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

Discussion in 'Amp Tech Center' started by King Fan, Nov 28, 2020.

  1. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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?

    NFB pot per Bruce Collins.png
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Since you want to stay somewhat consistent with the GA-5 schematic, the 47k NFB resistor could be replaced with a ~20k resistor in series with a 50K pot (variable resistor). Heavy NFB would be ~20K and light NFB would be ~70K.
     
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  3. FenderLover

    FenderLover Poster Extraordinaire

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    Bruce's idea will work, but notice the 47 ohm/3.9K gives you only a little over 1% feedback of the signal voltage, and the pot reduces the level from there. I'd short the 3.9K and determine your own minimum/maximum and sub in a resistor value to match.
     
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  4. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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?
     
  5. FenderLover

    FenderLover Poster Extraordinaire

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    Without the bypass (standard 25uF full-bypass) you'll loose about 6dB (half) the signal in amplification with the same input level, BUT... it will also take that much more signal at the input to overdrive the stage. It's one of those clean headroom-tuning places.
     
  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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?
     
  7. FenderLover

    FenderLover Poster Extraordinaire

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    Same effect on both gain stages. The smaller the cap, the higher the frequencies effected, where those frequencies will have greater amplitude.
     
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  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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?

    NFB pot simple style.png
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Well let's slow down and look at this a little more carefully.
    The stock 5F1 and 5F2A use a 12ax7 for those two gain stages. By comparison the GA-5 uses a 12ay7. The gain factor of a 12ax7 is 100 where a 12ay7 is 40. The Fender provides more gain than the Gibson even when the Gibson has the 220k plate resistors and different preamp bias resistors.

    Wait a minute this makes the NFB differences not so even Steven.:)

    EDIT: KF pointed out my error. The GA-5 does indeed have a 12ax7 in V1
     
    Last edited: Nov 28, 2020
  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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:

    upload_2020-11-28_14-48-22.jpeg
     
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  11. FenderLover

    FenderLover Poster Extraordinaire

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    Your minimum feedback resistance (highest feedback voltage) in post #8 will be 22K and in parallel with the 1.5K cathode resistor (conducted through the speaker to ground). As long as it is at least 10X the value, it will not affect the bias. However, there is a DC path to the speaker jack, so maybe a DC blocking cap on the wiper would be a good idea. The value an be experimented with; large value would be transparent to all feedback frequencies, small value can offer a tuning effect.
     
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  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Hah. My bad eyes got the best of me. The fuzzy GA-5 schematic I was looking at does indeed say 12ax7.
    So like Emily Litella, I will say *Never Mind*. :oops:
     
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  13. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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?

    upload_2020-11-28_18-32-12.png

    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.
     
  14. FenderLover

    FenderLover Poster Extraordinaire

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    You are correct. Another detail we can obsess over for legitimate reasons, or copy the old schematics and be just as happy.
     
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  15. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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?
     
  16. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    One side of the 22k NFB resistor will be at +1.5V or so due to the 12AX7's cathode.
     
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  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The 1.5 VDC at the cathode wouldn't be much at the speaker as most would go to ground thru the 1k5 cathode resistor. In your diagram of post #8 you could keep even more of the DC off the pot wiper by swapping positions of the pot and the 22k NFB resistor. A cap would work but I am not convinced it is necessary. OMMV.
     
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  18. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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.
     
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  19. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    I can think of two reasons to use a DC-blocking cap: one is to keep DC off the speaker, but as LLC points out, it'll be much less at the speaker and it wasn't a problem for millions of Fender amps. Another reason might be to prevent a dirty/worn NFB pot from crackling as you turned it. Neither would be very compelling to me. :)
     
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  20. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    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:

    NFB cut or boost switch.png
     
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