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MusicMan HD130. Tube PI. Upward ramping Bias

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  • #16
    Juan, its thanks to people like you and the other regular contributors to this Forum that makes the repair life bearable when you are alone in a workshop day-after-day. The incredible knowledge and experience that you guys contribute is amazing. I think the day you guys go, or this Forum closes down, I'll hang up my soldering iron, turn off the light and feed the pigeons.

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    • #17
      Nope. Then it'll be time for you to pass all you've learned on to the next generation of MEF'rs.
      "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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      • #18
        The cathode drive tubes do indeed run much cooler, a lot closer to class B.
        Education is what you're left with after you have forgotten what you have learned.

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        • #19
          I'll try Dude, but I won't ever be a Dude, Nick, Juan, Helmholtz, Justin, G1 and the usual guys that offer their experience. And God knows, I'll never be an Enzo!

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          • #20
            Hi G1, I set the amp to 20mA at 695V, about 56%. I hope that's OK. I have left in his original Sylvania EL34s, 3 of which are still close, while one is a bit down at 16mA compared to 20mA for the others. I decided to leave them all in, because I don't trust a modern tube to sit happily with 700v on its plates with a grounded cathode.

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            • #21
              Originally posted by christarak View Post
              I'll try Dude, but I won't ever be a Dude, Nick, Juan, Helmholtz, Justin, G1 and the usual guys that offer their experience. And God knows, I'll never be an Enzo!
              Thanks for the guilt by association, but don't put me in that crowd! Everything I know I pretty much owe to them, and a healthy dose of hacking up my own junk (not always for the better) and blowing some stuff up, and in one case setting my amp on fire.

              I just find I have a pretty good memory & a knack for translating concepts.

              I used to spend literally all night several nights a week reading every single post when this site was still Ampage. Most of it I didn't understand. But I got there. Sorta. I'm usually pretty good until you bring in Impedance. Then I just scream "TELL ME WHAT TO PUT WHERE & FOR THE LOVE OF PETE DON'T ASK ME TO DO ANY MATH!"

              Justin & Jusrin
              "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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              • #22
                You're a funny man, Justin. You have my respect because you offer generous help to your fellow techs without any judgement or negativity. Everyone has their own online personality that I like, from Juan's fiery Latino, but spot-on, comments, to Enzo's "how can one man possibly know so much" incisive advice. Wouldn't it be so cool, if one day we could all meet up?

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                • #23
                  Originally posted by g1 View Post
                  wonder if a tube is just running away?
                  The reason I ask this is the vast difference in idle dissipation between the grid drive and cathode drive versions of this amp.
                  Both versions have around 700V plate, 350V screen.
                  Grid drive version idles (spec) at 25mA per tube (17.5W or 70%)
                  Cathode drive version idles at 3mA per tube (2.1W or 8%)
                  I don't think the big difference would just be due to cathode drive?
                  My take on this is that power tubes are very nonlinear at low current, so when cold they sound choppy ugly "no sustain", like a poorly adjusted noise gate, so biasing them hot brings the mellow smooth quality out.
                  Problem is that by ear musicians bias them crazy hot.
                  You have noticed that most all Factories (usully run by Engineers) bias amps quite cold (in my book, "properly") .
                  While in cathode driven amps tube is VERY linear , plate current is exact same as cathode current (grid voltage is irrelevant) and this comes from a transistor, which is very linear even for very low idle current.
                  So in this hybrid cascode combination you can bias for very low current, a few mA, and have no problems.

                  Which in due time, since you lower idle dissipation so much, you can "safely" rise HV a lot.

                  Now "conventional PI" plus crazy high voltage , in my book, is an accident waiting to happen, or at least requires *careful* selection of components ..... or else.
                  Juan Manuel Fahey

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                  • #24
                    Wouldn't it be so cool, if one day we could all meet up?
                    That would be great, but I understand that down under the beer has its head on the bottom?
                    Education is what you're left with after you have forgotten what you have learned.

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                    • #25
                      And yet most of these amps have been working fine for decades.

                      Hmmm, maybe the screens running at half the plate voltage has something to do with it.

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                      • #26
                        Originally posted by J M Fahey View Post
                        ...and feeding corn and bits of bread to the pigeons at closest public square.
                        You don't have to spend all your time feeding the pigeons

                        If it still won't get loud enough, it's probably broken. - Steve Conner
                        If the thing works, stop fixing it. - Enzo
                        We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                        MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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                        • #27
                          Originally posted by drewl View Post
                          And yet most of these amps have been working fine for decades.

                          Hmmm, maybe the screens running at half the plate voltage has something to do with it.
                          You bet, I know the screen voltage definitely plays a substantial role. But I'm specifically asking about the 2 versions of the amp, both of which share the same plate and screen voltage, but are biased very differently.
                          I've probably worked on at least a hundred MM amps, but have only seen the PI tube variety maybe a handful of times. If you're telling me the grid drive/PI tube type biased hot are no harder on power tubes than the cathode drive type, I'm all ears.
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


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                          • #28
                            Don't be ridiculous Enzo. When you come to Australia, do what the locals do: turn the glass upside down so the head is at the top!

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                            • #29
                              Sorry, I don't know enough about your culture, I guess...
                              Education is what you're left with after you have forgotten what you have learned.

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                              • #30
                                Hilarious!

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