Local feedback loop question

[chocopower]

Hi!

Just a question..

I want to add a local feedback loop to V1 of my Pro-Reverb clon.

In layouts i saw a 22M resistor and a .05uf cap.

But.... What´s the reason for the 200K resistor (some times paralelled with a 390pf cap)?

Could i ignore it? Is that resistor part of the local feedback loop?


Best regards from spain, and sorry for my english.....

[mlp-mx6]

No, do not skip that. That resistor is the grid stopper for that stage. The small cap is a "bright" cap for that grid stopper.

[Fischerman]

If I'm understanding your question correctly...the 220k is the grid stopper for the grid of the next stage and it is bypassed with a cap (390pF or 500pF). But if the schematics I've seen are correct...that 220k and cap are not 'inside' the loop when the feedback loop is for CL2. The 220k and cap are in the loop when the feedback loop is put on the Return stage for a Dumbleator...but the cap value for the feedback loop is different as well (.22uF instead of .05uF).

Also note that the resistor value can be as big as 44M and at least as low as 22M maybe lower.

[chocopower]

I want to solder it to a 3 position-center off swicht with another 22M resistor... soo
1.- F.Loop On. 22M
2.- f.Loop off
3.- F. Loop On. 44M

My question is if the 220K/390pf will pair affect the sound in the bypass mode..


thanks for your fast positive feedback loop....

[Fischerman]

What I'm saying is that the 220k and the cap are part of the 'basic circuit'...regardless of whether you use the feedback loop. Those parts are not part of the loop...they're just part of that particular preamp circuit.

[chocopower]

so.... if i´m only interesting in the preamp feedback loop, i should skip them. isn´t?

[Fischerman]

Yes, just disregard the 220k and cap. They are not part of the feedback loop.

[mlp-mx6]

For the purposes of the feedback loop, yes, they are not a part of THAT circuit. Do not leave them off the tube socket, however. They are much more important to your tone than the feedback circuit.

[Fischerman]

mlp,
Very important part of choco's op:

I want to add a local feedback loop to V1 of my Pro-Reverb clon.

Those just go straight from the volume control to the grid of the second stage...no stopper or cap.

[mlp-mx6]

Y'know, that IS important! Thanks for helping me see what I missed.

[jerrydyer]

what exactly does that 22m loop circuit do? thanx

[llemtt]

it's a negative feedback loop, so does whatever negative feedback does: reduces distortion, improves freq response, lowers output impedance,...

the right question is: why is there?

teo

[chocopower]

it's a negative feedback loop, so does whatever negative feedback does: reduces distortion, improves freq response, lowers output impedance,...

the right question is: why is there?

teo

I don´t know.....but i´m gonna install it in 2 hours and monday..... i´ll be back.....whit a lot more questions!!!


Thanks everybody for your help. This forum (and Ampage, of course) are the best tecnichal support in the world!!

ahhh!!

This is my amp layout with this mod in V1...

[IMG614]http://i101.photobucket.com/albums/m74/ ... AYOUT5.jpg[/img]

[chocopower]

Ok....

mod is done.......and only a subtle effect is audible when i move the swicht... and the most evident change is in the 22M position, not in the 42M!!!!

I had mount the swicht in the front panel and the cap and one of the resistors in the main board. The other reistor is pick in the swicht.

Should i mount them direct to the V1 pins, as showed in the original mod, for a more significant effect?


best regards from Spain...

[mlp-mx6]

Your description sounds exactly correct. It is a subtle effect.

22M offers more feedback than 44M, so I would expect more effect at that setting.

Closeness to the socket will not make a difference, except maybe for noise - but you did not mention noise. Sounds like things are just as they should be.