NFB resistor value - AB763 circuits

[osing]

Hi,

I was looking at some of the Fender AB763 circuits (Super Reverb, Twin Reverb, Deluxe Reverb) and noticed that they all have the same NFB resistor value of 820 Ohm. Since these amps have different output tube configurations and thus different OTs and speakers impedances (2, 4 & 8 Ohm, respectively), does that mean the three amps each then use a different amount of negative feedback? I have no idea if what is written on the web site below is valid, but it seems to suggest that the resistor value needs to change if you want to maintain the same amount of NFB if using a different OT tap on the same amp, and thus I wonder if that therefore would apply to the different OTs for these three amps.

See last paragraph under the "How much feedback to use"
http://www.aikenamps.com/NegativeFeedback.htm

Thoughts?

[sluckey]

noticed that they all have the same NFB resistor value of 820 Ohm.

Yes, they all have a 820Ω. But that's only half the NFB voltage divider. If you look close at ALL of the AB763 series amps you'll notice that the other half of the divider is either 100Ω or 47Ω. The output speaker impedance determines which to use. Eight ohm outputs (single speaker amps) such as Deluxe, Deluxe Reverb, Vibrolux, Vibroverb all use a 47Ω. Four and Two ohm (multi speaker amps) such as Super Reverb, Bandmaster, Twin Reverb all use a 100Ω. Changing the ratio of the NFB voltage divider resistors changes the amount of NFB in the loop. Basically, a higher impedance OT secondary sends a higher voltage to the loop, so they use the smaller 47Ω resistor to reduce the NFB. And a lower impedance OT secondary sends a lower voltage to the loop, so they use the 100Ω resistor to increase the NFB.

The Showman was available in a Single or Dual. The only difference in the electronics was that 47 or 100 ohm resistor. If you look at the Showman schematic you'll see a note stating this...

http://www.el34world.com/charts/Schematics/files/fender/SHOWMAN_AB763.pdf

[mule]

I'm planning a ab763 build with a multi tap 2,4,8,16 ohm Mercury OT. I have 4x10 and 1x15 extension cabs. What would be a good value to go with? If I went with the 47r would that mean less feedback and how would that change the feel?

[osing]

Thanks Sluckey,

I did not notice the 47 Ohm vs 100 Ohm resistors on 8 vs. 4 and 2 Ohm amps, so thanks for pointing that out. I guess in theory the amount of NFB would be a bit different on a Twin Reverb vs. a Super Reverb since they both use the 100 Ohm resistor but have different impedances. Perhaps the difference is inaudible? Asked another way, is there a rule of thumb for how much you would have to change a NFB resistor by in order to have a significant audible difference?

Also, just to make sure I understand, if I were to use an 8 Ohm OT on a Super Reverb and wire the speakers series-parallel for 8 Ohms, I would actually want to change the 100 Ohm resistor to a 47 Ohm resistor and theoretically the NFB loop would remain roughly the same as with a 2 Ohm OT with parallel-parallel speaker wiring?

Thanks!

[terminalgs]

I'm planning a ab763 build with a multi tap 2,4,8,16 ohm Mercury OT. I have 4x10 and 1x15 extension cabs. What would be a good value to go with? If I went with the 47r would that mean less feedback and how would that change the feel?

You can put the NFB circuit on ANY of the OT secondaries and use ANY of the  sec. taps for your correctly matched speaker load.  Lets say you've got a rotary switch for your speaker load of 2,4,8,16ohm loads.  you can pick either the 4, 8, or 16ohm sec. for the NFB.  (they don't have to the same).  If you are building an exact AB763 circuit: if you pick the 8ohm tap for the NFB, use a 47ohm R2. 

If you were to install a 50ohm pot  as a variable resistor instead of the 47ohm, and roll it down, you'd have less NFB until the pot got to zero and then you'd no NFB.

