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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. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Ever since I restored a 1955 GA-9 for a buddy, I've been interested in the Gibson 'tweed equivalents'. I see Mojotone has a GA-5-style kit for $500, and I've heard a lot of good things about how these Champ-alikes (there were many, I guess, not just Gibbys) can have a sweeter richer thing going on.

    And then there was this nice recent thread by @mjcyates who built a GA-5 on a Champ framework.

    OK, I know, I need another amp like a moose needs a hat rack (a moose who already has a number of hat racks). But, hmm, one of my hat racks is a 5F2a I really don't play all that often.

    Looking at the above build thread, and the GA5 schematic,

    gibson_ga-5_sch.pdf_1.png

    and this page about interconverting Champs and GA-5s...

    upload_2020-11-27_12-32-51.jpeg

    I realized the 5F2a is already well filtered (32-10-10 instead of the GA-5's 20-10-10) and already has a ~20uF bypass on V1a, so I tried drawing the remaining changes (in purple) on the layout of my slightly modded 5F2a.

    5F2a to GA-5 idea.png

    So that's what? swap out 7 resistors, plus clip out the 1M and the jumper from input 2 switch to 1 tip?

    Questions:
    1. Would the tone control still work? work the same?
    2. Is there a downside of leaving the Princeton's full 32uF reservoir instead of 20?
    3. What am I overlooking?
     
    Last edited: Nov 27, 2020
  2. muscmp

    muscmp Tele-Afflicted

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    even tho it is a later 63 model, i have the ga5t that i sometimes pair with either my 75 vibrochamp or 76 champ. makes for a great pairing! other times i'll pair it with my ga9. both have 10" speakers.

    i say give it a shot as you are already doing in essence a mod.

    play music!
     
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  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The tone control would be the same as long as you don't change the circuit there.
    The 32uF filtering will help with low end flub but the difference between 20uF and 32uF you may not even notice.
    I don't think you are overlooking anything. Gibson and Rickenbacker both had their version of the Champ.
     
    Last edited: Nov 27, 2020
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  4. mjcyates

    mjcyates Tele-Meister

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    I was going to say pretty much what LLC just said.
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I rather like both the GA-5 and the 5F1 that I have heard on youtube.

    I wish the youtube videos comparing the GA-5 and the 5F1 would use the same speaker. There are a couple of videos that give a fairly honest review/comparison but each uses the speaker in their respective cabinets. I would like to hear a true comparison between the two circuits.
     
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  6. Kevin Wolfe

    Kevin Wolfe Tele-Meister

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    I like the idea although I’d stay with the Giby filtering. That flubiness may be part of the mojo.
     
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  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The 5F1 had 16-8-8 filtering. Again not a big jump from 20-10-10.
    The 5E1 and 5E2 had 8-8-8 but there was a choke involved.
     
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  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thanks, you all. Good ideas. LLC is right, the youtube comparison videos are intriguing, but (almost always) tend to mix an apple here with an orange there, speakers being a very common example. My only hopeful thought is *if* they ran original speakers/cabs in the tests, then it might be more fair on average. Every old speaker is different, but if Gibby sounds a bit 'nicer' each time, maybe it's not all down to the speaker.

    Our friend @theprofessor posted a nice comparison vid in MCJ's build thread that at least has 'similar' vintage-looking amps, though I dunno if the speakers are similar, and I think the Skylark's slightly ??? bigger cab may be a variable.

    A GA-5 demo that impressed me some was this one from Mojotone with their kit, which is in the small LPJr. cab and at least in stock form doesn't have a highly boutique speaker (though this Celestion doesn't look like the Mojo one they sell in the kit):




    Couple more things I'm thinking about here. Does this make sense?
    • B+... the GA5 seems to want roughly 300-0-300 to get the voltages in the schematic above; Mojo supplies the kit with 290-0-290, which I'm guessing would work well. MCJ and others have used the Hammond 290AX, at 275-0-275, which seems to work well and give even lower B+. But my 5F2a has a CT 40-18019 @ 325, which would push B+ considerably higher. OTOH, I do have my 'vintage voltage adapter' which could drop it by either 6% or 12% if needed.
    • NFB... the 51K that my footswitch adds to the Princeton's 22K is a pretty subtle change. Kinda like 'one toke over the line' -- if that. Not sure what the effect will be with the bigger 47K in the GA-5 NFB. But not a serious problem, fun to find out...
    • The 5F2a has the bigger cab and a 10" speaker (Weber 10A100t) -- that could be a good thing?
    • I haven't analyzed the input array. Would the two jacks now be identical? (Gibson didn't switch input 2, but says you should always plug into input 1 if you're running a single instrument). I know our brilliant friend @D'tar mentions the inputs in that original build thread...
     
