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Hmm. Convert 5F2a to GA-5? ish?

Discussion in 'Shock Brother's DIY Amps' started by King Fan, Nov 27, 2020.

  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    KF, have a little fun.
    Check out the Rickenbacker M8E and B9A. The B9A input resistors with switches will twist your mind. Then what happens when you plug in a jumper between channels?

    o_O:twisted::confused:
     
  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Excellent ideas -- and a top tip re the switch/pot caution. But, I think I'll stick at 220K and 2.2K -- I guess I'm looking simply to find GA-5 in there, and not look for a more Marshall-y Fender. (EDIT: Oho, I see you're trying to tempt me down the path! Rickenbackers? Jumpers? Must ... Keep ... It ... Simple ....)

    Hmm, I do have some 220Ks in the stash. Nice 1/2W Dales -- or 3W (!!) metal oxides. Do they have to be 2W as suggested in that conversion article? Or is that just to reduce hiss? I'm pretty sure the Dales will be quiet enough...

    Lessee, back of envelope says if we had schematic 165V there, 220K would run at ~0.125W, and at (cautious doubling?) 330V, ~0.495W? So maybe the 1/2W could do the job?
     
    Last edited: Nov 28, 2020
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Carvin would use 1/4 watt.;)
     
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  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Speaking of noise...

    Since you once had a thread about 12a_7 heater noise, now would be a good time to clean up the V1 wiring and flop the input to pins 6,7,8 (section 1).
     
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  5. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    You have a good memory! But, given it's SE with a lot of CC, this amp is pretty quiet, and rerouting those wires *neatly* and not the way Lupe did it would not, for me, be feng shui or Marie Kondo or ... :)
     
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  6. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Done. The results are so great I'm almost tempted to do an "NAD." Reborn amp, anyway.

    I ended up following advice here -- thanks, all -- that the 1M input resistor and Fender jumper between the jacks adds function rather than changing sound. I didn't replace the 68K input grid stoppers with 47K either; they're hard to get at in my build and a little rough calculation suggested the audible (RC) difference would likely be inaudible to me.

    So it came down to swapping out five easily accessed resistors. And for the moment I just used the stock GA-5 47K on the NFB, since my brother the GA-5 owner (who also likes to tweak and improve everything on his amps) said it's pretty perfect.

    And it is pretty perfect. I went to some trouble to put a track on the looper and record it on a GarageBand audio track, leaving all dials and levels exactly the same. But when I got back with the 'new' amp, I found the different NFB ratio (and likely the different preamp bias) was louder, and so needed different recording levels, and pretty soon I couldn't get the recordings to be 'identical in settings'. I was kinda disappointed, actually, since on the recording the two tracks sounded fairly similar.

    But in the room? Way different. Just richer and sweeter. More balanced, less mid-heavy, and a sweeter, prettier, and even more magical OD.

    With the hotter bias (??) and slightly less NFB, the sound really rides the edge of breakup across the top of the dial, which is sort of like being in the perfect place on a wave. Walk the nose? Do a cutback? Hunker down and get in the tube? Heh, as if I was a SLC surfer. But the result is also very touch sensitive. Back off for sweet edge of dirt, dig in for chimy, ringing, singing drive. Very tweed, still, but a beautiful not-quite-Fender version.

    upload_2020-12-13_8-44-49.png
     
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  7. Mongo Park

    Mongo Park Tele-Afflicted

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    Nice description and layout for other to follow in your footsteps.
    Your description is so good I am wanting one of my own.
    Cheers Ron.
     
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  8. jmp81sc

    jmp81sc Tele-Meister

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    I'm thinking of doing one of Rob's micro amps and this may be a good starting point. Thanks for the detailed layout and description.
     
