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PT sub with higher B+

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  • PT sub with higher B+

    I have a 66 Super Reverb that has a non Fender replacement PT in it with no bias winding, and a higher B+ than I'd like. The bias supply is tapped off one leg of the secondary, and works well. However, I am getting a secondary of 436 - 436 vac, and B+ of 550v. When I got it, the rectifier was changed to diodes. I put it back to a TAD GZ34/5AR4. Not only am I questioning can I run this amp with such high voltages, I also don't seem to be able to find a datasheet on the TAD tube.

    I have adjusted the bias down to an acceptable safe level of around 60% for the USA made Baldwin branded 6L6GCs.

    what to do?
    It's weird, because it WAS working fine.....

  • #2
    Your amp or a customer's? Seems to me like the best course of action would be to get a PT that gets it as close to stock as possible.

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    • #3
      Customer amp that is already at about $350 repair bill. I'm sure he'd like to stick with this PT if possible.
      It's weird, because it WAS working fine.....

      Comment


      • #4
        Are there numbers on the replacement PT, or do you have specs? What is your filament voltage? Is it also high?
        "Yeah, well, you know, that's just, like, your opinion, man."

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        • #5
          Originally posted by The Dude View Post
          Are there numbers on the replacement PT, or do you have specs? What is your filament voltage? Is it also high?
          Randall said early on that there's no ID on the transformer.
          This isn't the future I signed up for.

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          • #6
            Got it and thanks. It would still be helpful to know if filament voltage is high. In other words, do we need to "fix" the primary or secondary voltage.
            "Yeah, well, you know, that's just, like, your opinion, man."

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            • #7
              The only designation I see is 24R07, which may or may not be this Thordarson,

              https://worldradiohistory.com/hd2/ID...-Page-0142.pdf

              I am seeing filament voltage of 6.8v
              It's weird, because it WAS working fine.....

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              • #8
                Almost 10% high, that may be due to the difference between what the transformer was designed for 115, 117V common in the good ol' days. And now when 120V or more is common.
                This isn't the future I signed up for.

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                • #9
                  So, filament voltage is only slightly high.

                  Options as I see them if keeping this PT:

                  1) Use good quality 6L6 and let it buck running on the extremes. Many manufacturers run high plate voltage.
                  2) Install a bucking transformer in the primary to lower secondary voltages overall.
                  3) Modify bias circuit & change screen resistors for EL34's. Easy mod, and transformer filament winding is rated at 5A, so it should handle the increased filament current. EL34's can handle the B+. Increased filament current should lower filament voltage a bit.
                  4) Leave the only slightly high filament voltage as is and lower B+ with zeners or some other method.
                  5) I'm sure others will chime in.
                  "Yeah, well, you know, that's just, like, your opinion, man."

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                  • #10
                    Fits perfectly with 24R07 in the link: Input 115V, secondary 800V center-tapped.
                    - Own Opinions Only -

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                    • #11
                      Sounds too high even with tube rectifier but with silicon diodes itīs straight suicidal.

                      HOPE voltage tames down quite a bit under load.

                      Besides plates, I am quite worried about screens.

                      Cursing the earlier Tech who fit this tube eater PT instead of the proper one. What was he thinking?
                      Juan Manuel Fahey

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                      • #12
                        Originally posted by Randall View Post
                        Customer amp that is already at about $350 repair bill. I'm sure he'd like to stick with this PT if possible.
                        I'd tell him that there is no warranty with the current PT. Maybe the thought of an additional $350 repair in a month will convince him.

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                        • #13
                          Originally posted by J M Fahey View Post
                          Cursing the earlier Tech who fit this tube eater PT instead of the proper one. What was he thinking?
                          "Hey I got this old tranny lurking in my junk pile. Now I can sell it for some beer money, hooray!"

                          I have a few of those but mostly OT's.

                          Did manage to successfully revive an ancient Heathkit W4B power amp with a PT that I reckon came from a big old color TV. Still working after 21+ years, it serves as a real-world hi voltage output tube tester next to my workbench, plus I can play the radio through it to keep me amused. Sounds darn good! I've tested over a thousand output tubes with it, used and new, find out whether they can really take what a guitar amp will dish out.
                          This isn't the future I signed up for.

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                          • #14
                            Customer opted to go with a new PT. I opted for the Hammond 290D2X with a 710v CT secondary, options being what they are these days. That should cool things down nicely.
                            It's weird, because it WAS working fine.....

                            Comment


                            • #15
                              Originally posted by Randall View Post
                              Customer opted to go with a new PT. I opted for the Hammond 290D2X with a 710v CT secondary, options being what they are these days. That should cool things down nicely.
                              Had a nice 65 Sup Rev in recently, PT had open primary. It left feeling hale & hearty with a fresh Hammond 290DX from ce. Hmm what's the D2X? The sticker on the 290DX box says 325 - 50 - 0 - 325 aka 650V secondary.

                              Looking at what I can find of D2X version, might be better with the DX. Plus save a handful of $$$. Problem, ce shows out of stock. Oh well. Depends on your source, what's really available + hew fast you need it. ce's server also appears to be stumbling, running very slow last 24 hours.
                              This isn't the future I signed up for.

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