Power amp feedback

[dobbhill]

I have messed with output stage feedback on several of my previous builds and vintage amps. I believe (opinion) that there is an optimum feedback setting for each speaker arrangement/impedance. I have found that my #1 Dumble clone sounds/feels best into a 4ohm cabinet with the standard feedback resistor. Switching the same cabinet (2x12" alnico) to the 16 ohm setting changes the touch response greatly. My Marshall JTM45 came with a 27k feedback resistor that makes the amp sound constipated into a 16ohm cabinet, but when changed to 100k, the amp sings into the same cabinet. Has any one experimented with adjustable feedback on this design? I'd love to be exposed to the math used to calculate feedback resistors. For instance: if I like the way the amp responds into a 4ohm load, what feedback resistor would I have to use to get the same response into a 16ohm load? I understand that this relationship is not a simple linear one. Help, anyone?
Thanks for listening.
D

[Funkalicousgroove]

Well, if you're changing the load on the amp without a multi tap output tranny then you're actually changing the primary Impedance or plate-to-plate Impedance, if you do have an Impedance selector switch then the amount of negative feedback stays the same you're just using more of the winding.

[dobbhill]

Thanks for your reply.
Is the feedback coming from the same tap on the transformer, like a Marshall, or from the speaker/output jack? I am thinking that would make a difference, yes? Also, if the voltage ratio across the transformer is changing, shouldn't the feedback voltage be changed to compensate (via a change in the feedback resistor)? I know I'm feeling/hearing a difference in the amp.
D

[dobbhill]

I found a reference to feedback and how it relates to changing output loads, if anyone is interested. I knew I heard a difference, and now I have to pull out the calculator and do some experimenting.

http://www.aikenamps.com/GlobalNegativeFeedback.htm

It looks like the simplest way to do this is a 3 to 6 position rotary switch for selecting feedback resistors and, of course, my ears..........

D