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Bias Test Point for 5F6a Build

Discussion in 'Shock Brother's DIY Amps' started by nathanh, May 19, 2017 at 1:49 PM.

  1. nathanh

    nathanh Tele-Meister

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    Any good literature on how to make one or why I can/cannot?
     
  2. tubejockey

    tubejockey TDPRI Member

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    the bozone
    Google can probably help with more literature, but I am a big fan of test points on either side of a precision 1 ohm resistor inserted into the cathode circuit.
     
    PCollen likes this.
  3. nathanh

    nathanh Tele-Meister

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    @tubejockey, is just a 1m resistor going to ground? Does it matter than I'm not cathode biased?
     
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  5. tubejockey

    tubejockey TDPRI Member

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    Nov 25, 2015
    the bozone
    1 ohm not 1Megohm, between the cathode of the tube and ground, in place of the ground strap. This lets you measure the total current (minus the grid current) of the tube with a volt meter.
    This works great with fixed-bias ccts like the 5F6. For cathode biased ccts, I just measure across the existing cathode resistor.
     
    J Hog likes this.
  6. jhundt

    jhundt Poster Extraordinaire

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    I've said it before, and I'll say it again. The 1-ohm resistor is a fine idea and allows quick and easy testing of the bias current. But there is very little sense in adding test points on a Fender tweed-style amp. Unless you add test points for reading plate voltage AND an adjustable bias pot... and there just isn't enough room on the chassis for all that stuff.

    The best thing about the Fender tweed amp is that you can take the back panel off in less than one minute, and all the parts are right there. You can read your bias, and your plate voltage, and adjust your adjustable bias pot at the same time. The whole job will take about 10 minutes.
     
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  7. nathanh

    nathanh Tele-Meister

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    @jhundt great reply.

    I'm having trouble measuring the center tap resistance currently. The meter goes up to about 183 mega ohms then goes to "1 ." in the same 2 seconds which is usually the sign that I have it on the wrong setting. If I flip the power and standby to on (unplugged) I get 1m.

    I gotta get a handle on this. No doubt about it, multimeters and all they entail are my current weakness.
     
  8. Fiat_cc

    Fiat_cc Tele-Meister

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    What points are you using to try and measure OT resistance (I assume that's what you're measuring with the multimeter)? That resistance is way too high. I doubt you'd be getting noise out of th amp if that were the actual value. The ends of the OT winding connect to your power tube plates, and the centre tap connects to the B+ (or B+1 depending on amp/schematic) node. Try measuring resistance between those points.
     
  9. nathanh

    nathanh Tele-Meister

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    Yeah I'm doing something wrong here.

    I believe thats where I was measuring from. I'll give her another go. What kind of resistance should ball park be?

    EDIT: I just measured between the Center Tap going to ground and the plate of V6 (pin 3) and after several minutes it stopped climbing at 8m. A few minutes later it started to go back down. Is this normal?
     
    Last edited: May 19, 2017 at 9:12 PM
  10. Fiat_cc

    Fiat_cc Tele-Meister

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    I think you're measuring from the wrong centre tap. You should be measuring from the output transformer centre tap, which is not grounded. It gets fed B+ voltage, usually from the first node of the power supply section. As far as I understand, the primary side of the output transformer isn't grounded. It should measure a lot less than 8M. I would think well under 10K.

    It sounds like you're measuring one of the power transformer centre taps.
     
    Last edited: May 19, 2017 at 10:47 PM
  11. nathanh

    nathanh Tele-Meister

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    Yeah, but I think I tried that too. Hmm...should the amp be on for this one?
     
  12. tubejockey

    tubejockey TDPRI Member

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    the bozone
    When you measure resistance of an electrolytic cap, you will get exactly those results. The meter charges the cap, and the resistance steadily climbs.
     
  13. Fiat_cc

    Fiat_cc Tele-Meister

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    Amp shouldn't be on for resistance reading, and since we're measuring a transformer, there shouldn't be any cap involved. It's just a measurement of resistance of a single, long piece of wire wrapped around the transformer core.

    What layout did you use for the build? Can you post the layout image?
     
  14. nathanh

    nathanh Tele-Meister

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    Got it figured out! Phew that scared me. Now I just need a multimeter that reads a little more detailed.

    I used @robrob 's layout I was measuring the PT instead like a dummy
     
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  15. jhundt

    jhundt Poster Extraordinaire

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    that method of bias measurement works, but the other method (with the 1-ohm resistors) is much simpler, quicker, and safer.
     
  16. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I just pop in a bias probe. With the back panel off, everything is out in the open for measurements and adjustment.
     
  17. nathanh

    nathanh Tele-Meister

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    Eventually I want to buy a probe for ease of use. I'm just making sure I understand everything before hand.

    Also thanks @flat_cc and @clintj for the help. Now that I've got my head around it I feel a little silly.
     
    Last edited: May 20, 2017 at 1:37 PM
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