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5f1 PT test help.

Discussion in 'Shock Brother's DIY Amps' started by JuneauMike, Oct 9, 2017.

  1. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    I have installed the PT, lamp, on-off portion of the isolated volume knob and run the heater wires, 5v and the two 3255VAC HTs, grounded the CT and installed the chord. The chord is also grounded to the chassis. The volume pot/Power switch is not grounded.

    I've also got a .5 amp slow blow fuse on hand for testing.

    I'd like to power up and get voltages from my transformer primaries and secondaries, filaments and B+ voltages before installing the board and inputs. Trying to do this in phases.

    Does anyone see any problems here? With a smaller fuse, i dont see any danger to the components. The OT is not connected and no tubes installed (could I install tubes?). With no filter caps the B+ should be low, but I'd like to see where it is before I stalling it to the circuit.

    Anyway, anyone see a problem with this approach?
     

  2. CoyotesGator

    CoyotesGator Tele-Meister

    416
    Jul 12, 2014
    Gulf Coast
    Do you have a lightbulb limiter?
     

  3. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
    35
    510
    Oct 8, 2016
    Melbourne, Australia
    I don't see a problem taking measurements. Tube rectifier, yes? You could wire HT up to the rectifier socket, 6.3v to all your filament pins, and 5v to rectifier filaments, then take measurements with no rectifier installed. You should be able to measure all your AC voltages then.
     
    JuneauMike likes this.

  4. Nickfl

    Nickfl Tele-Afflicted

    May 24, 2016
    Florida
    Make yourself a lightbulb limiter and use it during testing to check for shorts and protect against damage if there are any. If your HT secondaries (and other secondaries) aren't connected to anything other than the socket pins you should be ok, just be careful with that 650v you will have across your meter probes.

    Also, with no tubes installed, your B+ isn't going to be low, it is going to still be AC. Go ahead and measure your HT and heater voltages, I can't see any point in installing the rectifier and checking B+ before you install the filter caps. Are you installing backup diodes on your rectifier socket? You should, its cheap insurance and easy to do.
     
    JuneauMike and Fiat_cc like this.

  5. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    Be warned you'll see higher voltages with no components installed. Usually +20-30 volts B+, maybe 6.6 or 6.7 heaters. Nothing to worry about, once all the board components and tubes are installed it'll drop back.
     
    JuneauMike likes this.

  6. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    I don't have a light bulb limiter, but sounds like this is the time to build one?

    How much protection is a .5 fuse in all this? Seems like that would be good insurance against damaging anything. Am I wrong?
     

  7. Nickfl

    Nickfl Tele-Afflicted

    May 24, 2016
    Florida
    Yup. Its a ten minute job a few $ worth of parts. A three prong cord, a junction box, an outlet, a socket, and a 100w bulbv(or so, maybe a 50W for a champ). Really is something you should have for amp building and repair as it can save you from smoking a PT. Ask me how I know...:oops:
     
    magic smoke and Fiat_cc like this.

  8. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    The light bulb gives you both limiting and indication. If it lights up bright, you can quickly cut power before the point of relying on the fuse. Beats going through a whole box of fuses trying to figure out an issue, too.
     
    JuneauMike likes this.

  9. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Thanks for all the helpful input everyone. Built my current limiter tonight and tested it on a healthy amp.

    Then I fired up my 5f1 amp and got a nice bright jewel light, no smoke and no light from the 150 watt bulb. Having a beer to celebrate a very small accomplishment. Tomorrow I'll start getting some A.C. readings and then move on to installing the board.

    Feeling much more confident about this build than I was with my first amp. Thanks again.
     
    Nickfl likes this.

  10. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Posting these for the benfit of other builders. This is what I was referring to in the first post.

    Most of the power section wired up. The power chord ground is attached to the lower right PT bolt. I later moved it to its own ground lug bolted to the chassis. I also made sure there was enough length to it so if the chord is jerked out, the ground is the last one to tear off.

    pt.gif

    Tube sockets wired. OT wires pushed out of the way and not connected.

    filaments.gif

    Power chord connection. Note, the connection is made at the ungrounded eyelet. Bolted down as tight as you can make it with a star washer. The power chord ground could be soldered to the center lug to be grounded.

    pt_union.gif

    Middle lug is ground. Outside lugs all jumpered to the middle lug.

    power_ground.gif
     
    Last edited: Mar 8, 2018

  11. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    Nice build, nice details, thanks for sharing.


    Would you recommend powering up a PT with no load hooked up?



    I did it once and regret doing it even though it's supposed to be OK.
     

  12. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Thats essentially what I did initially. The tube sockets were wired but no tubes installed. I didn't notice any problems. It's the OT that's fragile. PTs are tough as hell.
     

  13. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    That's what I did, removed the rectifier tube and measured. It's not good, there's no reason For it.

    What I understand the reason the OT is vulnerable is that it's fed by a tube and the tube can only provide but not take back, thus fly back.
     

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