disconnecting NFB in Fender amp

[Jef]

I just removed the pull boost circuitry and disconnected the master volume of my late '70s Pro Reverb. I want to make the NFB switchable with the pull switch on the master volume pot (which is a double pole single throw switch).

One question about the 'presence' resistor: can I just leave it between the PI tail resistor and ground when the NFB is disconnected? Or do I need to ground the tail resistor?

Thanks!

Jef

[hasserl]

It's just a 100 ohm resistor, I doubt there is any reason why you'd have to remove it. However I'll defer on that question to the more knowledgeable guys around here.

What I treally wanted to say was you can make that Master Volume control very useful, and it's super easy to do. Wire up a crossline type post PI master volume, it works very nice. I did just that with my silver face Super Reverb and am very happy with the result. It makes the amp much more versatile, allows me to use it on gigs I never would be able to without it as it'd be much, much too loud. But the MV lets me scrub off some volume and keep everybody happy.

The crossline may not be the best post PI MV, and one of the dual ganged pot style MV's may sound a little better, but the crossline is so easy to install, using the original pot, it's kind of a no brainer. You've got the pot sitting there, it takes about 5 minutes max to wire it up. Go for it, you'll be happy you did.

[Enzo]

Put it in context. Between cathode and ground, there is the 100 ohm and the other multithousand ohm resistor. That 100 ohms is such a tiny small percent, it won't matter at all. For instance, if it was in series with a 10k (10,000) resistor, the circuit would just see it as a 10,100 ohm resistor. That is one percent differnt than without the 100 ohms. Your amp has a 22k and a 470? The 100 ohms is less than half a percent of that. Since the parts in the amp are at most 5% tolerance, and possible larger amounts, we can safely ignore the 100 ohms.