NFB loop changes

[billyz]

Does it make any difference whether you change the 100k feed back resistor coming from the secondary side of the OT or change the grounded resistance I.E. the 5k pot to the PI circuit? Sound wise , gain , tone type of distortion?

[rhinson]

yep changing the nfb resistor makes a big change. as you lower it the amp gets less loud, tighter/more defined sounding and less full sounding.

i stick with the standard 5k pot or change it to the blonde bassman style of p.i. that takes dc off the pot for the most part. a big change can be had by increasing the cap value -----try anything above the .1uf up to .68uf-----i like a.18uf to .33uf better than stock in a wreck. rh

[novosibir]

No, as long as you keep the relation, but you also have to include the pres cap into the relation bill!

Example I: 100K FBR; 5K pot; 0.1 pres C

Example II: 50K FBR; 2.5K pot; 0.2 pres C

Larry

[rhinson]

No, as long as you keep the relation, but you also have to include the pres cap into the relation bill!

Example I: 100K FBR; 5K pot; 0.1 pres C

Example II: 50K FBR; 2.5K pot; 0.2 pres C

Larry

this is true---keeping the ratios the same make things work the same. BUT if you want to see how this affects your amp, tone, etc for experimenting and modding purposes it's good just to have at it and lower/raise the nfb resistor to get an idea how tone is affected--a low resistor with a series 100k pot is always fun---same with changing the tap from which the voltage is taken. as you know, vintage marshalls sometimes had a 47k and sometimes had a 100k. jtm45's had a 27k (copied from the tweed bassman) nfb resistor, but with the voltage taken off the 16ohm tap instead of off the 2ohm like the bassman, making for one of the big differences between the two. in all these cases they left the presence part of the circuit the same. so they were having fun too! rh

[billyz]

Hey thanks for the replies.
I have in the past varied the Resistor coming from the Output Transformer Sometimes known as the " Soul" control. But recently I have begun experimenting with the resistor going to ground from the Phase invertor input. And have found a lot of room to adjust the gain. So far nothing bad has happened except at the extremes. Just wondered if It the same as adjusting the " Soul" resistor.

[novosibir]

Just wondered if It the same as adjusting the "Soul" resistor.

You're right! The variable FB resistor some call 'Soul Control'.

Larry