5F1 variable NFB control

[Gregg Hermetech]

I'm a total newbie, only built one amp so far (it worked first time and sounds great), so please be kind!

I built a 5F1 based kit from AmpMaker in the UK, please see the attached schematic below. I was watching one of Uncle Doug's vids, and would like to add a variable negative feedback control to the amplifier.



From what I understand, replacing R7 (the 22k NFB resistor) with a 10k resistor and a 50k pot in series will allow me to sweep between even more NFB than I have now with the control at minimum (10k), to almost no NFB with the pot at full (60k). Please correct me if I'm wrong, or suggest alternative or "better" values.

Questions:

1) Let's say I did this, but would still like to know where the original value of 22k was on the pot's travel, to easily set it back to "stock". Where do I put my DMM probes on the pot to measure exactly where 22k is? Which pot lugs to measure?

2) Would it be possible to use a special "switched pot", so that when you turn it to maximum it "clicks" and the NFB is completely switched out of the circuit? If so please share any more relevant info, what this type of pot is named etc.

3) What taper would you recommend for NFB control? Uncle Doug doesn't mention taper. I've been spending time on the online Tone Stack Calculator a fair bit recently. I think I understand the difference between a linear and logarithmic (or "audio") taper pot, but on their calculator they have two different types of Log, called Log A and Log B. What is the difference and what should be I looking for when I order parts if I'm specifically looking for a Log A or Log B pot, are they named that in parts catalogues?

https://www.guitarscience.net/tsc/fende ... T_pot=LogA

Sorry for all the questions, but I want to do it once, and do it right. Thanks for any help!

[Gregg Hermetech]

And finally:

4) Where are good places in the EU to source components? Mouser, or are you using guitar or amp specific places like Tube-Town?

[martin manning]

From what I understand, replacing R7 (the 22k NFB resistor) with a 10k resistor and a 50k pot in series will allow me to sweep between even more NFB than I have now with the control at minimum (10k), to almost no NFB with the pot at full (60k). Please correct me if I'm wrong, or suggest alternative or "better" values.

Sounds like a good place to start. You will likely have to experiment to find a range of values that you find useful.

1) Let's say I did this, but would still like to know where the original value of 22k was on the pot's travel, to easily set it back to "stock". Where do I put my DMM probes on the pot to measure exactly where 22k is? Which pot lugs to measure?

Why don't you grab a pot and measure it? The middle lug is the wiper, and it sweeps from one outer lug to the other on the resistive track. Here's some reading: http://www.geofex.com/article_folders/p ... tscret.htm

2) Would it be possible to use a special "switched pot", so that when you turn it to maximum it "clicks" and the NFB is completely switched out of the circuit? If so please share any more relevant info, what this type of pot is named etc.

A volume control with a switch on the back (full CCW opens switch) could be used, but it may be hard to find one in the value you need. A pot with a pull switch would also work. Either way all you need to do is open the feedback loop.

3) What taper would you recommend for NFB control? Uncle Doug doesn't mention taperI think I understand the difference between a linear and logarithmic (or "audio") taper pot, but on their calculator they have two different types of Log, called Log A and Log B. What is the difference and what should be I looking for when I order parts if I'm specifically looking for a Log A or Log B pot, are they named that in parts catalogues?

The typical notation is B for linear taper and A for "log" or audio taper. Common audio taper pots are at 10% of total resistance at 50% rotation. Fender amp treble controls traditionally use a 30% of total resistance at 50% rotation taper, and here are several others including reverse log. For your purpose, I'd suggest a linear taper, so look for a 50kB pot.

[Gregg Hermetech]

Thank you for all the info Martin, lots to think about!

I don't have a spare pot, but next time I get the DMM out I'll have a play reading the resistance across the lugs on the amp's volume control for practise. Will read your link on pots.

[Raja_Kentut]

And finally:

4) Where are good places in the EU to source components? Mouser, or are you using guitar or amp specific places like Tube-Town?

www.reichelt.de
www.pollin.de
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