Negative feedback resistor value

[jeff]

 This is something that just dawned on me that I've never thought about before. If you're building an amp from a schematic but are using an output transformer with a different secondary(i.e. 8 ohm instead of 4 ohm) do you have to change the value of the negative feedback resistor?

Don't know why I never thought of that before.

Also I'm using a 4 power tube amp with 2 of the tubes out. I use a 8 ohm load instead of a 4 ohm. Should I also double the value of the NFB resistor?

[sluckey]

Theoretically, yes. Practically, doesn't matter enough to bother.

The NFB loop usually has two or more resistors involved that form a voltage divider for the feedback signal voltage. For example, the Fender AB763 series amps use an 820R and a 100R (or 47R). The amps that run an 8 ohm speaker use 820R/47R and the amps that use a 4 or 2 ohm speaker load use 820R/100R. The lower impedance OT taps produce a lower voltage for the feedback circuit, so the resistor divider ratio is changed to compensate.

Here are two almost identical AB763 circuits that operate different speaker loads. The Vibroverb uses an 8R speaker load and the Super Reverb uses a 2R speaker load.

http://www.schematicheaven.com/fenderamps/super_reverb_ab763_schem.pdf

http://www.schematicheaven.com/fenderamps/vibroverb_ab763_schem.pdf

[jojokeo]

Theoretically, yes. Practically, doesn't matter enough to bother.

I have to respectfully say it does to me. Too much (i.e. to small of a ratio) and you get too much cancellation of frequencies and lose too much signal choking your sound and tone.

You can experiment changing the dropping resistor by substitution or using a temporary pot to see how it affects the signal and solder in what you like. You can also put a switch in to cancel any feedback for signal gain and harmonic increase. Start off w/ appx a 20:1 ratio w/ 8ohm tap and work from there.

[jeff]

20:1 is 20:1 but what's the difference between 100K:5K and 820:41(Fender used 820:47)?

What is the point of negative feedback? Does It have anything to do with the speaker or output transformer's frequency responce? In otherwords what's the theory behind using it in amps?

[tubeswell]

20:1 is 20:1 but what's the difference between 100K:5K and 820:41(Fender used 820:47)?

What is the point of negative feedback? Does It have anything to do with the speaker or output transformer's frequency responce? In otherwords what's the theory behind using it in amps?

To smooth out the signal by dampening it with a bit of opposing phase signal. This can either be done locally - as in this example: http://www.freewebs.com/valvewizard/localfeedback.html

or 'globally' as in a tweed bassman etc.  As you have already recently realised, the global feedback mechanism in most amps takes a voltage swing from the OT secondary and feeds it back to an earlier stage in the amp where the signal is opposite phase. The resistors in the GFB loop form a voltage divider so that you can control the signal being fed back. In the case of a tweed bassman and other similar GFB loops, the leg of the voltage divider is made into a pot and used in combination with a suitable cap, to enable selective bleeding off of treble frequencies, i.e. a presence control.

The Pr:Sec turns ratio in the OT affects the voltage swing you will get for your feedback loop. (An OT with a 4k Pr and a 8R sec has a Pr:Sec turns ratio of 22.36:1, whereas an OT with a 4K Pr and a 2R sec has a turns ratio of 44.72:1, so the voltage swing on the secondary of the latter will be theoretically half as much as the voltage swing on the secondary of the former). So all other things being equal, you may want to adjust the resistor (and/or cap) value(s) in the voltage divider to ensure you are getting a GFB signal of appropriate strength to have the 'right' effect.