Build Your Own Clone Message Board

Making R10 switchable is simple enough. And this amp is really fun with the Negative Feedback disconnected. But to find the tones between "off" and "on" I'm thinking it'd be cool to put this on a pot. Simply adding a 10K pot in series w/ R10 ain't cutting it. Presumably because I'm adding resistance to the 22k that's already there. Would the solution be to change R10 to 12K, and then add the 10K pot in series?? Or am I over-simplifying it??

Looking at the '57 Twin-Amp layout & schematic from the Fender website (yes, a Champ ain't a Twin), the Neg Feedback resistor (R32) appears to be 56K, presumably a higher value due to the higher output of that amp. The Presence Control is a 5K pot situated between R32 and V4 with a .1 uf (?) cap between the pot and ground. (http://support.fender.com/schematics/gu ... ematic.pdf) Or might this approach be too complicated or unnecessary??

What do y'all think??

Thanks!
Erik

Make r10 a 25k pot.

Ummm....yea I guess I was making it too complicated. Hah! Thanks for the tip. Post results soon.....

Cheers!

That's not a presence control.
Just a variable NFB control.

Call it what you will. Wikipedia, for what it's worth, quoted the following from the book "Fender Amps" by John Teagle and John Sprung:

The original Fender presence control acted upon the amplifier's negative-feedback loop. As the level of "presence" was increased, so more and more of the higher frequencies in the negative-feedback loop were dumped to ground, leaving the low and mid-range frequencies. Increasing the presence resulted in there being less and less negative feedback on high frequencies. The effect varied according to amplitude.[5]

Later Fender amplifiers used a different presence control. The presence control on the 1975 Super Twin did not modify the negative feedback loop, but was a real equalization control, controlling the 3.9kHz frequency range. It had the ability to both amplify and reduce ("boost" and "cut" in amplification terminology), unlike the earlier presence control. It was active equalization.[5]

So.....that's why I called it a presence control.

A presence control is part of NFB. This mod brings NFB in and out.

There is no "presence" effect in the Champ. (you probably already knew that but I'm just stating the facts for others)

Check that wiki link you posted again, it's not a presence control:
As designed, there are no components in the NFB circuit that "dump high frequencies to ground". All you have is R10 which simply changes the outout of phase voltage signal going into the power tube cathode - at all frequencies. By adjusting it, most notabiily you will hear the output level change, and as it gets louder, become more "raw" as a NFB is reduced. In order to make a presence circuit, you need to add an aditional pot and cap to the NFB loop, see attached schematic clip (I'm pretty sure this is how you would do this, I've never done a presence circuit on a single output tube amp before, I have phase inverters to play with on the amps I work on).

The value of the cap can be whatever you want it to be. It and the value R10 will determine the corner frequency according to 1/(2*pi*R*C) - so if you have both variable NFB and a presence control, they will interact. The additional pot is a level control, ~5k (or 25K with a 10K parallel resistor, which is about 7K) is a standard value in big amps; I'm not sure what would be appropriate here.

The Presence circuit defaced describes is very similar to the one on the '57 Twin-Amp I referenced earlier. Must be because that's a true presence control!

I'll stand corrected.

Cheers!

Cool. If you decide to add the parts for a presence circuit, post up what values you end up with. IMO, this would be a good addition to the mods thread.

I was thinking about this while doing some things this morning and remembered something I should have said earlier. By turning R10 into a 25k pot, effectively you'll be taking the champ from whatever NFB ratio it has now, to "max" negative feedback. From an operational standpoint this may cause the amp to oscillate (reference: http://www.aikenamps.com/GlobalNegativeFeedback.htm, this is in reference to a push pull output stage, but the concept is the same); you would want to test to see if the amp oscillates with full NFB, the extra phase shifts in a PP stage will increase the likelihood of oscillation over a single ended stage like what the Champ has. From a tonal standpoint, this will reduce the output level and make the amp more "controlled" because of the increased speaker dampening.

IMO, it would be more interesting to look at the other end of the spectrum, using less NFB. To see the max of where this could go simply remove any of the wires in the NFB path. If that sounds good, then find a resistor that's large enough to give you something similar to that sound. maybe 100k, maybe 250K (keep them similar to stock pot sizes, and my guess is that you'll want these to be log (audio) pots). Cool, that's the minimum NFB you'll want. Now remove those and find the max NFB by subbing in smaller resistors, like the stock value of 22k, or see how much more NFB you can get away with/what sounds good, maybe reduce R10 to 10k or 4.7k (these are just suggested values I'm pulling out of thin air, experimentation will yield useful values). Cool, now you've book-ended the "no" NFB condition, and "max" NFB condition, so put the pot (wired as a variable resistor, "NFB Pot" on the schematic) and the resistor in series, now you have variable NFB that swings either side of the stock value for a more raw or more compressed sound. A pot with a switch in it would be cool to use if you want a true "no" NFB condition, or wiring a stand alone switch ("NFB Switch" on the schematic) in series.

That's some nice work defaced. I've only experimented with lifting the nfb (which I didn't like - made the bottom end distort as you might expect)

That's exactly what I'm after. Had a push-pull 10K pot handy. But the audible difference of the 10K pot at any setting in addition to the 22k NFB resistor was negligible to my ears using a Tele...nothing useable. So based on that I'd think going higher than the 25k pot you recommended won't make any real difference. Expecting to have a push-pull 25K pot in hand Monday. So I'll poke around a bit once that's here.