Ac Voltage

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by AL1, Jan 27, 2006.

  1. AL1

    AL1 Member

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    How do you check the ac voltage on pins 4 and 6 on the rectifier tube? The 2 red wires from the power transformer. Thanks.
     
  2. John Phillips

    John Phillips Member

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    WITH CARE.

    The voltage is typically in the 300V range.

    I'd almost be inclined to say that if you need to ask, you probably should stay away from stuff like this, but... assuming you are basically careful and understand the risks:

    Use a meter which is rated to at least 400VAC, not the 250VAC that many cheap ones are (in the UK, anyway) and set it to the highest AC range. Connect one probe to the chassis - use a clip if it has one, or poke the probe into something like a screw hole... the important thing is that you don't hold it. Touch the other probe carefully to the pin, WITHOUT holding the chassis with the other hand.

    With the meter set to volts it's unlikely that an accident could draw enough current via the meter to kill you, but be careful about slipping with the probe and touching something else with your hand, which could be dangerous - or at the very least you may give yourself a huge fright if the probe contacts something it shouldn't and causes an arc.
     
  3. AL1

    AL1 Member

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    John, I put one lead on pin 4 and the other lead on pin 6. The transformer is supposed to have 325 from each red wire. At 120.6 VAC I got 695 VAC instead of 650. This transformer also has a 50 volt bias tap which on this princeton reverb is not used. Would that account for the higher voltage reading? Thanks.
     
  4. John Phillips

    John Phillips Member

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    Be VERY careful doing that. You're metering across both secondary windings - the way I described measures each separately and is much safer. It's the single winding voltage which is important anyway.

    The schematics I have all show 340V for the secondary voltages, so allowing for the slightly higher supply voltage (120.6 vs 117, 3% higher) I would say your amp is actually under the expected voltage, which would be 350V. In any case, it's well within design spec. The unused bias winding won't make any difference, it doesn't draw any significant current even if it's in use.
     
  5. donnyjaguar

    donnyjaguar Member

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    The 650Vac is probably the value when under load. Its only 6% different from expected so I wouldn't lose any sleep over it. I have a 6.5Vac/5A winding on one of my amp's that measures 7.9Vac with no load - which worked out well as I put a voltage doubler on it, bagged 17Vdc and it now drives a 24V pilot lamp and two 24Vdc relays. Schematic here if interested.

    DJ
     
  6. AL1

    AL1 Member

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    John, this is an upraded pt from Allen Amps. It shows 325 from each red wire at 120vac wall outlet.
     

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