Negative Feedback - more questions!

[Bruce / Mission Amps]

I could be wrong, but I thought the tweed Princeton used an 8-ohm secondary, rather than the Champ's 4-ohm.

You are right.

[jimboyogi]

Very interesting, my mistake

So, for my previous Tweed Princeton build, which is running the speaker from the OT 4 ohm sec. tap, my 22k resistor is too large, and not giving the designed amount of negative feedback?

From the information above in this thread, I should use a 15K resistor instead off the 4 ohm tap to get the designed feedback?

[Bluefinger]

I have already experimented with the -fb loop in the preamp section and know what it does and how it effects sound but what can I expect soundwise from varying the -fb at the output section as described here?

[jimboyogi]

I'm probably the least qualified around here, but I would expect that introducing neg. feedback encompassing the output stage will -

Increase linearity of the output stage

Increase speaker damping

Delay the onset of output stage distortion until higher output levels


There are bound to be more effects as well, but someone else will know more.

[jimboyogi]

yes it is, I was mainly referring to where you would typically find it in a push pull amplifier using a typical LTP for the sake of simplification, although there are a number of places you can insert the FB.




no, you are thinking about it too much like you would just the output of a voltage divider, but what really happens is the open loop gain gets divided by the signal at the output of the voltage divider.... this is the factor of how much the output gets reduced by not what it reduces to, in other words, (and if this is what you are asking.) it's the reason that in your example if you used a 1.5K feedback resistor, you wouldn't be cutting the gain of the output stage in half, .. by a factor of 2 (6db), what you would be doing reducing it to a gain of nearly 2!. I hope it makes a bit sense. I can walk you through some more examples if you would like me to illustrate it further.

Alright, I'd like to resurrect this thread if I may..............

Silverface Fender Champ using 6V6 output valve. Uses 2.7K feedback resistor from 4 ohm OT secondary back to preceding 12AX7 gain stage cathode, with a 47 ohm tail resistor.

Now replace 6V6 with EL34, with just over twice the gm, and therefore just over twice the open loop gain.

To achieve the same amount of NFB as the 6V6 circuit, am I correct in thinking that I need to halve the NFB signal voltage at the 12AX7 cathode. And the simplest way to do this is to double the value of the NFB resistor, from 2.7K to 5.6K?

[Steve Conner]

Yes