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  • #76
    Originally posted by hylaphone View Post
    So this suggests the bottleneck is either OT losses, or plate supply current?
    As your B+ sags by only around 25V at full power, I don't see a supply current problem. Supply current is determined by the tubes, not the PT.

    Another question (sorry to ask): Are you sure your screen resistors are actually 47R and not 47k?
    Last edited by Helmholtz; 06-24-2021, 10:56 PM.
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    • #77
      Originally posted by Helmholtz View Post

      As your B+ sags by only around 20V at full power, I don't see a supply current problem. Supply current is determined by the tubes, not the PT.

      Another question (sorry to ask): Are you sure your screen resistors are actually 47R and not 47k?
      Yes they are 47 ohms.
      So perhaps an OT with lower primary Z would yield more power?

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      • #78
        Originally posted by hylaphone View Post
        So perhaps an OT with lower primary Z would yield more power?
        Use Nick's calculator to find out. I wouldn't go lower than 4k though to avoid plate overdissipation.

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        • #79
          Originally posted by Helmholtz View Post

          Use Nick's calculator to find out. I wouldn't go lower than 4k though to avoid plate overdissipation.
          4.2K yields about 10W more

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          • #80
            Originally posted by Helmholtz View Post

            Supply current is determined by the tubes, not the PT.
            Maybe I'm missing the context, but I don't understand this statement. Maximum supply current is determined by the PT, is it not?

            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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            • #81
              Originally posted by g1 View Post
              Maybe I'm missing the context, but I don't understand this statement. Maximum supply current is determined by the PT, is it not?
              A PT has a thermal current limit. Doesn't mean it can't supply more current, but will overheat.

              For a given B+ the current is solely determined by the load, i.e. the tubes and the PT has no influence. All the PT can do is sag the voltage.
              Last edited by Helmholtz; 06-25-2021, 01:44 PM.
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              • #82
                Originally posted by Helmholtz View Post

                …As mentioned earlier, you might have somewhat inefficient tubes.
                E.g. Prof. Zollner found that Chinese KT66s can produce less output than original GEC types because of higher "saturation" voltage.
                Don't know about KT88s, though.
                Yes, modern power valves just don’t seem to produce as high an output as old vintage production ones that are still serviceable. Beam types, at least. The only apparent explanation seems to be that the anode voltage on their Vg1 = 0 curve is a bit higher, thereby reducing the max p-p voltage swing.

                My band:- http://www.youtube.com/user/RedwingBand

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                • #83
                  Originally posted by pdf64 View Post
                  The only apparent explanation seems to be that the anode voltage on their Vg1 = 0 curve is a bit higher, thereby reducing the max p-p voltage swing.
                  Yes, that's what I called "saturation voltage" (had seen this English term used somewhere). Zollner calls it "residual plate voltage" (my translation).
                  It is the voltage drop across the tube at max. signal current.

                  A tube with an increased saturation voltage has a more rounded plate characteristic and increased plate dissipation at full power.


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                  • #84
                    I get that this might be an exercise in theory and a quest for knowledge, but I'd just like to point out that "squeezing out a few more watts" isn't something to be concerned about. A few more watts will be hardly noticeable with respect to SPL. You'd need to double the output power to get even a 3dB increase.
                    "Yeah, well, you know, that's just, like, your opinion, man."

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                    • #85
                      Originally posted by The Dude View Post
                      A few more watts will be hardly noticeable with respect to SPL.
                      See post #56.

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                      • #86
                        Originally posted by Helmholtz View Post

                        Yes, that's what I called "saturation voltage" (had seen this English term used somewhere). …
                        Yes, I’ve read that terminology somewhere, but wasn’t keen on it, as it might imply some relationship to operation in saturation mode, which is something else again.
                        My band:- http://www.youtube.com/user/RedwingBand

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                        • #87
                          Fully agree, that's why I typically use it between double quotes.

                          Now we're in need of a better term. Any suggestion?
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                          • #88
                            Originally posted by Helmholtz View Post

                            A PT has a recommended thermal current limit. Doesn't mean it can't supply more current, but will overheat.
                            Ok. So the measured sag seems to tell us it's not a PT issue.
                            But I've re-read the entire thread and one data point that is missing is the make & model of PT. It would be good to know.

                            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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                            • #89
                              Originally posted by The Dude View Post
                              I get that this might be an exercise in theory and a quest for knowledge, but I'd just like to point out that "squeezing out a few more watts" isn't something to be concerned about. A few more watts will be hardly noticeable with respect to SPL. You'd need to double the output power to get even a 3dB increase.
                              Yes, appreciated. I'm trying to have a more "holistic" understanding of all the factors discussed here so that I may design better output stages. This thread has been extremely helpful.

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                              • #90
                                Originally posted by g1 View Post
                                Ok. So the measured sag seems to tell us it's not a PT issue.
                                But I've re-read the entire thread and one data point that is missing is the make & model of PT. It would be good to know.
                                It's custom from Hammond, with a separate winding for the screen supply. I couldn't find an off-the-shelf transformer that fit the bill. But now I have a bunch of these to try out in different designs..

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