Figuring out negative feedback

[mightymike]

Sticky

[vanhalen5150]

Agreed ^^^^Sticky this one.

1)Why not use some shielded cable for the "purple" wire?
2)How is the switch connected? If your set to 16ohms, how does the wire(FB) attached to a 4ohm work?

[rgorke]

Agreed ^^^^Sticky this one.

1)Why not use some shielded cable for the "purple" wire?
2)How is the switch connected? If your set to 16ohms, how does the wire(FB) attached to a 4ohm work?

I am not sure the you can get 4 Ohm if your switch is at 16ohm. Meaning, if your NFB is on the speaker jack, I think it is determined based on which setting your selector is on.

So, on the speaker jack you can switch from 16-8-4 for you NFB just by switching the selector, right?

[vh junkie]

You can use shielded wire for the purple NFB wires.
All 3 FB attachment points on the switch are live at all times, regardless of what position the selector is in... so you do not need to put the nfb wire on the speaker jack.

[vanhalen5150]

Ok.

So basically the "purple" wire is "because that's the way Marshall did it".

[mightymike]

They really need to make purple sheilded wire.

[stef]

I allways tend to get a kind of thin spiky tone with anything higher than 27k... it ALLWAYS ends with me putting the 27k back in... If I want more gain or fatness and still keep the whole tone, I work with the taps instead... if that's not enough, the problem, or gain issue lays somewhere else...
Increasing the tap value isn't the same as increasing the resistor value... eaven if the two are basiclly in the same possition in the circut... I think it has something to do with the sound getting better if there's as little resistance as possible in the NFB circut... it's better if it's controlled with the OT's taps... With a small resistor the OT detirmines how much is feed back into the PI circut... the resistors value detirmines how much the OT will feed back...

Please correct me if I'm wrong...

I've had the 47k/8ohm combo for years - just switched to 27k (same brand of the resistor - Piher), still on the 8ohm tap.
Nice and tight sounding, but sounded bigger with the 47k - similar to the sound of older set of power tubes (27k) compared to brand new (47k), sort of
Anyhow, I'm keeping the 27k in for further testing
27k/4 should be close to 47k/8, therefore I'll keep the 27k and maybe play with OT taps

[jnew]

It is interesting. Going down to 27K or even 47K does make the amp tighter but to me, seems too much with maybe too much focused, or scooped type effects. I like the looseness and hairy rasp that gives the distortion some character. Have you tried lifting it all together. That's a fun ride but noisy and really tough to tame with the hands. I like 100K on the 8 ohm. (I can only use the 8 ohm anyway because of the way the OPT Trannie is)

[rgorke]

I liked the tone at 100k/4 ohm but it makes the bass too muddy.

[jnew]

I wonder why that is. Are you still working on that muddy bass issue? It's gotta be something simple, right under you nose. I know you've got all the specs and gear dialed. Clips?

[EddyInChicago]

Agreed ^^^^Sticky this one.

1)Why not use some shielded cable for the "purple" wire?
2)How is the switch connected? If your set to 16ohms, how does the wire(FB) attached to a 4ohm work?

[parkhead]

As someone mentioned much earlier
you have reversed the primary wires on the output transformer.

the clip you posted is exactly what that sounds like, you have positive feedback
you cannot dial in the correct NFB resistor till you resolve this as all your listening tests are useless

p

[AndyK]

I'm wondering how much using a PPIMV in the lower settings reduces NFB? Does the stock 47k/8 ohm (with PPIMV set low) become 100k? 200k, etc?

[Jeremy1283]

why don't you try 27k-16ohm (that NFB setup was most likely in Ed's amp back in 1977) or 47k -8ohm

Why do you think eds amp had this? I am really confused on nfb

[stef]

why don't you try 27k-16ohm (that NFB setup was most likely in Ed's amp back in 1977) or 47k -8ohm

Why do you think eds amp had this? I am really confused on nfb

According to amp archives, this site etc, the usual stock NFB was 47k-16 or 47k-8 for 1968 lead amps. 27k is from the JTM era. In fact 27k-8 is almost the same NFB (mathematically) as 47k-16 but the OT tap is different, hence it should sound slightly different (more open or brighter maybe)