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Critique My 5F2A Build

Discussion in 'Amp Tech Center' started by Sylas Wojo, Oct 14, 2020.

  1. Sylas Wojo

    Sylas Wojo TDPRI Member

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    Thanks LLC, yes I am getting 68k and 34k ohms now measuring as you described.



    Now to check the output tube bias voltage. With no tubes installed, I measured between the tube grid (pin 5 on 6v6) and ground. Does it sound like I am measuring this wrong? I get an dc voltage of 0v. Shouldn't it be around negative 30-60 vdc?


    On a different note, is it really necessary to develop the capacitors with a lightbulb limiter? How much of a difference does it make than to not use one?
     
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The negative bias voltage is for fixed bias amps. The 5f2a has cathode bias. Two different animals.

    New caps generally do not need to be formed or re-formed.

    Just use the LBL for initial start-up to make sure there are no shorts in the wiring.
     
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  3. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    What he said! but let me underline you *do* want one for startup. Do you have one?
     
    Last edited: Nov 16, 2020
  4. Sylas Wojo

    Sylas Wojo TDPRI Member

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    I thought fixed bias didn't sound right in my head.

    Let me get on that :oops:
     
  5. Sylas Wojo

    Sylas Wojo TDPRI Member

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    I'm curious as to what the use of the light bulb limiter is? Wouldn't the 2 amp fuse blow with any shorts?

    I also build drones, and we use small 12 volt light bulb limiters / smoke stoppers for the same reason, but in those cases a fuse is not present in the circuit.
     
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Yes, mainly it keeps the fuse from blowing. It can keep other parts from saving the fuse.

    How many fuses are you willing to use?
     
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  7. Sylas Wojo

    Sylas Wojo TDPRI Member

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    Quick noob question, does the polarity of the speaker matter? The weber signature 10" came in today, and I want to wire it real quick... I'm still double checking voltages so I won't plug in the speaker yet. I haven't blown a fuse yet, but I'll still put together a light bulb limiter (the toughest part is trying to find a 100 watt incandescent).
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    60Watt will do.

    Speaker can be wired either way.
     
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  9. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    As usual, LLC is just right -- the +/– only matter when wiring more than one speaker. You will see millions of opinions (and even some citations from stereo sites) to the contrary, but a single speaker can't be out of phase with itself.

    And as he says, a 50-150W incandescent will do. You may find 'em sold as 'tough bulbs' for garage / shop environments. My *grocery* store was carrying a few as recently as last year -- haven't looked recently. Amazon carries some -- just make sure they're real 60-100w bulbs, not 'Edison-style' LEDs...
     
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  10. Blue Bill

    Blue Bill Poster Extraordinaire Ad Free Member

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    KF, I just found this thread, looks really good so far, nothing to critique. I'm looking forward to catching up and watching the show!
     
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  11. Sylas Wojo

    Sylas Wojo TDPRI Member

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    Ah, good to know

    I just found a 60 watt incandescent... All built. :)
    20201117_182034[1].jpg
     
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  12. Sylas Wojo

    Sylas Wojo TDPRI Member

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  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Your post has me concerned you may not have the speaker connected when the tubes are in...

    FYI. Once the tubes are installed the speaker (or dummy load) needs to be connected.
     
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  14. Sylas Wojo

    Sylas Wojo TDPRI Member

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    Can you elaborate?... Its fine to power up the amp without a speaker connected when only 1 or 2 of the 3 tubes are installed, correct? (Thats what the amp startup procedures made it sound like). I only installed all tubes once my speaker was plugged in.
     
    Last edited: Nov 19, 2020
  15. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Do not run the amp with the power tube(s) installed unless there is a speaker or dummy load plugged in.
     
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  16. Sylas Wojo

    Sylas Wojo TDPRI Member

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    Pfew, I think I'm safe then. Now that I think of it, when I tested the power tube, I only had the filament wires hooked up.

    What kind of damage could that have cause if I had powered up the amp with the power tube fully wired up, and no speaker plugged in?
     
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    As a safety device a tube amp generally has a shorting speaker jack. Shorting the secondary is better than having it open.
    The damage that can happen... The OT will heat up melting the insulation shorting the windings. This can take out the power tube(s) as well.
    Will it happen every time? No, but it is best not to tempt fate. Transformers and power tubes are the more expensive bits so it is best to take care. I think we have all turned on an amp without connecting the speaker. Usually we get away with it. If the amp has no signal there shouldn't be a problem but a hum or oscillation is a signal so it is rather a cardinal rule to always have a load connected.
     
    Last edited: Nov 20, 2020
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  18. BigDaddy23

    BigDaddy23 Tele-Holic

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    @Sylas Wojo - Make a dummy load. All you need is a large resistor at the same resistance value as your speaker (eg 8 ohms/10w for your 5F2A or bigger if you plan on building bigger amps) and a mono jack. You connect one leg of the resistor to the jack tip lug and the other end to the jack ground lug. When you insert that into the output jack of the amp, the OT sees that load and you can make your measurements without damaging the OT or going deaf (or falling off your bench stool) when you get a loud pop through the speaker! I have a few different ones to cover my amp herd and find them invaluable when testing.
     
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  19. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    That’s a good point. If you’re eager to get started (a little!) after you double check all the things Sluckey and Rob mention, one last time, you don’t need a dummy load *right this moment*, though. Sluckey tells you exactly when to plug a speaker in, and his next steps, as Paul Ruby points out, reward and even require having a speaker to hear your amp.
     
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  20. Sylas Wojo

    Sylas Wojo TDPRI Member

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    I triple checked all of my connections / voltages and plugged in the speaker and all tubes. With the amp plugged into the bulb limiter, the bulb shines bright, then dims... then slowly gets bright again (but not at full brightness). It doesn't squeal, so I must have gotten lucky with the negative feedback polarity.

    I plugged in my guitar and I get sound! And it sounds fantastic! Is there any checks I should do next?

    Thank you guys for all your help, I'll post some pictures of what I have so far. And I'll also start building the cab.
     
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