Cut control questions

[Fred G.]

I have a Fender Bandmaster which I have modded with a 4-cap post-pi MV - .1uF caps in, 33k resistors off the pots into 1.0uF caps going out. I have completely disconnected the negative feedback. It soulds pretty good with the exception of some high end slice which always seems to be lurking around.

I'd like to install a cut control to tame this - I have an extra pot position open. But - I attempted a x-line MV, and it worked SOOOOO poorly that I'm apprehensive about the Cut control. The Xline MV jumped from 0 to MAX pretty dang quickly. I don't want a Cut control that does this.

All info and ideas greatly appreciated!

[MWJB]

Before you disconnected the NFB did you try larger feedback resistors? Say 1.5K to 4.7K, assuming it's a tolex Bandmaster.

[Fred G.]

Yep, sure did. Right now I have a 100K/4.7k feedback/shunt combo, but they're disconnected at the output jack.

I honestly haven't noticed THAT much difference between that particular resistor combo and having it totally disconnected...but I'm curious as to where you're heading with this!

Thanks,

Fred G.

[Enzo]

You are complaining of the high end, and he is talking about keeping some feedback, whuch should serve to control that high end.

[Richard]

Could a capacitor be placed in the NFB loop to only let the high frequency through?

[Enzo]

yes.

[Fred G.]

Well, it was my understanding (from a previous exchange here) that the post-PI Master volume essentially either removes most of the NFB, or screws about with it so much, that it's best to just disconnect it when using this type MV.

I must confess to a great deal of confusion about NFB at times.

The addition of NFB would, then, tame the high end problems? With this amp, it has seemingly been the opposite - it has been the MOST shrill when having the Fender Blackface NFB resistor combination, and better behave with progressively LES NFB. At least that's how it has seemed...in frustration, I have turned to just disconnecting it altogether and contemplating a Cut control.

More info certainly appreciated!

Thanks,

Fred G.

[MWJB]

The values you mentioned earlier are not typical blackface values. Fender usually used 47ohms (sometimes 100ohms) to ground with an 820ohm feeback resistor. But it's the ratio between the 2 resistors that's critical.

Typically more negative feedback (smaller dropping resistor to the speaker tap) softens high end response. Larger resistor values (less feedback) sound more aggressive.

I wouldn't necessarily expect your master to have a big impact on the NFB specifically and there's no mention of disconnecting it in the Trainwreck diagram.

[rathmann]

Not necessarily advocating reconnecting the feedback, but is it possible some modification has reversed the phase of your output section? That would turn NFB into positive feedback causing the behavior you describe (shrill, better sound with reduced feedback) Not all positive feedback will immediately result in runaway screeching oscillation.

Anyway, I've used the Cut control successfully on a couple of projects. You may need to experiment with the pot value to get an optimal range of control. I've used 100k linear and 250k audio values with good results. Cap values to start with might be something like .01, .0068, or .0047. The larger the cap the more "meat" will get cut when turning up the control (as the pot value approaches 0), so larger pot values would be in order. Smaller caps just to tame "fizz" can use smaller pot values.

Steve