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Weird values in heater virtual ground? SF Princeton Reverb

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  • #1

    Weird values in heater virtual ground? SF Princeton Reverb

    Hey guys,
    I'm working on a princeton reverb that had some odd hum (that disappeared and reappeared as the volume knob is turned). I found a broken solder joint on one of the resistors for the heater virtual ground, and now that I'm looking at it the values are strange. I wonder if anyone has ever seen this before?

    One resistor is 330K and the other is 1M. I'm only familiar with seeing 100ohm for both (and this is what the schematic shows).

    These are carbon resistors matching the originals, but I have no clue if this is original. Any idea what's up?
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  • #2
    Highly doubtful it's original... how easy to just fix? I've seen 100-500 for that purpose. And it's a 250k resistor for the total...

    Justin
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    • #3
      Originally posted by Justin Thomas View Post
      Highly doubtful it's original... how easy to just fix? I've seen 100-500 for that purpose. And it's a 250k resistor for the total...

      Justin
      Hi Justin. Thanks for the info, I don't understand part of your reply though. 100-500 for what purpose? And what 250k resistor?

      Comment

      • #4
        Sorry to confuse you... I've seen a pair of resistors used for that center tap, anywhere from 100 to 500 ohms in value, each. Since they're in parallel, that would basically be anywhere from 50-250 ohms to ground. I did the math for your 330k + 1M in parallel, and it makes 250k to ground. Or something like that. Or maybe it doesn't matter what they add up to. Either way, the whole point is to balance the sides, and I don't think a 3:1 ratio is exactly "balanced..." And I've never seen such crazy high values used there.

        Maybe it was a really really happy day at the factory...

        Justin
        "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
        "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
        "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

        Comment

        • #5
          Are those oddball resistor values an actual out of circuit ohm reading?

          How about the color code markings on them?
          What are the colors?

          There is no reason for such high value resistors on a low resistance winding.

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          • #6
            Are the resistors configured as a voltage divider to provide a positive reference voltage off a high-ish voltage supply? If so that would explain the unusual values. I use a similar technique in SRPP amps where the heater reference voltage needs to be high to keep the tube within the heater-cathode voltage limits.

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            • #7
              Originally posted by Jazz P Bass View Post
              Are those oddball resistor values an actual out of circuit ohm reading?

              How about the color code markings on them?
              What are the colors?

              There is no reason for such high value resistors on a low resistance winding.
              That was with the colour codes. I swapped them out for a pair of 100 ohm resistors and the buzzing noise is now gone.

              The amp still hums, with the volume and tone controls at 10. The B+ filter cap is new (2 years old) and I don't suspect it's faulty, but I did clip in an extra 47uF at this stage and the hum was reduced considerably.

              B+ ripple is 15Vpp with the stock filter capacitor. Is that within spec? The temporary addition of 47uF reduced the ripple to about 5Vpp.

              Comment

              • #8
                I's say that the cap is bad. There should not be anywhere near that much ripple.

                Comment

                • #9
                  As B+ current draw increases, so will the ripple magnitude. So a ripple measurement to be of much use, it really needs to be measured alongside the B+ current, to give it some context.
                  My band:- http://www.youtube.com/user/RedwingBand

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                  • #10
                    Fully agree but in this case we already have useful data: if clipping 47uF in parallel reduced ripple 3:1 on the spot (no matter the absolute value and changing nothing else) we see that original cap *real* value is about 1/3 of that, or around 15/20uF

                    Since I doubt such a low value was used, then original must be shot.

                    Now IF they used 15/20uF as standard value and somebody is still using that today, maybe because of superstition (a.k.a. "mojo") , then may God have mercy of their souls.

                    Personally, I'd leave the 47uF there and tell nobody, specially at TGP
                    Juan Manuel Fahey

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                    • #11
                      Originally posted by J M Fahey View Post
                      Fully agree but in this case we already have useful data: if clipping 47uF in parallel reduced ripple 3:1 on the spot (no matter the absolute value and changing nothing else) we see that original cap *real* value is about 1/3 of that, or around 15/20uF

                      Since I doubt such a low value was used, then original must be shot.

                      Now IF they used 15/20uF as standard value and somebody is still using that today, maybe because of superstition (a.k.a. "mojo") , then may God have mercy of their souls.

                      Personally, I'd leave the 47uF there and tell nobody, specially at TGP
                      The cap that is in place is a 20/20/20/20 can that's been replaced about 1 year ago with a CP Manufacturing can.

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