5f6 neg feedback

[ernie_jr]

Currently building a 5f6-a with an O/P tranny having 4. 8 and 16 ohm taps. If is use the 16 for the neg feedback, what change to I have to make in the resistor for that loop.
thanks,
Ernie

[sluckey]

Increase resistance if you want to maintain the same amount of feedback.

[ernie_jr]

Thanks Slukey,
 if it is 27K would i go to 75k or 100k?
Thanks,
Ernie

[HotBluePlates]

Currently building a 5f6-a with an O/P tranny having 4. 8 and 16 ohm taps. If is use the 16 for the neg feedback, what change to I have to make in the resistor for that loop.

Only the tap used for sourcing feedback voltage matters in the equations below.

Raw speaker voltage:
√(Power * Impedance) = Voltage

Original 5F6-A Bassman (assumed 50w output):
√(50w * 2Ω) = 10v

New Bassman (assumed 50w output):
√(50w * 16Ω) = 28.28v

For stock feedback, the new series resistor should be 28.28v/10v = 2.828 times the original resistor value.  27kΩ * 2.828 = ~76kΩ.  Closest standard values (for E12, 10% tolerance parts) would be 68kΩ (more feedback) or 82kΩ (less feedback).

You can sort this yourself more directly by observing the change in tap impedance.  If you move from a tap to another with 2x the impedance (2Ω to 4Ω), the speaker voltage (and source for feedback) increases by √2.  Go up another doubling, and it increases √2 * √2 = 2, or double voltage.  So the step up from 2Ω to 16Ω is a source voltage increase of 2√2.

Marshall appeared to observe this and raise the feedback resistor from 27kΩ to 47kΩ in their 50w amps, which sourced feedback voltage from the 8Ω tap. 56kΩ would have been closer to 2*27kΩ, but perhaps they wanted a touch more feedback.

[ernie_jr]

Thank HotBluesPlate,
I have plenty of 68K to plug in.
Ernie