 82k there since it's a standard E12 value (no need to mess with 2% components to get 78.7k). at 10% tolerance it'll very likely be 79k anyway, and unless you're a 'golden ears' audiophool you probably won't hear the difference anyway

[ViperDoc]

That's what I thought. This might also give me an excuse to swap the second output jack for a variable NFB pot and minimum resistor setup. Since I'll surely never run an extra cab off this 85-watt combo, it seems like I should.