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  9. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Going from the "How to" list in the first post, if I had a 5f2a already built and wanted to experiment with it, I think I would simply
    • swap the 100k -> 220k
    • 1.5k -> 2.2k
    • play with the NFB value to taste. If I remember correctly, if you have an 8ohm speaker on it now, the NFB would need to be around 39k for the correct equivalency as the 22k @ 4 ohms. Something that was 47k-56k there would be sure to open it up more for some grind.
    And I would probably call it a day on the rest of the list. They just don't look that drastically different to be worth the total rebuild in my opinion, but that's just me.
     
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  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    You're exactly right, I think. And you're right, FWIW mine has a 15W OT (CT 40-18031) and a 10" (8 ohm) Weber speaker (10A100t).

    I'm glad you mentioned the NFB. That was my next thing to report. As above, I thought about pots, and my existing footswitch, but decided for a first go to build it stock GA-5, 47K / 2.2K on 8 ohms, since that seems to be such a well-loved sound. And in fact it's awesome -- no, I haven't tried other values, but so far it makes you not want to very much!

    That said, the amp sounds great with my Strat, insanely musical but gutsy (I suddenly think of Nathaniel Rateliff's voice???). But it may be a bit bright on the bridge pup of the Tele. Nothing the good old 5F2a tone knob can't fix (which is a big plus over the original GA-5 IIRC).

    But now I gotta try it with my P90 and HB guitars. So far I'm thinking I may rig the switch to have the option to run 33K as a slight "bright tamer" -- I've heard 33K is a good fit here, and it'd be very much the same NFB ratio (~15:1) I ran as the 'cleaner' tone in my 5F2a. Might give you a cleaner clean for country and jazz too. We'll see; I may mount the pot just to dial in the right 'alternate value' and then swap out the pot for a fixed resistor that size.
     
  11. mjcyates

    mjcyates Tele-Meister

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    Good work KF, I preferred the GA5 over the 5F1 as well.
     
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  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thanks. Actually, you get all the credit for making me realize I'd had a sleeping GA-5 right here in my music room all along. :)
     
  13. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    FWIW, all this thinking about NFB made me revisit something I knew *zero* about when I built my 5F2a, the relationship of NFB to speaker (OT) impedance.

    I was told at that time that the original 5F2a actually / usually had an 8-ohm speaker, unlike the 5F1. That mighta seemed odd (since the original Princeton had an 8" speaker just like the Champ). But I didn't think twice, and I stuck the schematic 22K NFB resistor in with my 8-ohm OT and speaker. And it sounded fine. But it was pretty polite at normal room volumes.

    So my first mod was to run a (full 25uF!!!) bypass cap on V2b, but that was 'much too much.' So I rigged the footswitch and stuck in a supplemental resistor instead (51K, for reasons I don't recall). It worked well, and was actually pretty sweet -- just took you from sorta 'rhythm' to sorta 'lead' with a tap on the button. But as I started to learn more about NFB and impedance (Rob has a nice discussion on his 5E3 mods page) I had to wonder: Was that 22K / 8 ohm setup 'correct?' Did the original 5F2a actually run a 22K NFB (same as the 5F1) even though it (apparently) ran an 8-ohm speaker?

    Many of you no doubt know this, but it's one of those Fender/Triad/Schumacher mystery stories, with the Flying Dutchman 6G10 sailing past at the end. For a nice rundown and insights, you can't do much better than this classic TDPRI thread from 2015, featuring insights from our wise friend @Wally and the legendary @CoolBlueGlow , among others.
     
  14. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Try a 12ay7 and your favorite overdrive/boost pedal in front if you've already changed the 2.2k & 220k resistors. Let us know if you do.
     
  15. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Yay, the NFB footswitch is back in the game. After some thought and some experimentation with a 50K pot and the resistors I had on hand, I settled on this as a slight ‘tamer’ as in post 30 above. It actually sounded good all the way down to 10K as a ‘base’ NFB resistor, but I didn’t have a 37K in the bin, so I went with 20K and 27K — nice, not glaring, but noticeable.

    NFB cut final version.png

    IMG_6412.jpeg
     
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