GNFB resistor tuning

[erwin_ve]

Hi all,

For a while I have the RF mods in my amp.
But I've never been happy with the presence control of my amp. I literally had to crank the presence control to make the amp balanced.

I had a nice discussion with forummember Airniles about this because he experienced the same.(different build)

First thing I did was to make the presence pot 1K= no good.
Airniles suggested to make the GNFB resistor 10k instead of 4,7k. I was a bit reluctant to try this becuase the frequency knee shifts when doing that.

Anyway tried it and for me it works; The amp feels better. And there's a better finetuning range for the sweetspot. I'm really happy with this one.

Anyone else tried or experimented with this?

[phalanges]

I have a D'lite head, high plates, skyline stack,(though with 400pf treble cap value) on which I changed the GNFB resistor to 10k carbon comp and it seems to have "opened" the sound some. I was bothered by a bit of constriction or nasal-ness to the sound and this helped.
What really dialed in the sound for my style (rock) was changing the V2 snubbers to 120/330.

My thanks to all.

[ampdork]

You have the 2.2Uf cap and it needs to be cranked?

[erwin_ve]

You have the 2.2Uf cap and it needs to be cranked?

Yes in the old setting the presence pot needed to be.

[Structo]

Not sure I understand the logic of that tweak?

By increasing the 4K7 feedback resistor to 10K, aren't you decreasing the feedback?

[bluesfendermanblues]

Not sure I understand the logic of that tweak?

By increasing the 4K7 feedback resistor to 10K, aren't you decreasing the feedback?

Yes,10k is less feedback. And therefore a looser sound.

[Structo]

But how does that affect the presence part of it as far as top end goes?

Are you keeping the 2.2uF cap in addition to the 10K NFB resistor?

The stock Brown Note D'lite NFB resistor is 6.2K and that is with their 6V6 layout.
Not sure if the 50 watt 6L6 amp should use that but I have mine at 4.7K.

[erwin_ve]

10k GNFB has less dampening effect on the high frequencies because less signal from the OT is fed back to the PI. (think phase cancellation)
The more you open the presence pot, the more signal from the OT is send to ground by the presence cap.( Very similar to snubbers on V2)

The presence cap determines which range is dampened.
A 2,2uF presence cap is affecting mid-high freq.
If the presence cap is 1uF, the overtone range is affected.

IMO opinion this is what happens. I think tuning the GNFB resistor and presence cap is very useful for tuning your sound when choosing a particular speaker. EVM12L vs G12-65 f.e.

[Structo]

Thanks, that helps me greatly.

[erwin_ve]

Thanks, that helps me greatly.

I'm glad I could explain; thats what this forum is all about

[ampdork]

Sorry I am an idiot....I completely glossed over some details in this post.

[erwin_ve]

Sorry I am an idiot....I completely glossed over some details in this post.

Did you ever tried other values for your D-builds Shad? I'm curious...