NFB on multi-tap OT

[fp2000]

Does anyone know of a good way to wire the right amount NFB to be reflected on each secondary tap of a multi-tap OT? Meaning if you switch from 4ohms to 8ohms the NFB resistance will change as well?
I was thinking of wiring a separate resistor to each impedance tap but would that work?
Any help on this is appreciated. thanks

Frank

[Funkalicousgroove]

What difference would that make?? You could use a switch with two decks, wire the resistors directly off each tap to the next deck, and simply run the lead from the pole of the second deck into your PI.

[llemtt]

NFB is "reflected" from secondary to PI not "vice versa". Secondary taps are all always active, if you take NFB from 4ohm tap and connect the speaker to the 8ohm tap NFB is still in place in the correct amount, you don't need to switch anything!

teo

[Funkalicousgroove]

That's what I'm sayin!! Unless you wanted to have varying degrees of NFB, But I personally would go about that with a trimmer. I was doing my duty and answering his question........

[fp2000]

The main reason why I was asking about multiple resistor values is because when you switch from 4 ohms to 8 ohms the voltage drop will be different on the speaker. So, a single resistor will not provide the same feedback for different ohm settings, is that right?
But maybe a trimmer resistor wouldn't be bad.
Thanks

[dogears]

Wrong... I think Teo and Funk explained it well in that the NFB comes off of the secondaries which are all "on". Which tap you plug your cab into has no, or insignificant, bearing on the voltage that is feeding the NFB circuit.

The main reason why I was asking about multiple resistor values is because when you switch from 4 ohms to 8 ohms the voltage drop will be different on the speaker. So, a single resistor will not provide the same feedback for different ohm settings, is that right?
But maybe a trimmer resistor wouldn't be bad.
Thanks

[Funkalicousgroove]

Exactly, if you take the FB off the 4ohm tap that's where it always comes from(assuming you wire the switch right! ) no need to switch for output impedance. The only reason you'd want to switch the NF resistor value is to change the amount of NF you are getting.

[fp2000]

Guys, I could not be more thankful. You did two things for me, clarified this one question about feedback and got me off the hook with trying to figure out how to wire NFB with multiple impedances/resistors. This is my first time dealing with tweaking NFB, so I appreciate Teo's, Funk and Dogears' help a lot. I know there's been a lot of discussion about NFB, but I just had to make sure that I understood the concept in theory and in practice.
I guess then I can say that my first D'clone build is almost done. Clean channel sounds awesome, OD channel needs some tweaking. I am getting some squealing noise on some volume/master settings, when playing loud. Any suggestions on where to start?

Thank you all

Frank

[Funkalicousgroove]

Tubes and lead dress!!

[BobW]

Re-check your solder joints too.

[Tdale]

I had some squeel, and it turned out to be the primaries on the OT that was too close to the speaker jack/secondaries wires.

Tommy

[fp2000]

I check all the solder joints, cables, lead dress and everything seems to be fine. I did replace the EH12AX7's on V1 and V2 with some re-issue mullards from GT and the noise improved a lot.
I am waiting for a new mullard re-issue of the 12AT7 for the reverb ( I have reverb) and see what it does, right now is sounds very thin and not too deep. I have the one tube reverb design. Does anyone know what I should expect for plate voltage?
I will also replace the PI tube with a tung-sol 12AX7. Not for nothing but now a days there are so many choices at decent prices. The EH's seem to get noise and microphonic over time, I don't know if it was just the batch that I got.
The squealing still happens on some settings but it's more subtle. I need to keep working on the cable re-arrangements.

Thanks again to everyone. I will post some pics soon on another thread.

[drz400]

I had some squeel, and it turned out to be the primaries on the OT that was too close to the speaker jack/secondaries wires.

Tommy

Happened to me too, I just routed the wires differently for the plates and it went away, Also odly enough shielding the input jack makes it go away

[espinete]

Hello everyone ! My technician installed a multi tap ot in my super reverb style amp.
One of the yelow wires comes from 100 resistor and then to mini switch where 820 ohms NFB resistor is. Then goes to the 8ohms tap jack on the left. Jacks from left to right : 8 ohms, 4 ohms and 2 ohms outputs.
Is it connected properly ? I thought nfb goes in the 2 ohms jack. Because the original ab763 super reverb.

Is he right ? Is he wrong ?

[Roe]

100k NFB @ 4 ohms= 141k @ 8 ohms = 200k at 16 ohms.

I prefer a 27k resistor in series with a 220k pot at @ 16ohms