Amp Guys: How do you remove the NFB loop in SF Pro Reverb?

[Lockbody]

I'd like to experiment with removing the negative feedback loop in my '79 SF Pro Reverb (pull-boost master vol and Ultra Linear trans, natch). Lots (too much actually) of info ranging from clipping resistors to removing wires form the speaker outputs, so I'd rather ask someone who has experience doing it. I'm not afraid to work with a soldering iron and have done minor amp mods in the past (and yes, I'm plenty aware of the lethality of the voltages inside of the amp.).

Also, I know there's limited mods that can be done to these UL Fenders, but besides the NFB mod, removing the master volume (or changing it to different type) seems to come up a lot. I might be interested in this too, as I always run the MV on "10" and control the volume with the channel vol.

Thanks!

[RoyB]

I'd like to experiment with removing the negative feedback loop in my '79 SF Pro Reverb (pull-boost master vol and Ultra Linear trans, natch). Lots (too much actually) of info ranging from clipping resistors to removing wires form the speaker outputs, so I'd rather ask someone who has experience doing it. I'm not afraid to work with a soldering iron and have done minor amp mods in the past (and yes, I'm plenty aware of the lethality of the voltages inside of the amp.).

Also, I know there's limited mods that can be done to these UL Fenders, but besides the NFB mod, removing the master volume (or changing it to different type) seems to come up a lot. I might be interested in this too, as I always run the MV on "10" and control the volume with the channel vol.

Thanks!

Disconnecting the wire going from the speaker jack to the board it's self is take if out of the system, but be aware your 79 may not be stable without it and may just sound unusable and nasty. Just unsolder the wire from the jack and tape it off and give it a try. Personally I don't like the sound of Blackface or silverface amps with it removed, but that's just me.

[Lockbody]

Thanks Roy. I been digging through the FenderForum archives back to 2002 and I found a post where a member, Steve Dallman, suggest this mod:

With a bright amp like a twin, you might try a "resonance" control instead of lowering the NFB. The resonance decouples the NFB at the low end leaving the high end tight and controled.

Disconnect the NFB line, and solder in a .0047uF cap. Now solder a 1Meg audio taper pot across the cap, so turning down the pot will short out the cap. There's your resonance control. Turn it up and the low end will loosen up and get nicely floppy.

Now that I know what I'm looking for I might give this a try. Thanks again.

[RoyB]

Thanks Roy. I been digging through the FenderForum archives back to 2002 and I found a post where a member, Steve Dallman, suggest this mod:

With a bright amp like a twin, you might try a "resonance" control instead of lowering the NFB. The resonance decouples the NFB at the low end leaving the high end tight and controled.

Disconnect the NFB line, and solder in a .0047uF cap. Now solder a 1Meg audio taper pot across the cap, so turning down the pot will short out the cap. There's your resonance control. Turn it up and the low end will loosen up and get nicely floppy.

Now that I know what I'm looking for I might give this a try. Thanks again.

It's an easy mod and worth a try. Some people love it and it works great with some amps. What are you trying to achieve in your tone (more gain, open tone fuller sound, quicker breakup)?

[Lockbody]

What are you trying to achieve in your tone (more gain, open tone fuller sound, quicker breakup)?

Yeah, all of that Mainly I wish it was a 40w Pro Reverb instead of a 70w UL, but it is what it is.

Like I said, I know I'm really limited what I can do short of gutting the thing, but I would like a little earlier breakup as, like many UL's it doesn't sound half-bad when you get it cooking. Gain, I got pedals, but a tad more open tone at a more manageable volume wouldn't be bad either.

I do really think I'd like to get rid of the MV, too. I've never played a Fender amp with one I've liked.

If it hadn't led such a terrible life before I inherited it (it was abandoned at a friends practice space for the last five years, and not taken care of very well before that), I'd sell it to a pedal steel player and get something else.

[RoyB]

What are you trying to achieve in your tone (more gain, open tone fuller sound, quicker breakup)?

Yeah, all of that Mainly I wish it was a 40w Pro Reverb instead of a 70w UL, but it is what it is.

Like I said, I know I'm really limited what I can do short of gutting the thing, but I would like a little earlier breakup as, like many UL's it doesn't sound half-bad when you get it cooking. Gain, I got pedals, but a tad more open tone at a more manageable volume wouldn't be bad either.

I do really think I'd like to get rid of the MV, too. I've never played a Fender amp with one I've liked.

If it hadn't led such a terrible life before I inherited it (it was abandoned at a friends practice space for the last five years, and not taken care of very well before that), I'd sell it to a pedal steel player and get something else.

Really I had one of those for years and I really like their sound better than the early models. Some cool tricks you might try.

#1 Change the resistor on the bass control to a 10k

#2 disconnect the tremolo intensity control

#3 change the phase inverter tube from a 12AT7 to a 12AX7

These things will really help.

[Lockbody]

Really I had one of those for years and I really like their sound better than the early models. Some cool tricks you might try.

#1 Change the resistor on the bass control to a 10k

#2 disconnect the tremolo intensity control

#3 change the phase inverter tube from a 12AT7 to a 12AX7

These things will really help.

Cool, Thanks!

So #2, I just desolder the pot and cap the leads, or is there more to it?

[RoyB]

Really I had one of those for years and I really like their sound better than the early models. Some cool tricks you might try.

#1 Change the resistor on the bass control to a 10k

#2 disconnect the tremolo intensity control

#3 change the phase inverter tube from a 12AT7 to a 12AX7

These things will really help.

Cool, Thanks!

So #2, I just desolder the pot and cap the leads, or is there more to it?

I kept notes on what I did I'll look into it tomorrow and let you know what to do.

[Lockbody]

Thanks Roy. I can't wait to try it out.

[velorush]

If I can jump in: Roy, what does the higher gain tube get you in the PI position?

[Lockbody]

If I can jump in: Roy, what does the higher gain tube get you in the PI position?

I think it's a hotter signal that pushes the power amp harder, so less headroom.

[RoyB]

Correct. It drives the power tubes harder.

[RoyB]

Oh one other quick trick to try. Remove the first 12AX7 preamp tube that controls the normal channel. Looking at the amp from the back it's the first tube on the right furthest from the power tubes. It lets the Vibrato channel break up faster and sound a lot my responsive. Then you can take that extra 12AX7 and put it in the PI position.

[Lockbody]

I did know that one, thanks.

Maybe I shouldn't be doing this as I pulled out the chassis and (a) can't find the nfb loop, and ( the tremolo intensity has two wire, the speed does have one, but neither are yellow. I've got some pics of the insides if you had a minute to look at them.

[RoyB]

I did know that one, thanks.

Maybe I shouldn't be doing this as I pulled out the chassis and (a) can't find the nfb loop, and ( the tremolo intensity has two wire, the speed does have one, but neither are yellow. I've got some pics of the insides if you had a minute to look at them.

Send me the photos at support@rsguitarworks.net