Boothill GA5 build

Discussion in 'Shock Brother's DIY Amps' started by LPJr54, Oct 31, 2019.

  1. LPJr54

    LPJr54 TDPRI Member

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    Just finished my first build from a Boothill 5F1 kit that included a few part changes in circuit to make it a GA5 with larger transformer. The cabinet is from Guitar Cabinet Outlet on Reverb - using a 10" Weber 8A speaker. This build taught me about how a basic amp is put together and functions. The Boothill instructions are clear and it helped to compare other schematics. I deviated from instructions with the grounding (hum is quiet), kept the 1m resistor on input and eventually removed the NFB from the circuit as to reduce dampening. I'm interested in building a British 18w now.

    IMG_3300.JPG IMG_3309.JPG
    IMG_3294.jpg
     
    Last edited: Oct 31, 2019
  2. D'tar

    D'tar Friend of Leo's

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    Sweet little amps! p90 and a GA-5!!!! perfect practice combo!

    Congrats!

    And welcome to the Forum!!!
     
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  3. theprofessor

    theprofessor Friend of Leo's

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    Awesome! And as @D'tar said, Welcome! Folks are wonderful at supporting builds around here. Keep us posted if you do decide to go for another one.
     
  4. LPJr54

    LPJr54 TDPRI Member

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    Thanks for the replies. My nephew played through it with his Tele and he wants one for himself so that'll be the next project. Forgot to mention that the Boothill instructions, as it pertained to the OT wires to speaker jack, caused an out-of-phase ringing that went away when switching the red and blue. I'd get another Boothill kit - his email support was good.
     
  5. King Fan

    King Fan Friend of Leo's

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    Very nice. Super tidy for a first build... or any build in the sardine can of a 5F1. :)

    I don't think the instructions were faulty. AFAIK OT leads aren't capable of being color coded -- it ends up being 50:50 whether you'll get OT squeal.
     
    Last edited: Nov 1, 2019
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  6. D'tar

    D'tar Friend of Leo's

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    This is common. You really have a 50/50 chance of getting it right. You may even end up just the opposite with your next build and the same parts.

    What Output tranny did you use? And what value for the NFB?

    yeah... What KF said!!!:)
     
  7. LPJr54

    LPJr54 TDPRI Member

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    I didn't know that about the 50/50 OT wiring gamble. Good to know going into next project. I believe the NFB resistor was a 47K which is GA5 spec. I ended up removing it from circuit which brightened up the tone and also makes it feel more like my fingers are playing the speakers. Transformers are the Classic Tone 40-18019 and 40-18115.
     
  8. LPJr54

    LPJr54 TDPRI Member

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    Has anybody experimented with the bypass cap values on first stage? I currently have a 25uf cap w/1.5k res in place and I've read that a higher value can further brighten and tighten the high mids? Any thoughts on a bright cap across volume lugs? What the rule of thumb as far as determining a value?
     
  9. D'tar

    D'tar Friend of Leo's

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    This is true, however...... The quote in read is from my post in another forum where someone was building a GA5 Mojotone kit. There are minor discrepencies when mixing the parts together.

    You may like it just fine without any NFB No problem. If using the correct value for the 4 ohm tap you would be increasing the NFB effect.




    Mojotone schem shows multi tap OT using 4ohm tap /4ohm speaker and a 47k nfb resistor

    Ga5 uses 8ohm tap/ 8ohm speaker and 47k nfb resistor

    5f1 uses 4ohm tap/ 4ohm speaker and 22k nfb

    So just fyi if you build to that schematic, for original sonic value, use the 8 ohm tap with the 47k nfb or use a 22k resistor with the 4 ohm tap. Better yet, use the 4ohm tap and a 10k nfb resistor in series with a 50kl pot for a nice variable nfb.


    New to tube amps and building....

    IMO, Robs site is a must and he is an active member here

    https://robrobinette.com/Amp_Stuff.htm
     
    Last edited: Oct 31, 2019
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  10. D'tar

    D'tar Friend of Leo's

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    more champ values:)

    Edit... it looks like you have the correct (GA5) 220k plate and 2.2k cathode resistors in your picture


    https://robrobinette.com/Voicing_an_Amp.htm#Cathode_Bypass_Cap

    https://robrobinette.com/Voicing_an_Amp.htm#Bright_Caps

    Have fun... Be safe... Good luck!!!
     
