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Stupid power supply question

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  • Stupid power supply question

    Built a single channel deluxe reverb. Have been playing it for a while to get to know it first, and now I am tweaking it a bit to dial it in to my taste and getting it gig ready and all.

    To make a long story short, I noticed that preamp voltages were higher than the spec, while voltages elsewhere were just right. So I did some calculations, and figured that I'd need another 10k resistor in series with the last one in the B+ rail. I put it in et voila, voltages are just about right.

    It can't be that easy, can it? By adding another resistor onto the b+ rail, and increasing the total resistance on the b+ rail by 50%, I am not hurting anything, am I? I guess I am just dissipating a little extra power (that would normally go to the plates of the other channel), right?
    In the future I invented time travel.

  • #2
    You're right.
    What you experiencing is the same as if you run an amp with no tubes in it. The voltages would be much higher because of the missing current draw of the tubes.
    Your amp has only one channel, hence less current draw than a regular DR - and you adjusted it by adding a resistor thanks to ohms law.


    • #3
      What he just said.

      The tubes you eliminated are no longer there drawing current, so the current remainin through that resistor causes less voltage drop. SO your voltages wind up higher. Increase the resistor, and the voltage drop increases as well. That compensates for the lost tubes.

      If you knew the target voltages, you could have calculated what you need. Trial and error worked out for you, but for future reference... The existing tubes drew current through that 10k resistor. Measure the voltage dropped across it, and Ohm's Law will tell you the current. Now how much voltage do you need that resistor to drop to get the voltages where you want them? Enter that voltage and the current we just figured into Ohm's Law and it will tell you the resistor you need.
      Education is what you're left with after you have forgotten what you have learned.


      • #4
        Thanks guys, for confirming my work.

        Enzo, I calculated the voltage drop across the second 10k resistor and figured out the current flowing through it. I then used the target voltage and the current to calculate the resistor value. 10K is what I calculated would be needed. It worked out to being pretty close, close enough at least. Still 10-12 volts high but I am not going to lose any sleep over that. Tolerance, after all. I was kinda proud of the fact I did it scientifically rather than trial and error!
        In the future I invented time travel.