NFB and selectable impedance

[StrummersOfThunder]

Quickie for the venerable amp gurus

NFB circuits when you employ selectable output impedance

If for instance you have a 58k NFB resistor that heads to the 8ohm tap (connected to the output jack) but you interupt that with a switch to select between 8/16 ohm, what should one do with the NFB ? Presumably the value will be incorrect for the 16ohm tap now?

There is an unused 4 ohm tap on the OT . Is that able to be used to connect an adjusted NFB resistor to so that things remain 'stable 'as it were.

Thanks

[StrummersOfThunder]

Is the advice within this thread kosher ?
https://el34world.com/Forum/index.php?topic=21511.0

Would that mean leave it attached to the 'hot' of the output jack or to the 8ohm side of the STDP switch?
Im assuming switch lug is the correct answer.

[GrantB]

I was trying to find a Ralph Wiggum picture of "my nose makes its own bubble gum".

Mainly to express, despite my desire to help, how utterly lost I am in the electrical theory world.

[StrummersOfThunder]

I was trying to find a Ralph Wiggum picture of "my nose makes its own bubble gum".

Mainly to express, despite my desire to help, how utterly lost I am in the electrical theory world.

We are in similar snotty boats

[TmcB]

Does it change the sound if you plug a speaker into a different tap than the nfb is on? Yes.

You can use that to your advantage though - a JTM45 has the nfb on the 16ohm tap but using the 8ohm give a different tone, bit more grit.

If I was you I would either pick the tap I’m more likely to use OR just build it as stock to hear what it has classically sounded like. I wouldn’t faff about with changing NFB resistors as you change the impedance - you’d likely have to muck around with different values to get it feeling the same anyway


Disclaimer: not a guru

[StrummersOfThunder]

Does it change the sound if you plug a speaker into a different tap than the nfb is on? Yes.

You can use that to your advantage though - a JTM45 has the nfb on the 16ohm tap but using the 8ohm give a different tone, bit more grit.

If I was you I would either pick the tap I’m more likely to use OR just build it as stock to hear what it has classically sounded like. I wouldn’t faff about with changing NFB resistors as you change the impedance - you’d likely have to muck around with different values to get it feeling the same anyway


Disclaimer: not a guru

Good advice. 8ohm tap it is I think
Sub-guru status

[murky]

*slaps hood* you can fit so many ohms in this bad boy…..

[Reg18]

I don’t know anything about the technical side of your question, but I do like the feel of the 68 deluxe and even 68 twin over the 65 versions which from what I’ve read is mostly due to decreased NFB giving it a more aggressive feel. More noise though too!
I’d like the option of switching it personally if I was in your position.

[StrummersOfThunder]

I don’t know anything about the technical side of your question, but I do like the feel of the 68 deluxe and even 68 twin over the 65 versions which from what I’ve read is mostly due to decreased NFB giving it a more aggressive feel. More noise though too!
I’d like the option of switching it personally if I was in your position.

Yep thats a big part of the voice (and noise) of those amps. Comes down to taste.
Im totally happy to play round with the idea of a switch to have variable NFB
It was more 'where to hang the connection' . But I think I have this sussed now.