    Last edited: Oct 31, 2019
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  11. LPJr54

    LPJr54 TDPRI Member

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    Circuit is wired for 8ohm and speaker is also 8ohm.

    Question: I might want to put the NFB resistor back in - without, it's quite transparent but a tad splatty and it seems that the NFB addressed this at the expense of dampening. What value resistor would you recommend installing to reduce the NFB as I'm considering going the route of keeping the NFB (lower value) and replace the bypass cap on the 1st filter from a 25uf a 68uf as to brighten and tighten the tone. Your thoughts?
     
  12. D'tar

    D'tar Friend of Leo's

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    LPJ,

    The classic tone 40-18115 has a 8k primary and 4ohm secondary windings. Unless you have a different model I would use a 4ohm speaker and the 22k nfb like the 5f1 uses. If you are up to modifications a 10k nfb resistor in series with a 50kl potentiometer can give you a wide range of options. Your interpretation of the sweet spot may be between the two set values. With NFB the smaller the resistor value=more feedback and more signal cancelling. The larger the resistor=less feedback and less signal cancelling.

    Look closer at your bypass considerations. here again a large 25uf cap boosts the full guitar signal. using a larger value such as 68uf will do more of the same thing.

    Decreasing this value to say 10uf, 5uf:), or even 0.68uf which I assume you are seeing. will not boost the lower frequencies and will emphasize the mids and higher freqs.
     
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  13. jtcnj

    jtcnj Tele-Holic

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    I have the v1a bypass cap on a 3 way switch to ground 25µf /none / 0.47µf and the NFB on a 25k pot in series with @10k. (2 4.7kΩ in series IIRC).

    Credit Dave Hunter and Uncle Doug.

    Like both a lot.
    The mods I mean.

    I guess the dudes too.
     
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  14. LPJr54

    LPJr54 TDPRI Member

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    The idea of a variable control to dial it in is a good one.

    I fell for the 57 Champ Reissue a few years ago and this GA5 variation is more versatility than that Champ - particularly with humbuckers; perhaps its the cabinet w/10 and it would seem that the speaker might be the darker culprit as its sensitivity is around 91-92 and being a smooth cone - would be a tad darker than most ribbed. I'm inclined to keep the circuit as-is so I'll be able to remember how my first build sounded years downstream - though I might bright mod the volume pot. Thank you for all the advice.

    VARIABLES: LP Jr (Pigtail wrap) wired 50s style (500k vintage taper/250k tone) to a Wolfetone Meaner/Throbak A4 magnet; 12-54 nickel round core Pyramid strings w/wound G; RCA 5Y3; WWII JAN RCA 6v6; RCA Blackplate 12AX7; Klotz cable.

    1954 Jr Dealer Sheet.jpg
     
    Last edited: Nov 1, 2019
  15. LPJr54

    LPJr54 TDPRI Member

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    0BFE3DEA-17B4-4917-AB4A-BDBC1A9EC481.jpeg F713D101-3916-4889-89A5-68847D34D702.jpeg BootHill kit is great - don’t hesitate to build your favorite amp.
     
  16. theruley

    theruley TDPRI Member

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    that was what happened when I powered mine up for the first time. Almost had a panic attack over it until a google search showed it is a common occurrence and easy fix.
     
  17. LPJr54

    LPJr54 TDPRI Member

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    I'm gonna try a NFB switch. Seems that a SPST switch is all that is needed to disconnect the resistor from the circuit.
     
  18. D'tar

    D'tar Friend of Leo's

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    .
     
  19. LPJr54

    LPJr54 TDPRI Member

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    Diagrams GA5 grounding1.jpg I have a grounding question. I've attached 2 diagrams for thought regarding 5F1/GA5 preamp grounding. I have the power section isolated and grounded to the transformer and the transformer directly bolt to the chassis along with the AC wall ground in that bolt. My question pertains to the preamp section. I'm not sure if Diagram 1 or 2 is right (red lines denote ground). Im uncertain if the preamp ground ends at the potentiometer/switch or if the ground continues to another isolated spot on the chassis. Are there more than one preamp grounds? For some reason, I get a lot of static on my guitar volume knob (does same on another) and I'm thinking it might be a ground problem within the amp. Currently, the amp is wired per diagram 2.
     
  20. D'tar

    D'tar Friend of Leo's

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    IMO...

    Diagram 2= potential ground loop

    Diagram 1= Mo betta, grounds to chassis through the input jack
     
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