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5F1 transformers, tubes, and headroom - how and what to measure

Discussion in 'Shock Brother's DIY Amps' started by pugnax, Jan 21, 2021.

  1. pugnax

    pugnax Tele-Meister

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    I'm building a 5F1 using the Hoffman layout. Because the ClassicTone transformers suddenly exploded in price, I ended up splurging a bit on some Mercury Magnetics transformers instead. I ended up with a FTCP-59-CT (330-0-330, 5V winding PT) and a FTW-CHMP-OS (the oversized 8Ω OT). I know it's semi-sacrilege, but I'm happy to let this amp get a bit cleaner/more headroom than it's ancestor - it's going to be a lower volume recording amp and I'd like it to be able to get some decent cleans from a guitar with vintage output single coils.

    This PT seems like it might be a bit hot - I've attached the schematic. Wondering if there's any resistor values I should change, as well as whether this might make it possible to use a bigger OT (KT66?).

    Most importantly, how do I measure some of this stuff (plate voltage, etc). I'm not totally new to electronics, but this is my first tube amp build from scratch.
     

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  2. jsnwhite619

    jsnwhite619 Friend of Leo's

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    JJ 5y3 usually does well at getting the voltage as low as you can hope, but I would still expect 380v-390v range B+ probably. It will still probably sound great, but in my experience, it will probably just have a harder "edge" to the tone/notes while the lower vintage schematic level seems to have a fatter tone and a bit softer/warmer sounding. If it's too clean, raise the 22k NFB resistor to 37k-56k range and it will gain some dirt.
     
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  3. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Since you already spent the MM-money, I’d build the amp stock and see what happens. You could possibly also use an NOS 5Y3 to drop volts; Jason knows a ton so his JJ there may be perfect. And I’d also start with JJ 6V6s — 14W rating is very conservative, IOW, they can handle high voltage.

    Before we get into voltage measurement (fairly simple), you asking makes me ask :): Do you know how to drain the fiiter caps before working on the amp? Better yet, do your plans include a bleed resistor to do it automatically?
     
  4. pugnax

    pugnax Tele-Meister

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    Thanks - that helped me connect a few dots. Any reasons (besides effort) not to just swap that for a variable resistor to easily adjust the negative feedback?

    I actually do have a NOS 5Y3 and a copper cap from another amp, was planning on using the 5Y3.

    Heh, yes, I'm good with draining filter caps, although when I've done it in the past I used a big ceramic resistor and some hefty alligator clips grounded to the chassis. But, no, this schematic doesn't have a bleed resistor and I wondered if that was for some reason. That would be some heavy resistor in parallel with the big filter capacitor, right?
     
  5. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Yep. :) 100k 2W is pretty standard.
     
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  6. jsnwhite619

    jsnwhite619 Friend of Leo's

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    No reason not too. However, I used to always build my 5f2a's with a 3 way switch, but as most people on here will tell you, you'll eventually find one place you're happy & leave it there. It may be worth rigging it temporary and not drill any extra holes and just experiment some. Upper 30k range with an 8 ohm speaker equates roughly to what stock value is with a 4 ohm speaker. I like 56K with an 8 ohm, but it definitely gets the good dirt, kinda 5e3-style at that point. So, if you are wanting a cleaner amp, you probably don't want higher than upper 30k range at most. A little more can add some nice top end bite, but by 56k and above it's going to be short on headroom.
     
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