    Last edited: Nov 27, 2020
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The GA-5 input does not have the 1M to ground like the Fender product. When plugged into the non-switched jack the other 47k is grounded. Similar to the 5F1 when the 68k is grounded.
    I would just leave the Fender 1M in the build. The 47k or 68k difference won't be noticed IMO.

    The NFB resistor will be on a 8ohm tap for the GA-5. 4ohm tap on the 5F1. So a little lighter NFB on the GA-5. I assume you will just find a NFB resistor that suits your ears though.

    Between these two circuits the main difference of what you will hear will be because of the 220k plate resistors IMO.
     
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  10. James Knox

    James Knox Tele-Holic Silver Supporter

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    It will be fun to see what your analysis of the finished product is! I listened to the comparison video and my first impression was that I liked the 5F1 mo betta. Your experiment with the same cab and 10A100T will be more apples to apples.
     
  11. mjcyates

    mjcyates Tele-Meister

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    And once again I concur with Mr. LLC!
     
  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thanks, super helpful. I did wonder if 47K v. 68K was a big deal. On a practical level, it'd be nice not to be soldering up there in the crowded space under the pots.

    And am I right thinking the 1M just provides the 'Lo' for input 2? Keeping or losing it doesn't change the actual sound on input 1?

    But NFB, dang. I certainly didn't understand it when I built the amp -- my first. I still don't really dig the relationship to 4 v. 8 ohms. I used an 8ohm speaker and the 8ohm tap from the OT, but I just took the 22K NFB resistor from the stock schematic. Does this mean my NFB has been 'wrong' all this time? (I know this is pretty basic, but if I'm honest I have to work hard just to recall that more resistance means less NFB and less NFB means more raw signal and more noise and less clean and....) :D

    Hey, maybe that's why I like it with the 56K switched into the NFB? Oddly enough, it ain't a huge jump -- more Mama Bear (just right). Shoulda used a pot, may do so now, at least to find a 'stock' value I like.
     
    Last edited: Nov 27, 2020
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  13. jrblue

    jrblue Friend of Leo's

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    Oh, dream on. Thart's the problem with virtually every online so-called "review." They're generally not valid for numerous fundamental reasons, including the one you mention. But I guess people seek guidance, even from nimrods pushing flawed observations and conclusions.
     
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  14. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    There really is no wrong or right or normal.

    what is normal.jpg
    IDK who to credit for this but it makes me smile.:)
     
  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Actually, the 1M would constitute the high input. The 1M input impedance would make it difficult for the signal to go to ground. More signal will be available to V1.

    The low input would make a voltage divider with the two 68k resistors. 1/2 the signal would go to ground.

    The GA-5 has the two 47k resistors as a voltage divider, again 1/2 the signal would go to ground.

    There will be some difference due to the 47k vs 68k grid stopper. In this circuit I doubt a difference could be heard and I doubt the additional RF rejection of the 68k would be of any benefit. For all intents and purposes it is pretty much same same IMO.
     
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  16. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thank you. That is helpful -- and *clear.* I always read about this but don't quite get it. You really helped.

    Hmm. I'm a visual learner; I just found this from über-genius Steve Luckey's "amp scrapbook." I think it shows what you're talking about:

    Input jacks.jpg
    Thanks again. I may be emerging from the fog. :)
     
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  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yep, that is what I was talking about. I was thinking you might want me to make a drawing. Good of you to find the *amp scrapbook* so I didn't have to. lol.

    I would say this is a better option than the GA-5 design and as you say it saves you from dismantling that part of the circuit.;)
     
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  18. gusfinley

    gusfinley Tele-Afflicted

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    There are many "GA-5" circuits out there. I think the most common differ from the champs by just a few resistor values and an extra cathode bypass cap on the preamp.

    I think I calculated about a 7dB increase in gain with the GA-5 Circuit.

    I was looking to teach a class on building tube amps once and termed the circuit the "GA5F1" since its less than a dollar in the extra parts necessary to build either amp.
     
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  19. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thanks. Helpful again!

    In addition to the plate loads, I suspect the cathode resistor change would be audible too, yes?

    Lessee (grabs hand-cranked adding machine)... would four new resistors (or just two? -- I may have some 220Ks) be cheaper and simpler than building or buying a GA-5? :D
     
  20. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The cathode resistors would make the load lines look better. I haven't heard much difference when I have changed them. The plate resistors do make a noticeable change though.

    To be more adventurous, you could put in 470k plate resistors. It would then be easy to bypass them with another resistor for 220k equivalent, 100k equivalent, or anywhere in between. Same with the cathode resistor start with a higher resistance of say 5k and bypass to taste.

    Don't start thinking of putting a switch or pot in for quick change plate resistance. There is too much voltage to safely put your fingers on a switch.;)
     
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