Adding adjustable NFB to 6L6 Blackvibe

[pjd3]

Hey there,

It's a good time to add some adjustable negative feedback to my 6L6 blackvibe (as seen on Rob Robinettes site).
I'm hoping someone here who has played with the NFB on an AB763 circuit could relay their experience with NFB in this circuit.

So Rob shows a nfb mod with a 3 position switch giving you:
530 ohms - heavy nfb
820 ohms - standard AB763 nfb and,
1500 ohms - light nfb

This sounds like a nice way to get some shifts in nfb but,
has everyone had good results with using say, around a 530 ohm resistor in series with a 2K ohm pot?
What resistance would you choose for the heaviest but usable nfb and how much resistance would you use for the least usable nfb?
I will be surely experimenting with range and values but it would be nice to hear what folks before me have settled on as a usable range for this type of circuit.
So you think Rob's ranges as stated above pretty much covers a useful range?
Thanks!
PJD3

[Stevem]

No one will know better what your needs are for the amount of feedback applied then you!
The way the amp reacts and falls over into hard clipping and how loose the bass responce gets, along with the level of back ground noise is all very dependent on how loud you play it and what output Transformer impeadance taps you pull the feedback voltage off from.

I would build the AB763 circuit as stock and then play the amp at 4 different volume levels while carefully noting the amps responce, then I would fully disconnect the feedback circuit and make the same test and notes, then you will have a much better idea as to how much feedback you may want for any given volume level you play at.

While you make these test I would strap voltmeter across the feedback connection point and make note of the minimum and maximum AC voltage you see.

[mojotom]

You could use a 10k pot in serie with the 820R feedback R and be done with it.

[pdf64]

...The way the amp reacts and falls over into hard clipping and how loose the bass responce gets, along with the level of back ground noise is all very dependent on how loud you play it and what output Transformer impeadance taps you pull the feedback voltage off from...
I would build the AB763 circuit as stock and then play the amp at 4 different volume levels while carefully noting the amps responce, then I would fully disconnect the feedback circuit and make the same test and notes, then you will have a much better idea as to how much feedback you may want for any given volume level you play at...

All pertinent points and good ideas but the term 'volume levels' is a little ambiguous, eg does it refer to volume control setting of instrument, volume control setting of amp, sound pressure SPL, preamp gain, system gain etc, or combination thereof.
Probably meant the amp volume control setting, ie preamp gain. Problem with that is when the NFB loop on the power amp is opened, the power amp gain will increase, which will greatly confuse the comparison.

To counter that, I suggest that a Type 4 master volume be fitted; set to full with NFB loop in place, adjust to normalise system gain when the NFB loop is opened.
The MkI Boogie has an example of a Type 4 master volume control https://el34world.com/charts/Schematics ... _marki.pdf

https://robrobinette.com/RR763_BLACKVIBE.htm
https://robrobinette.com/AB763_Modifica ... e_Feedback

[Stevem]

I was referring to the actual volume output of the amp.

[pdf64]

Ah, sorry I got the wrong end of the stick, thanks for the clarification.
Using the amp vol control to 'normalise' system gain would work, but need to make sure the bright switch is off.

[pjd3]

Thanks everyone for the inspiring replies,

I'm actually pretty confident in testing resistance ranges for nfb and seeing what I think will be useful to me. Its fairly straight ahead on how its done, and what some of the electrical limits entail,
I was mostly wondering what your personal experiences were with doing it, why you did it and what you liked or didn't like about it, and what range you found useful and why.
Whats going on here is that the Blackvibe is very, very clean, which is what I wanted since it is hugely a pedal platform. The pedals I use for distortion and emulation certain amp families doesn't get along with an amp that is distorted, even mildly sometimes.
I'm looking to actually rat up the sound a bit for when I'm not using pedals. I'm moderately familiar with the kind of effect less NFB can bring to an amp, and find a bit of it here and there could make the otherwise totally clean (no pedal) tone more useful for some things.
I know blackface designs have quite a bit of NFB to begin with so, I was more interested in creating less nfb than standard. It could very well be just a small amount of standard to moderately less NFB will be enough for my purposes but, I was really interested in how and how much NFB worked for you, and your performance sounds

I may very well just add a series pot to the 820 ohm resistor to cut it back some, and work with that.
Just curious - any warnings about pot/wire positioning for minimizing noise? I wanted the pot in the front panel and was just going to keep wiring away from anything carrying power supply voltage/current.

Thank everyone!
PJD3

[pdf64]

A JTM45 has significantly more NFB than any BF model.
A variable NFB control would be handling an output signal, so I suggest to keep it and its wiring away from inputs and early stages.

[Roe]

A JTM45 has significantly more NFB than any BF model.
A variable NFB control would be handling an output signal, so I suggest to keep it and its wiring away from inputs and early stages.

yes the nfb wire can be critical - it can squeal and add noise

[mirage_indigo]

I did this mod literally yesterday to my DRRI along with the robrob Reverb mod and the 3way FB mod. Honestly, at bedroom volumes i didn't hear that much of a difference in any of the three positions. Although I seemed to like the Heavy setting the best. It's a bit tighter and less flabby. I used the ground lift hole with a big toggle on-off-on I had lying around. Although I may not be hearing the mod in all it's glory.

The preamp revoicing to plexi specs works great, although it's really really hard to work on the back of the solder board for the v1a preamp side. I had a blob of solder go stray which led to very odd behavior on the normal channel. The reverb two channel mod also works great.

This particular DRRI has a bad, blatty low end that seems to follow volume levels. Not sure if it's badly biased or what so I'll likely do the PS cap mods next. Then I have a mouser order i need to compete today for a dual 250kA pot to put in the master, then I can figure out if it's the preamp blocking distortion of some kind or if its the PA running out of something.