Show me your 5F1 / 2a NFB pots. Or critique mine...

Anybody got pics or (extra credit) schemes of how they did this?

I'm thinking about a few minor mods to my 5F2a to make it more GA-5 (this thread if you care) and got to thinking I always wished I'd put in an NFB pot instead of a single value on a switch. At least use the pot to find a 'just right' value by ear.

FWIW my 5F2a uses an 8ohm speaker on the 8ohm tap.

I found a Bruce Collins post on MEF -- from 2010 -- where he suggests this:

… lift the second cathode biasing resistor (1K5), from ground with a 47 to 100 ohm resistor and insert NFB at their junction. Use a 2700-3900 ohm resistor at the speaker jack and over to a 10K-25K linear pot wired as a variable resistor. Then from the NFB pot, connect a wire to the junction of the 47-100 ohm resistor and the 1500 biasing resistor. You can still bypass the 1500 ohm resistor with a 1uF to 4.7uF cap but only bypass the 1k5 resistor not the 47-100 lifting one... make sure the cap is not grounded except by the lifting cap {I think he means 'resistor' here?}. Now you can tone tweak with different bypass caps and have a little or a lot of NFB because it is variable.
*The mod I do is a 3900 ohm resistor on the speaker jack hot lug over to a small 10Kb pot {‘b’ is linear taper} mounted on the rear chassis, a 47 ohm resistor under the standard 1k5 and a 1uF to 2.2uF bypass cap and that's all.*

I tried to draw his mod per that last paragraph. Did I get it right? Your thoughts?

Oh, and is there a simpler way, using just the stock 1.5K and existing NFB resistor -- like say if you wanted to end up finding a single "boost" value to switch in?

 

Excellent points -- gotta aim for that 47k GA-5 target in this project. In fact, from what I'm reading, somewhere in the 33k-56k range is actually pretty popular on the 8-ohm 5F2a.

Thanks, very helpful. You're right of course -- it's all about the NFB to tail ratio, and the percent of feedback.

Now I'm thinking that working in the original parameter space (22K up to 'mucho'K) with a 50k or 100k pot) would also be easier than trying to find a fixed value to sub in with a 10KL pot. A 10K pot might have advantages if you planned on leaving it there and dialing it as needed.

One more question. Do I really need or want a bypass cap on the NFB? I get the purpose -- choose which frequencies to suppress more v. less -- but how bad would it be if I just left it out?

 

Thanks, @FenderLover . D'you mean the cathode bypass cap on V1a? I was looking at the NFB bypass cap on Bruce's plan -- assumed that was about frequency shaping? Or does gain come in there too?

 

Oh, duh. An example of getting fooled by layout thinking. This was my first amp and I never saw back then that the 1.5K isn't there as an NFB 'tail' resistor, *it's the V2b cathode resistor.* Plus of course I'm about to replace that cathode resistor with a slightly bigger one. Double duh. Great example of how scheme thinking beats layout thinking, though.

All this gets me wondering; my amp-wise brother, who actually owns a 1959 GA-5, points out I want to see how *that* circuit sounds with *its* stock NFB before I decide if I need a pot or an alternate value on a switch. It has more gain? And less NFB than the 5F1/F2a? So maybe I'd even possibly want the option to *increase* NFB (clean it up, son, clean it up).

Pending more "duh" moments (not rare for me; call them "learning opportunities") I might just think of building it to GA5 stock and then see how I feel. IF I was still NFB curious, would this work?

 

Slowing down is always good -- I shoulda done that like 10 times already on this project.

But now recall another thought. "There is never just one Gibson model. Each Gibson model is infinite; it contains multitudes." The GA-5 I'm looking at uses an AX7 in V1:

 

Thanks, @FenderLover . I sense another chance to learn coming on. Bear with me and let me break it down.

I get that minimum resistance (max NFB) will be the 22K + 0 setting (in fact what I was aiming for)... and I can increase that to 22K + 100K (very little NFB?).

*In parallel, through the speaker, with the 1.5K?* Ah, right, they're parallel paths to ground. And (thinks) capacitance in parallel is additive. So make that 22K+1.5K. No huge diff?

*As long as it's ≥ 10x bigger (ie, ≥ 15K??), it won't affect the bias.* Aha, didn't know that! So using a 10K there *would or could* change the bias. Ding ding!

*But there's a DC path to the speaker jack?* OK, did I recently read that DC on a speaker is no bueno? So DC blocking cap on the wiper? I could draw that. And large value is transparent? Right, like a 25uF bypass cap -- all bypass all the time.

Aha, but now I get confused. Isn't my path just like Fender's, with different fixed resistor values (and the pot)? Did they have DC on the speaker?



Not challenging what you say here -- just trying to learn how/how much it matters. I'm *guessing* the DC block would be a nice detail, not a rigid requirement? but I'm seriously interested in learning more.

 

Ding ding ding. Thank you!

Learning stuff may be the best part of not knowing stuff.

Final question: Should it be obvious to me where the DC comes from?

 

Heh -- you mean the cathode voltage I write down every time I do a complete voltage table? That gets both a 'ding' and a 'duh' for me! But I'm both turkey-full and thankful for all the kindly help you guys are giving me.

 

FWIW, I was thinking I already have a nice footswitch jack wired up in this amp, and it'd be a shame not to use it. Someone mentioned you might rig a cathode bypass 0/1 switch. Especially if the GA5-ification makes this a dirt machine.

Now I have heard GA5s tend to have nice cleans in the lower volumes *and* nice dirt in the upper, so if I don't need to lift the bypass cap, and if I *do* find an alternate NFB value I like -- to either cut or boost the stock 47K -- here's how (I think) my current switch wiring would work: