5F2A Build

Discussion in 'Shock Brother's DIY Amps' started by mabinogeon, Jan 2, 2020.

  1. mabinogeon

    mabinogeon TDPRI Member Silver Supporter

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    Hi all,

    Finally getting around to (slowly) building my next amp. I've wanted a 5F2A-type amp for quite a while, but I hate working in those tight Tweed-style chassis - so I ordered up a Zach Hunter 6G2 chassis on eBay and will make that work. Unfortunately (and I didn't notice 'til after I bought it), his 6G2 chassis is drilled for a Deluxe-sized PT & OT, so I will have to do a little drilling to make my PT & OT fit. Turns out this has come up before.


    On the front panel I added an SPDT switch and another pot to fill the extra holes. The switch will flip between two different value tone caps, and the pot will be a presence control. The extra holes in the back will have a switch to flip between the two different pairs of PT HV secondary windings (ClassicTone 40-18027), and another SPDT switch, this one to switch between two different value cathode bypass caps (25µF and 1µF) on V1A. (Maybe. I may just use an SPST to switch the 25µF cap in and out of the circuit.)

    In the past, I have built amps point-to-point and on Fender-style eyelet boards. I wanted to give turrets a shot, so I picked up a few pre-drilled boards. I may drill my own in the future, but I have to say I absolutely love the turrets and will never go back to eyelet boards.


    This is my first attempt at building a turret board, and I've already found a few things I'd do differently next time.

    I really don't like how the signal caps are on top of the plate resistors. I left an 1/8" air gap, just in case, but in the future I will space out those turrets an extra hole or two. Probably wouldn't be an issue if I used M150s, but those Jupiter caps are fairly large.

    I probably could have scrunched everything up a little tighter for the sake of neatness - but the chassis is plenty big and the extra space between components is nice for my clumsy fingers.

    The extra turret on the bottom left is for the PT heater center tap. I thought it'd be handy to have a spare hole to drop it in once the board is installed in the chassis.

    I haven't decided on the cab yet. I will build it, but I'm not sure out of what yet.

    For the speaker, I have a Jupiter 12SC in the mail.

    Will try to remember to update this thread as I make progress!
     
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  2. Mongo Park

    Mongo Park Tele-Holic

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    Looking good
    If you have a switch for the cathode cap consider having a different resistor
    to go with the cap. On mine I have the standard fender and a .68uf cap with a 2.7k resistor. This gives it a little more cooler bias sound with that resistor.
    Cheers Ron.
     
  3. mabinogeon

    mabinogeon TDPRI Member Silver Supporter

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    Thanks for the tip!
     
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  4. chas.wahl

    chas.wahl Tele-Meister

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    I was going to try to build a 5F2-A in a 5E3 chassis (to fit a 5E3 cabinet), and ran up against the transformer cutout/mounting spacing issue too. Here is how I was going to solve it:

    Screen Shot 2020-01-04 at 13.11.12 .jpg
    The 5E3 mounting holes were on 2-1/4 x 2-3/4 centers (with oversize holes), while the 5F2-A transformer I have is 2 x 2-1/2. The plate, I figured, could be aluminum or phenolic or G10, something reasonably easy to fabricate. The original chassis would need to be provided with a bit of butchery between its cutout corners and mounting holes, but wouldn't be disfigured in a way that would be visible, either with a 5F2-A or 5E3 transformer mounted. If you'd like a PDF in grayscale, or a DXF drawing, just ask.
     
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  5. monkeybanana

    monkeybanana Tele-Meister

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    mmhmm
    I just finished one yesterday but in a 5F1 chassis. I used one less filter cap and upped the other's values. I also kept the one input resistor (68k/2) off the board. That gave me room.

    I also used a separate power switch and this is by far the quietest amp I've made in that chassis.

    For the Jupiters, what a about pushing them off to the side like you did with the 25uf?

    Looks nice and neat! Probably better cab with that chassis.
     

    Attached Files:

    Last edited: Jan 4, 2020
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  6. mabinogeon

    mabinogeon TDPRI Member Silver Supporter

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    Thanks for the input, and the template. I attacked the chassis with my angle grinder and made a couple of quick brackets out of some aluminum stock I had in the garage:


    Not the prettiest, but it works. I also filled the gaps with silicone which will hopefully help keep everything steady.
     
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  7. mabinogeon

    mabinogeon TDPRI Member Silver Supporter

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    I mentioned previously that I added an SPDT switch to one of the extra holes in the front so I can switch between two different value tone caps. I used a .001μF and a .005μF Jupiter. I imagine these are close enough in value that I won't hear much a difference between them - but I had to fill the hole with something. The resistors on the switch are 1M 1/2W carbon film for pop suppression.


    I also fixed the board as best I could to accommodate the two cathode bypass caps:


    I'll run a wire from pin 3 of V1 to the bottom turret of the cathode resistor, and a wire from that lug back to the center terminal of the SPDT switch (mounted on the back panel of the chassis). From there, each lug of the switch will get a wire up to one of the bypass caps. I had originally planned to run the caps directly to the switch with a wire from the center switch lug to the top turret of the cathode resistor, which is why I didn't make room on the board for the caps to begin with. The more I thought about it, though, the less I liked the idea of floating the caps off the board - so this is the best solution I could come up with in the space I had.
     
    Last edited: Jan 5, 2020
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