Negative Feedback, Where To Get It From?

[DuaneOh]

Hello All,   My first post. I have no real formal education in electronics, just from reading books and searching the internet so please bear with me.

I am designing a new amp and have a question about feedback. The amp will be able to use 6V6's and 6L6's and I plan to have a switch that will select the correct OT tap for the tube. I also plan to have a negative feedback loop with a LTP phase inverter. I am planning on using a 4k primary OT which will be connected to a 8 ohm speaker, the 8 ohm tap for 6L6's and the 4 ohm tap for 6V6's. Should the feedback be taken off the the speaker or connected to one of the taps? I would also like to know why.

Thanks for your help.

[thetragichero]

i found this helpful to wrap my head around global negative: https://www.aikenamps.com/index.php/designing-for-global-negative-feedback

short answer: you can take it from whichever tap you want, as long as you factor that into your resistor calculations

[2deaf]

I am designing a new amp and have a question about feedback. The amp will be able to use 6V6's and 6L6's and plan I to have a switch that will select the correct OT tap for the tube. I also plan to have a negative feedback loop with a LTP phase inverter. I am planning on using a 4k primary OT which will be connected to a 8 ohm speaker, the 8 ohm tap for 6L6's and the 4 ohm tap for 6V6's. Should the feedback be taken off the the speaker or connected to one of the taps? I would also like to know why.

The feedback signal should be taken from the speaker because the open loop gain turns out to be nearly the same at the speaker in both scenarios.

[DuaneOh]

i found this helpful to wrap my head around global negative: https://www.aikenamps.com/index.php/designing-for-global-negative-feedback

short answer: you can take it from whichever tap you want, as long as you factor that into your resistor calculations

I have read that article before but I wasn't sure of how the gain of the different tubes and OT taps would effect it. I guess I should have done the math!

The feedback signal should be taken from the speaker because the open loop gain turns out to be nearly the same at the speaker in both scenarios.

I was thinking that the speaker would be the better choice, I just wasn't sure how close the gain would be between the 6L6 and 6V6 at the speaker. Looking at blackface Fender amp schematics they use different values for feedback resistors but that's probably because of different speaker impendence as well as the gain?

Thanks for all your help, Duane.

[d95err]

How much feedback you want is just a matter if taste. Use whatever sounds best to you.

You could have a switch to select different levels of feedback, to add to the tonal palette of the amp.

(I have on one biild, but frankly, I don’t hear much difference)

[sluckey]

Looking at blackface Fender amp schematics they use different values for feedback resistors but that's probably because of different speaker impendence as well as the gain?

It's the impedance tap of the OT that determines the value of resistor that Fender used in the blackface amps. Amps (such as Deluxe Reverb or Vibroberb) that use a 8Ω impedance tap with a 8Ω speaker load, used a 820Ω and 47Ω NFB divider. Amps (such as Super Reverb or Twin Reverb) that use a 2Ω or 4Ω impedance tap with a 2Ω or 4Ω speaker load, used a 820Ω and 100Ω NFB divider. This was done to keep the amount of feedback approximately equal for all amps.

When I use an OT with a multi-tap secondary, I always connect the NFB to a secondary tap (not the speaker). This way the amount of NFB remains constant, regardless of which tap the speaker load is connected to. Others seem to think that when the speaker load is changed, you should also change the amount of NFB. I'm not so sure about that.

If I build a Fender AB763 Twin Reverb clone with a multi-tap OT, I will use the 820Ω and 100Ω NFB divider and I will connect to the 4Ω tap, regardless of which speaker load I may use. And if I build a Fender AB763 Deluxe Reverb clone with a multi-tap OT, I will use the 820Ω and 47Ω NFB divider and I will connect to the 8Ω tap, regardless of which speaker load I may use.

Many people like different amounts of NFB. They even use switches or pots to control the amount. As long as the amp is stable there is no right or wrong. Experiment to taste.

[2deaf]

It's the impedance tap of the OT that determines the value of resistor that Fender used in the blackface amps. Amps (such as Deluxe Reverb or Vibroberb) that use a 8Ω impedance tap with a 8Ω speaker load, used a 820Ω and 47Ω NFB divider. Amps (such as Super Reverb or Twin Reverb) that use a 2Ω or 4Ω impedance tap with a 2Ω or 4Ω speaker load, used a 820Ω and 100Ω NFB divider. This was done to keep the amount of feedback approximately equal for all amps.

The AB763's did follow that pattern, but it didn't keep the amount of NFB approximately equal for all amps.  If the intent was to have similar NFB for all amps, then the voltage divider would have followed the open loop gain.  The 8 ohm models did have more open loop gain by and large, but there are two glaring exceptions.  The Twin Reverb and the Dual Showman had open loop gain that was on par with the 8 ohm models, but they used the 820/100 NFB voltage divider.  Had they used an 820/47 NFB voltage divider, then the gain reduction with NFB would have been right in there with the other models.
 

[2deaf]

When I use an OT with a multi-tap secondary, I always connect the NFB to a secondary tap (not the speaker). This way the amount of NFB remains constant, regardless of which tap the speaker load is connected to.

The OP is changing three things at once:  The secondary tap, a matched to mismatched load, and the tube type.  All three of these things change the variables that determine open loop gain.  Changing the secondary tap from 8 to 4 ohms reduces the gain of the OT.  The mismatched load doubles the primary impedance of the OT which increases the gain of the power tubes.  The 6V6 has a lower transconductance than the 6L6, so the gain of the power tubes is reduced with the change to 6V6.  It turns out that the three variables nearly balance out and the open loop gain is nearly the same in both cases.  Therefore, the NFB signal should be taken from the speaker in order to have the same amount of gain reduction in both cases.     

[DuaneOh]

The OP is changing three things at once:  The secondary tap, a matched to mismatched load, and the tube type.  All three of these things change the variables that determine open loop gain.  Changing the secondary tap from 8 to 4 ohms reduces the gain of the OT.  The mismatched load doubles the primary impedance of the OT which increases the gain of the power tubes.  The 6V6 has a lower transconductance than the 6L6, so the gain of the power tubes is reduced with the change to 6V6.  It turns out that the three variables nearly balance out and the open loop gain is nearly the same in both cases.  Therefore, the NFB signal should be taken from the speaker in order to have the same amount of gain reduction in both cases.     

That makes it very clear for me, thanks.