NFB Resistor Value 50W vs 100W

[richon]

Hi,

Just read lots of post of different amp makers and lots said that at the same NFB resistor the 100W amps had more negative feedback than 50W models

so I was wandering if my Dumble inspired 50W amp (with 390R and 4.7K) would be noticeable changed when using a 3.3K instead of the 4.7K resistor in the NFB

[Roe]

try it. I like a pot in series with a resistor

[martin manning]

Very easy to find out- just clip a 10k across the 4k7 and you have it.

[erwin_ve]

I tend to adjust the NFB resistor in relation with the speaker used. Fe. Celestion G12-65 does sound happier with a larger resistor and a EV 12M with a smaller resistor. So darker speaker; bigger resistor. vice versa.

[Structo]

Interesting topic.
When I built my Brown Note D'lite 50w, the layout had the FB resistor as 6K2 rather than the customary 4K7 you see on the 100w Dumbles.

On the other thread about it, Gary (Glaswerks) mentioned he liked to keep the ratio between the FB resistor and tail resistor the same scale.

So 12:1 is the ratio with a 4K7 and 390R resistors.

I actually put a switched pot for my presence pot in my 100w ODS clone so that I can turn off the NFB.

When I do that the amp gets quite a bit louder and raw sounding. More treble and high mids.
Works great when playing at a lower volume.

What I don't like about the usual 4K7/ 390R 1uF Dumble presence circuit is that it is pretty subtle until I get above 2:00 on the knob.

I need to experiment with this more.
I have a resistance decade box that will help.

Am I correct in thinking that the larger the FB resistor is, the less feedback is sent back?

Also how does the output transformer tap that the NFB is attached to affect the feedback?

[ToneMerc]

Am I correct in thinking that the larger the FB resistor is, the less feedback is sent back?

Also how does the output transformer tap that the NFB is attached to affect the feedback?

http://www.aikenamps.com/NegativeFeedback.htm

TM

[Structo]

Thanks for that, I forgot about that article.

This is Gary's post about the NFB ratio.

https://tubeamparchive.com/viewtopic.ph ... 020#224020

[roberto]

Of course a 100W amp will have more NFB if you use the same NFB resistors as the 50W. A 100W amp will have sqrt(2) times the voltage on the output, so you have to multiply the NFB resistor by sqrt(2) times (or lower the tap at whick you keep the NFB).

I like to add a pot in series with the NFB resistor just to fit the PA response to the cabinet you use, and give different feelings to the amp.

[rogb]

I'll be approaching this tricky subject this month, what is the consensus on the 50w NFB, start at 4k7 and work up/down/ add a trimmer/pot in series?

I'm more confused now than at the start of the NFB epic voyage.

In my Deluxe I leave it in circuit for Fender and switch it out for Marshal/Vox - that was easy compared to this