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odd screen current resistor question

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  • #1

    odd screen current resistor question

    I have a 50's Wurly juke box on the bench that came in among other things with its screen resistor open. It's an odd one, 69A (1650R) and 69B (9000R) are the same large wirewound heatsink mounted piece with a middle tap about two thirds to one side. The 9000R side is cracked open. I got an Ohmite 9090R 10 watt and flew it off the socket since there was no way to deal with the heatsink mount. Well, I have only had the amp on for short bits of time to service it, and the new resistor is already discoloring. I measured 331.5v across it and by my math I get 12.1 watts.

    331.3 / 9090 = 0.0364 x 331.5 = 12.09 watts

    So I looked at the schematic and saw that R69A is a 1650R 15 watt, and R69B is a 9000R 10 watt. How can this be if they are the same tapped resistor?

    I then did the math for the print and came up with 11 watts.

    315 / 9000 = 0.035 x 315 = 11 watts

    SO, I don't know why the original resistor cracked open, but the currently installed one per the schematic seems to be out of it's league. Any thoughts?

    Whurly schematic.pdf

    BTW, For dissipation the cathode resistor measurement gives me 41.6 watts:

    25.7v across 240R = 0.107A x 388.5v = 41.6w / 2 = 20.8watts per tube, which is about what I get measuring current with my socket bias tester.
    Last edited by Randall; 06-13-2018, 10:03 PM.
    It's weird, because it WAS working fine.....
    Tags: None
  • #2
    I'm guessing you used an Ohmite 80 series? Above 25C you have to derate linearly to 100% 275C. So 10W at 25C. How hot is it in Florida plus the chassis temp? 45C? Anyway 12W is far too much. Go for a 20 watter.

    The cathode calc is off: P= V^2/R = 25.7^2/240 = 2.75W
    Experience is something you get, just after you really needed it.

    Comment

    • #3
      Is this a brownish-gray cylinder mounted to the chassis sorta like a can cap, sticking up?

      It isn't a tapped resistor, it is two resistors in one package. They have a common lead between them. This allowed them to get a hot part out of the chassis and into the air. it is a custom part. it may be constructed by tapping, but we still have the two sections separate inside.

      Unlike a lot of guitar amps, the main B+ to the OT then goes through the screen node resistor to the screens and preamp stages, but R69B is a brute force voltage reducer. They don't just use preamp current draw through a resistor to reduce the B+, they add this resistor to ground to make a voltage divider.

      Yes, I have had to replace those and similar many times.
      Education is what you're left with after you have forgotten what you have learned.

      Comment

      • #4
        Originally posted by nickb View Post
        I'm guessing you used an Ohmite 80 series? Above 25C you have to derate linearly to 100% 275C. So 10W at 25C. How hot is it in Florida plus the chassis temp? 45C? Anyway 12W is far too much. Go for a 20 watter.

        The cathode calc is off: P= V^2/R = 25.7^2/240 = 2.75W
        We were at 88 degrees F so 31C in NW FL. But he's in S FLorida so maybe a bit warmer.

        nosaj
        soldering stuff that's broken, breaking stuff that works, Yeah!

        Comment

        • #5
          Originally posted by nickb View Post
          The cathode calc is off: P= V^2/R = 25.7^2/240 = 2.75W
          He was solving for the tube dissipation there, not dissipation of resistor.
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          ​

          Comment

          • #6
            I wondered about that being a voltage divider after I posted. But even with the print values a 10 watt doesn't seem a good idea. The original on this one is green and horizontally mounted with a large screw through it. So what to do? I wonder if it is a misprint?

            And, not sure it matters what the temperature is down here, it will live in a comfortable air conditioned home at, I'm guessing mid 70's F.
            It's weird, because it WAS working fine.....

            Comment

            • #7
              Don't worry about matching it, 20w aluminum resistors are cheap and easily mounted, just wire something in.
              Education is what you're left with after you have forgotten what you have learned.

              Comment

              • #8
                I'm not having much luck finding what you describe in that value Enzo. How true to the 9000R value would you be?
                It's weird, because it WAS working fine.....

                Comment

                • #9
                  You will have a real hard time finding wither 1650 ohms or 9000 ohms. They are not standard values. But...

                  https://www.mouser.com/ProductDetail...yTuj7ZRQ%3d%3d

                  https://www.mouser.com/ProductDetail...CpzzqwZA%3d%3d
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment

                  • #10
                    The 15 watt 1650R is intact. It's the 9KR I'm concerned with. If I were to deviate from it, which would be better 10K 20W alum. or 8.2K 25W alum.?
                    It's weird, because it WAS working fine.....

                    Comment

                    • #11
                      I just looked up both. One of my links was a 9k 20 watt.

                      9.1k is a standard value. 10k is probably close enough.
                      Education is what you're left with after you have forgotten what you have learned.

                      Comment

                      • #12
                        "One of my links was a 9k 20 watt."

                        Yes, but how to heat sink it? I favor the idea of an alum. 2.5" that I can fasten to the chassis topside and not trapped under it. It must be mounted close to the 6L6s, so it will be more of a temperature factor than the weather in SWFL. Also wondering which way to go that might tweak the voltage divider in a favorable way?
                        It's weird, because it WAS working fine.....

                        Comment

                        • #13
                          I love the aluminum ones, remember to cover the end connections.
                          Education is what you're left with after you have forgotten what you have learned.

                          Comment

                          • #14
                            Originally posted by nosaj View Post
                            We were at 88 degrees F so 31C in NW FL. But he's in S FLorida so maybe a bit warmer.

                            nosaj
                            To 31C you have to allow warm environment of the chassis. That's where the guess of 45C comes from.
                            Experience is something you get, just after you really needed it.

                            Comment

                            • #15
                              Which way should I go with respect to effecting the voltage divider in a favorable way, from 9000R to 8200R or 10K?
                              It's weird, because it WAS working fine.....

                              Comment

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