Help - New Amp Build - AC Voltage on B+

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by seedlings, Mar 20, 2008.

  1. seedlings

    seedlings Member

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    On this amp build I'm getting 386VDC coming off the rectifier, as I expected, but also seeing about 700V on the AC setting with either meter I have.

    I started with the 5Y3GT, but the AC was there, so I switched out to diodes, and thought the problem was gone because the amp played, but the AC is still there, so I'm stumped.

    To be clear (since I'm still learning), each lead of the secondary is soldered to a diode, then those diodes come together for the bridge. At that point, with or without any further wiring, there is 386VDC as well as about 700VAC.

    Can anyone point me in the right direction?

    CHAD
    [​IMG]
     
  2. seedlings

    seedlings Member

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    I switched from the 340-0-340 to the 170-0-170 winding and now there is 160VDC and 351VAC coming off the rectifier.

    And this is weird, too: the voltmeter neg lead to ground reads 351VAC, but reversing the leads (red to ground, black to rectifier) there's 0VAC?!?

    Oh, and I've removed the voltage regulator portion of the circuit until this is straightened out.

    CHAD
     
  3. Lanesplitter

    Lanesplitter Member

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    This is a wild guess, so listen at your own risk...

    The node you're measuring has huge current spikes passing through it to charge the capacitors. Perhaps the current spikes mess with the way the meter measures AC?
     
  4. phsyconoodler

    phsyconoodler Member

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    When you put AC voltage through a rectifier it turns the AC into Pulsing DC current.What your meter is telling you is that the two 386v windings together add up to about 700v AC.The AC voltage is not what you need to worry about.
    Try measuring other high voltage DC elsewhere in the amp.You'll find you get a reading with your meter on the AC setting as well.
    Your meter is fooling you into believing there is a problem.If you have doubts,replace the rectifier diodes and measure it again.Chances are it will be the same.That 'voltage regulator' you have is power scaling.
    Digital meter,right?
     
  5. seedlings

    seedlings Member

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    Yes, both meters are digital, and the voltage regulator is for power scaling, but I've unhooked that because my mosfet (and the spare!) found a path to ground even with the included insulating pad and washer.

    So will a digital meter tell me there's AC present at the rectifier (before the first cap) on any tube amp?

    CHAD
     
  6. mark norwine

    mark norwine Member

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    Just a thought....Are you sure that your meter doesn't say 700 mV? Milli-volts....700/1000 of a volt?
     
  7. seedlings

    seedlings Member

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    The meter has a 300VAC and a 600VAC setting. 700mV would show up as .700 and I see 700 (no decimal).

    Thank you, though!
    CHAD
     
  8. phsyconoodler

    phsyconoodler Member

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    It's not actually AC that you are measuring.It's 'pulsing DC'.
    Your meter cannot decipher which is which.Your meter can,however,tell you if your diodes are working properly.If they are,taking AC readings is just going to confuse you.Do your measurements in the DC range.
    It gets smoother the more filtering it gets.AC and DC ride on each other in a tube amp.The DC is what you need to concern yourself with,unless it's filament voltage,which is AC.
     
  9. VikingAmps

    VikingAmps Member

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    I would check the freq of the AC, it could be that the regulator has turned into an oscillator. If so it probably will be something much higher than 60 hz.
     
  10. seedlings

    seedlings Member

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    OK. Thanks all for the info everyone. Now it's time to tidy up a bit and start enjoying the amp!

    CHAD
     

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