removing the negative feedback loop on Dumbleator?

[Joost]

Today I've been experimenting with some different values in my Dumbleator. The thing I found to offer most difference was taking down the value of the 220k grid resister on the return. However I didn't like what it did to the high-end.

I also took out the negative feedback loop on the return side by shorting it to ground between the resistor and the capacitor. There is a subtle sound difference. I assume it's there to stabilize something, so two questions:

1) will my Amplifier become more unstable if I remove it?
2) will it sound any different if I cut it out altogether instead of shorting it to ground between the resistor and capacitor?

[erwin_ve]

the lnfb works together with the grid. If you remove the lnfb and then lower the grid resistor it might end up nice. Omit the 500pf cap thats in parallel with the grid resistor. This doesn't make the amp unstable it just reacts different on larger signals. The non lnfb situation is more towards a non coloring loop, although it wil always color.
I dont think there is any difference between removing or shorting the lnfb to ground.

[Joost]

Interesting. I did some experimenting with lowering the grid resistor on the return by taking a 100 K pot and soldering alligator clips to the input and wiper. I put that pot in parallel with the 220k grid return. Then I listened to it with and without connecting it as well as finding the optimal spot on the potentiometer while it was connected.

Together with grounding the feedback loop, the high-end became a bit too sharp, but maybe I would just have to tune my bright caps to the new situation. plus I didn't try removing the 500 Pf capacitor.

Did you try this? I ask this because it's more involved surgery to take out the feedback loop, lower the resistor and take out the 500 PF bypass capacitor. Messing around close to heater wiring is always a pain. But more importantly, there's no easy way to A/B the situation, which I was able to do with the alligator clips lowering the grid resistor and another pair of alligator clips grounding out the feedback loop.

You know, I've also built in a switch so I can ground out the feedback loop on V1b and I definitely like it better without the feedback loop there. The feedback loop just cleaned it up a bit and makes it a bit too sterile for me. The difference is quite audible there. The same thing is going on with the feedback loop on the return of the Dumbleator however the difference is much more subtle. But again, without the feedback loop there's just more tone to me. I guess I don't like the feedback loop canceling out frequencies.

[erwin_ve]

Yeah the 500pF bypasses all high frequencies. removing it will make it easier what resistor to pick. You can go to as low as 15k and don't use a bypass cap at all.
I love the sound of a "normal"dlator, tried a few things. I still have 2 dlators for test situations like this.

[Joost]

Interesting stuff Erwin. thanks a lot for your input. I think I will bypass the negative feedback loop. It just sounds better to me.

I'm not sure about changing the grid resistor with the bypass cap. Maybe I should put 2 configurations on a breadboard or something with a switch so I can try out a low resistor value without bypass cap and the 220K resistor with 500pf bypass cap.

Since that's a bit of work, can you tell me from your own experience how to describe the difference between the two?

[erwin_ve]

Interesting stuff Erwin. thanks a lot for your input. I think I will bypass the negative feedback loop. It just sounds better to me.

I'm not sure about changing the grid resistor with the bypass cap. Maybe I should put 2 configurations on a breadboard or something with a switch so I can try out a low resistor value without bypass cap and the 220K resistor with 500pf bypass cap.

Since that's a bit of work, can you tell me from your own experience how to describe the difference between the two?

Sent you a PM, maybe we can meet.

[Joost]

Great. PM replied. Looking forward to meeting you!

[Teleguy61]

Interesting thread guys.
Please post your results and opinions.
Thanks.