As for your audio preference,  instead of  picking fixed NFB R1/R2 resistors, put a presence control pot on the amp and dial in the sound you like (be it front or rear facing, or a screwdriver controlled trim pot inside)...   


more NFB means less distortion in the power amp stages, flatter freq. response., a more stable amp that "behaves itself". 

less (or no) NFB means the opposite, in a lot of ways: some unruliness, an eagerness to distort at the PI and power amp stages, a bit unstable.

no NFB: think Neil Young cortez.
stock Fender NFB: think Wes Montgomery four on six.

[mule]

Awesome. Thanks terminalgs.

Owing sorry for the hijack...your thread was timely to what I'm working on.

[osing]

If you were to install a 50ohm pot  as a variable resistor instead of the 47ohm, and roll it down, you'd have less NFB until the pot got to zero and then you'd no NFB.

I thought in order to get less NFB you would increase the resistor value to the point where infinite resistance =~ open circuit = no NFB? Thus, you would really want to have a fixed resistor (for example 47 Ohm) and then add a pot in series with this resistor to increase the overall resistance to dial down the amount of NFB?

[sluckey]

If you were to install a 50ohm pot  as a variable resistor instead of the 47ohm, and roll it down, you'd have less NFB until the pot got to zero and then you'd no NFB.

I thought in order to get less NFB you would increase the resistor value to the point where infinite resistance =~ open circuit = no NFB? Thus, you would really want to have a fixed resistor (for example 47 Ohm) and then add a pot in series with this resistor to increase the overall resistance to dial down the amount of NFB?

There are TWO resistors involved in the NFB loop for an AB763 amp. If you increase the value of the 820Ω resistor, you will decrease the amount of NFB. If you decrease the value of the 47Ω (or 100Ω), you will also decrease the amount of NFB.

[osing]

Thanks Sluckey. I think I am slowly getting it. Increasing the ratio of the two resistors (820/47) will reduce the NFB. This can be accomplished by either increasing the numerator (820 Ohm) or decreasing the denominator (47 Ohm).

[Geezer]

Thanks Sluckey. I think I am slowly getting it. Increasing the ratio of the two resistors (820/47) will reduce the NFB. This can be accomplished by either increasing the numerator (820 Ohm) or decreasing the denominator (47 Ohm).

What!!?? MATH!!!??? Eeeewwwww 

Just kidding......you do now, in fact, "get it"

[sluckey]

Using the amps you originally mentioned, ie, Super Reverb, Twin Reverb, and Deluxe Reverb, let's set up a scenario. Let's say that all three amps are putting out 20 watts.

SR @ 20W into 2Ω is 3.16V. This voltage is sent to a 820Ω/100Ω divider. The output of the divider is taken across the 100Ω resistor and is the actual NFB voltage. This voltage will be 0.34V. (Remember this number)

TR @ 20W into 4Ω is 2.24V. This voltage is sent to a 820Ω/100Ω divider. The output of the divider is taken across the 100Ω resistor and is the actual NFB voltage. This voltage will be 0.24V. (Remember this number)

DR @ 20W into 8Ω is 1.58V. This voltage is sent to a 820Ω/47Ω divider. The output of the divider is taken across the 100Ω resistor and is the actual NFB voltage. This voltage will be 0.086V. (Remember this number)

So, to answer your original question, "Since these amps have different output tube configurations and thus different OTs and speakers impedances (2, 4 & 8 Ohm, respectively), does that mean the three amps each then use a different amount of negative feedback?", I would say yes those three amps use a different amount of negative feedback.

It's perfectly OK that they all use different amounts of NFB. The correct amount of NFB is what the designer (Leo) says it is. Using different amounts of NFB is a factor in the particular sound each of these amps has. I don't think Leo was interested in keeping the amount of NFB exactly the same for all of the AB763 amps.

I have no idea if what is written on the web site below is valid, but it seems to suggest that the resistor value needs to change if you want to maintain the same amount of NFB if using a different OT tap on the same amp, and thus I wonder if that therefore would apply to the different OTs for these three amps.

That's correct when you're talking about one amp. But IMO it doesn't really apply to the AB763 series of amps since they were designed with different goals in mind. A DR is not a SR is not a TR.  It's three different amps with three different (but similar) sounds, all from the same circuit. Crazy, ain't it!