Negative Feed Back 8 ohm tap?

[zfiles1701]

Close to getting sound out of AB763.  Have  4 and 8 ohm taps off the OT.  Going to use two 8 ohm speakers in parallel, so I'll use the 4 ohm tap to the speakers (easy enough).  Question:  Is it OK to hook the remaining 8 Ohm tap to the NFB?  That's how my ax84 p1x mod is.  It has 4, 8, and 16 ohm taps wired to jacks but the NFB is off the 8 ohm tap, even when using 4 or 16 ohm taps.    Also, all my PS caps (illinois from Hoffman) have the neg arrow pointing to the ground.  Is that right?  Figured it would ok, but would appreciate some input.  Thanks

[phsyconoodler]

If you want the negative feedback to work,I suggest you hook it to the speaker jack with whatever impedance you are using.
  You would find out rather quickly if you had your power supply caps negative ends going anywhere else but to ground,except the first two caps.They are wired in series and you need to be very familiar with the schematic so you don't wire them wrong.
  If this is an AB763 DEluxe,they are all in parallel,if it's a Super or Pro or a 6L6 amp,they are in series.

[tubeswell]

all my PS caps (illinois from Hoffman) have the neg arrow pointing to the ground.  Is that right? 

Yup

[Fresh_Start]

If you want the negative feedback to work,I suggest you hook it to the speaker jack with whatever impedance you are using.
  You would find out rather quickly if you had your power supply caps negative ends going anywhere else but to ground,except the first two caps.They are wired in series and you need to be very familiar with the schematic so you don't wire them wrong.
  If this is an AB763 DEluxe,they are all in parallel,if it's a Super or Pro or a 6L6 amp,they are in series.

The first sentence above is not correct.  NFB works fine with NFB connected to the 2 ohm tap and have the 8 ohm tap selected.

Which OT tap to connect the NFB to depends on which AB763 you built.  A Deluxe would use the 8 ohm tap.  A Super Reverb uses the 2 ohm tap.  A Twin Reverb would use the 4 ohm tap.  The tap selected for NFB has an effect on the level of the NFB.  I'm sorry but I forgot what that relationship is exactly.  IIRC it's not critical unless you're going from a 2 ohm tap to an 8 ohm tap - Super Reverb, Twin Reverb and Deluxe all use same 820 ohm NFB resistor. 

EDIT:  However, the AB763 amps that use the 2 ohm or 4 ohm tap for NFB have a 100 ohm ground leg on the NFB voltage divider while the AB763 amps that use the 8 ohm tap for NFB (Deluxe) have a 47 ohm resistor on the ground leg.
 
HTH

Chip

[jjasilli]

If you want the negative feedback to work, I suggest you hook it to the speaker jack with whatever impedance you are using.  Lot's of people do it this way.  E.g., if there's an impedance selector SW, it might be wired so that the NFB connection follows the speaker impedance which is selected.  

However the correct amount of NFB is subjective to your taste.  The higher the secondary ohm tap used to originate NFB signal, the higher the start-off voltage of the NFB signal will be.  The NFB signal voltage is then cut-down through a voltage divider (and maybe some more complex circuitry), then injected back somewhere: in the PI or preamp.  It's the NFB voltage you end up with that's critical.  The starting voltage in conjunction with a commensurate voltage divider will get you there.  It's easiest to plagiarize a known circuit, then tweak from there to taste.

Different speaker cab impedances may call for more or less NFB in the amp.  Psycho's suggestion is a good shortcut way to accomplish that without having to change the values of the components in the NFB circuit.

[Fresh_Start]

JJ is correct - a higher ohm secondary OT tap produces higher voltage going into the voltage divider (etc) for NFB.

If you wire the NFB connection so that it follows whichever tap on the OT is selected, then you'll have more NFB when you select the 8 ohm tap than you would with the 2 ohm tap selected.  You may want that result; however, I would start by figuring out what the appropriate NFB should be regardless of the OT secondary tap actually connected to the speaker(s).

Hope that's clear,

Chip

[zfiles1701]

Hope that's clear,

Well sorta.  I have a 4 and 8 ohm tap.  I wired the 8 ohm tap to the speaker.  Now if I select the 8 ohm tap I should have the level of NFB that the amp was designed to have.  Here is the question:  If I keep the speaker tap at 8 ohms but switch the NFB to the 4 ohm tap, will I get more or less NFB?
Thanks

[Shrapnel]

Hope that's clear,

Well sorta.  I have a 4 and 8 ohm tap.  I wired the 8 ohm tap to the speaker.  Now if I select the 8 ohm tap I should have the level of NFB that the amp was designed to have.  Here is the question:  If I keep the speaker tap at 8 ohms but switch the NFB to the 4 ohm tap, will I get more or less NFB?
Thanks

Switching the NFB from the 8R tap to the 4R tap, irregardless of where the speaker is connected AND not changing any of the parts will give less NFB, as it will have less drive voltage entering the NFB voltage divider network.

[J Rindt]

If you want the negative feedback to work, I suggest you hook it to the speaker jack with whatever impedance you are using.  Lot's of people do it this way.  E.g., if there's an impedance selector SW, it might be wired so that the NFB connection follows the speaker impedance which is selected.  

However the correct amount of NFB is subjective to your taste.  The higher the secondary ohm tap used to originate NFB signal, the higher the start-off voltage of the NFB signal will be.  The NFB signal voltage is then cut-down through a voltage divider (and maybe some more complex circuitry), then injected back somewhere: in the PI or preamp.  It's the NFB voltage you end up with that's critical.  The starting voltage in conjunction with a commensurate voltage divider will get you there.  It's easiest to plagiarize a known circuit, then tweak from there to taste.

Different speaker cab impedances may call for more or less NFB in the amp.  Psycho's suggestion is a good shortcut way to accomplish that without having to change the values of the components in the NFB circuit.

This is how I have always understood NFB. Think of all the amps (an Express comes to mind) that have selectable impedance. The NFB is frequently strapped to the 8 ohm tap in an amp with a 4, 8, and 16 ohm OT.......

[zfiles1701]

Now I understand.  Appreciate all the input and thanks again guys.