Added Doug's NFB Control to my 5F1 - Not Working!

[Gregg Hermetech]

I built a 5F1 clone from a kit a few months ago and love it. I changed the fist preamp coupling cap to a lower value to get a bit less sub flub, and it sounded great.

Next I wanted to see what more NFB would do, so I changed the fixed 22k NFB resistor to a 10k and also loved the results: cleaner, more hi-fi.

Today I followed Uncle Doug's advice in this video, to be able to vary (and very much reduce) the NFB for raunchier tones:

https://youtu.be/H5fFb5_9xyA?t=124

I added a Linear 50k pot before the new 10k NFB resistor, which should theoretically allow me to sweep from just 10k resistance with the pot at zero, to 60k with it at full.

I wired it as a variable resistor like this, pot lugs 1 and 2 shorted together, with the OT coming in on lug 3 and the attenuated signal going out to the fixed resistor on lug 1:

Clipboard01.png

However, the pot doesn't do anything at all to the sound. The amp sounds the same as it did with just the fixed 10k resistor, whichever position I put the pot in. The amp sometimes crackles a bit when I move the new NFB pot.

Firstly, is my wiring correct?

Secondly, where should I start with the fault finding?

Thanks in advance for any help!

Gregg

[martin manning]

Nothing wrong with the concept. Can you put a meter from ground to lug 1 on the pot and measure the change in resistance as you rotate it?

[Gregg Hermetech]

Thanks Martin. Glad it should work in theory!

Switched off amp, unplugged everything, pulled tubes, made sure it was safe.

I am getting about 11.5k ohm resistance with one probe on pot lug 1, and the other probe on chassis ground, with the DMM set to resistance, wherever I move the pot, no change. Any ideas? I measured the pot before I soldered it up yesterday and was getting the expected 0-50k resistance along its travel.

[Gregg Hermetech]

OK I wiggled all the solder connections I had made/unsoldered/resoldered and took readings again. Now with lug 1 and Ground I'm getting from around 3 ohm to 9.3kOhm when turning the pot. Can I ask what should it be reading?

[sluckey]

OK I wiggled all the solder connections I had made/unsoldered/resoldered and took readings again. Now with lug 1 and Ground I'm getting from around 3 ohm to 9.3kOhm when turning the pot. Can I ask what should it be reading?

That sounds perfect.

[martin manning]

That sounds good. At full resistance you should have the 50k in parallel with the 10k fixed resistor + 1k5 cathode resistor, which is 9k3. 😋 At minimum resistance you are essentially grounded through the OT secondary.

[Gregg Hermetech]

It's working! Thanks to you both for hand holding me through this (I'm a total newbie). I don't know what was going on at first (stray solder whisker or something?), but after wiggling the solder connections all is working as expected. I get the nice clean tone with the pot at lowest, and a bit more volume, grunt, noise and breakup with it cranked to the top. Just what I wanted!

If I could ask one more question: What would be the lowest value you would recommend for the fixed NFB resistor? It's 10k at the moment. I REALLY like the cleaner tones, but read you shouldn't make the resistance too low or something terrible might happen (oscillation?) What might be a safe minimum?

I also intend to add a Framus Mid Scoop control for more Blackface style tones next, but will start another thread about it. I have a few questions...

[martin manning]

What would be the lowest value you would recommend for the fixed NFB resistor?

I would say 7k5, which is about 5x the cathode resistor value on the triode the NFB is going to. If you go lower, you will be moving its bias point significantly.

[Gregg Hermetech]

I would say 7k5, which is about 5x the cathode resistor value on the triode the NFB is going to. If you go lower, you will be moving its bias point significantly.

Thank you. I might just give that a go!

[Gregg Hermetech]

2 x 15k resistors in parallel will give me that, right? Literally just solder them next to each other using the same solder connections? Can't find 7.5k resistors on the Tube Town site.

[martin manning]

Yes. You could also use (7.5 x 10)/(10-7.5) = 30k in parallel with the 10k you have. It doesn't have to be exact, a standard 33k would make 7.7k

[Gregg Hermetech]

Thanks once again Martin, I'll order the 33k!