schematic for nfb defeat switch please

[funkmeblue]

anyone here that can point me in the direction of a schematic for a negative feedback removal switch? thanks

[Colossal]

anyone here that can point me in the direction of a schematic for a negative feedback removal switch? thanks

FMB,

All you would need is a SPST switch. Just place it between the tap on your output transformer supplying the feedback and the negative feedback resistor or between the resistor and the insertion point in the power amp receiving the feedback. Flip the switch and voila', it opens the circuit and no more feedback. If you are unsure of what I mean, post a schematic of your amp, I will add a drawing showing the change and repost.

[funkmeblue]

right on! Just wasn't sure if it was cool to leave the resistor in place coming from the phase inverter that would normally just go to ground on a non nfb amp. thanks

[DonMoose]

Just to be clear - losing the NFB also disables the presence control, if present.

[Baxtercat]

...then it becomes and absence control.

[wallybob]

Nice one Bob!

[DonMoose]

...then it becomes and absence control.

HAH! I believe you owe me a new keyboard.

[KT66]

I usually use a SPDT center off switch so that you can select 2 different NFB resistor values, or have it completely out of the circuit.

[paolojm]

I usually use a SPDT center off switch so that you can select 2 different NFB resistor values, or have it completely out of the circuit.

This is a great way of getting the open/loose vibe of Vox type power amp, or the the tightness of 6L6 type power stage, or somewhere inbetween, like an early Plexi, with one amp. Works great when the right resistor values are selected.

[Colossal]

I usually use a SPDT center off switch so that you can select 2 different NFB resistor values, or have it completely out of the circuit.

This is a great way of getting the open/loose vibe of Vox type power amp, or the the tightness of 6L6 type power stage, or somewhere inbetween, like an early Plexi, with one amp. Works great when the right resistor values are selected.

I have a quick question on the topic of NFB:

The Deluxe Reverb, Vibroverb, Vibrolux, and other amps in that series all have an 820R feedback resistor. Would there be any benefit to adding a pot there to vary the feedback (increasing it slightly) or is that something just better left alone (in other words, don't mess with a good thing - it ain't broke...). If it would add some value or interest, any ideas on a good value for the NFB pot? 5k-L? My goal is to ensure the clean, 3D quality and response of the amp stays intact.

Thanks.

[KT66]

Sure there is a benefit to monkeying with the NFB resistor : if you like it better with a different value, then it is better. As far as the standard Fender Blackface value goes, is it really the same ? For example, The Deluxe Reverb and Super Reverb both have the same 820 Ohm resistor, but the DR's is connected to an 8 Ohm tap and the SR is connected to a 2 Ohm tap. I think a 5K pot would be good to experiment with, any bigger might be too sensitive. Just hook it in series with the 820R resistor so that is the minimum resistance.

[Colossal]

Sure there is a benefit to monkeying with the NFB resistor : if you like it better with a different value, then it is better. As far as the standard Fender Blackface value goes, is it really the same ? For example, The Deluxe Reverb and Super Reverb both have the same 820 Ohm resistor, but the DR's is connected to an 8 Ohm tap and the SR is connected to a 2 Ohm tap.

Ahhh....good call, I hadn't noticed the different taps on all of the other models. But I really only looked at the Deluxe, Vibrolux, and Vibroverb, as a point of reference since I am basing a couple of builds around those. The Twin is NFB'd off its 4ohm tap. So, you are right, there is a lot of variance but the Rk stays the same.

I think a 5K pot would be good to experiment with, any bigger might be too sensitive.

Cool, thanks!