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First Amp PT Worries

Discussion in 'Shock Brother's DIY Amps' started by Archie343, Jan 26, 2021.

  1. Archie343

    Archie343 TDPRI Member

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    Currently in the process of buying parts to build a 5F1 clone, and trying to do it a bit cheaper than buying a £500+ kit. Bought a second hand PT on Ebay that said it would be a 'drop in replacement' for fender tweed amps, however I am having second thoughts about trusting the guys words.

    The secondary is 260-0-260 VAC output, which is significantly lower than the champ's stock 325-0-325. I plan to ditch the 5y3 for a ss rectifier, would a full wave provide a high enough B+? or Would removing the CT and implementing a full bridge be better.

    I couldn't find my answer else where so this is my last resort. Thanks!!
     
  2. Nickfl

    Nickfl Friend of Leo's

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    That transformer with a solid state rectifier should be about right for the champ layout voltages. it'll probably be a bit lower than the voltages a lot of people get on their builds because they're using modern transformers that probably aren't quite right.

    Your transformer with the solid state rectifier will probably be somewhere in the range of 340 to 360v depending on how much it sags under load.

    You do not want to cap the center tap and use a bridge rectifier, that would give you something bonkers like 700 volts.

    The published B+ for the original 5f1 was 340, a lot of kits tend to be higher than that because of transformer selection.

    IMO, anything from 340 to 380 probably falls into the range of "close enough".
     
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  3. Archie343

    Archie343 TDPRI Member

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    Thanks for the answer, I appreciate it. I think the math behind the calculation confused me a little, as B+ is anywhere between the Rms and p2p ac voltage with varying consistency. I'll get the hang of it eventually. Anyway.. I've seen plenty of posts asking how to lower B+, so that seems like an issue in its own right.

    Thanks again!
     
  4. Dacious

    Dacious Poster Extraordinaire

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    Don't be too concerned with B+. Fender's tolerance was 15-20% on all figures then.....

    The good thing about single ended cathode biased amps is they tend to be self regulating. If B+ is higher, all that happens is the cathode bias resistor will dissipate more heat and your plate volts will drop. If B+ is lower it will dissipate less heat, allowing a bit more voltage. It's good to check once it's built, but I bet you find it'll be spot on.

    If you're have a centretap I'd use that with two diodes to form a solid-state rectifier. Solid-state rectifier going into a 20uf filter cap, should be ~ 350vdc. 10 or even 20 volts either way is nothing.

    On a single ended amp not using a tube rectifier will let your transformer run a tad cooler.

    What you can do is install the rectifier socket, then make yourself a ss module using a valve base and couple of diodes, which you can buy, and change back and forth with a 5Y3 and GZ34 and note the effect.

    My friend has a Joyo Sweet Baby, a 5F1 clone. It comes with a GZ34, swapping a 5Y3 in doesn't make much difference, it still sounds like a 5F1.
     
    Last edited: Jan 27, 2021
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