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Series-Parallel Heaters

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  • #1

    Series-Parallel Heaters

    Hi Again All,

    I have a transformer & tube heater question. I have a Baldwin organ PT. It powered 3x12A_7 tubes & 2x6L6GB alpng with a 5U4. But as an added bonus, there's a 12V winding that powered 57x12A_7 tubes. No, that's not a typo.

    I have a BOATLOAD of various 6V power tubes that use lots of heater current, along woth all kinds of weird preamp tubes. So I'm wondering about wiring up various combinations in series-parallel. I've heard that it's "impractical" to do so but I'm not too worried about that... I also know that when wiring heaters in series that the current draw is the same among the tubes in the string - is that how it goes?

    Though, don't Champs have series heaters & a 6V6 & 12AX7 don't have the same current ... Or is that not quite the same?

    I'm going to go read Merlin's article again & other literature in the meantime, but I'd like to try out some of these dual-triode & pentodes that all suck up between 1A & 1.5A in heater current if I can. This tranny is kinda a behemoth so I think I can pull it off with this one. Thanks for any advice or considerations everyone might have.

    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 -
    Tags: None
  • #2
    Originally posted by Justin Thomas View Post
    ...don't Champs have series heaters & a 6V6 & 12AX7 don't have the same current ... Or is that not quite the same?...
    There are various of Fender Champ circuits but I have never seen one that used series heaters.
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    • #3
      Okay, thanks. I'm confusing series heaters with something else then. They're still parallel, but only using one leg of the winding, correct? Or, using the chassis as the other heater wire?

      I'm also looking at Peavey Classics. Seems the power tubes are in series; there's no value given but it's a 24V winding, correct?

      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 -

      Comment

      • #4
        No issue for placing 12A-7 tubes across 12Vac winding, but I'd recommend only placing two 6V heaters (in series) across 12Vac winding if they are 'the same' tube (ie. model number and manufacturer). As a check that any/all of the series connected heaters are 'ok' then just measure Vac across each valve's heater - it should be 6V3 +/- say 5%. If you have an odd 6V3 valve on its own then you could insert a resistor in lieu of the missing series heater, and adjust the resistor value to get 6V3 across the valve's heater.
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        • #5
          This is what I was thinking. So I could put say, (random examples of tubes I pull out of my head here) 2x6SJ7 in series, & 2x6BK7 in series, & put those 2 pairs in parallel, & so on, correct?

          It's okay if the series pairs are different types, correct?

          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 -
          • Likes 1

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          • #6
            I've observed with series heaters in pairs that even with the same brand/type of tube, one tube can get more voltage during a cold start - the worst affected are those quick-start tubes that flash when initially powered. Significantly different tubes show different voltage drops when hot due to different heater resistances, so each pair ought to be the same tube type.
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            • #7
              A lot of people have trouble understanding series heaters but it's not that hard if you think about current . With parallel heaters the voltage stays the same and you add currents . With series heaters the current stays the same and you add voltages . With series parallel you have to use Ohm's law . As an example , I have an amp with 2 x 50L6 and 3 x 20EZ7 . The 50L6 are 150 ma and the 20EZ7 are 100 ma . I have line voltage full wave rectified for 160V . The 50L6 are wired straight through and I parallel the 20EZ7 with a resistor to eat the other 50 ma . There are many possibilitys , you just need the currents through the string to be the same and the voltages to add up . Treat the heaters like resistors.
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              • #8
                The 12A*7 series tubes have 2 heaters each, which can be wired series or parallel. The Champ runs them parallel off 6.3V, which is most common.
                Like trobbins said above, it is best if each pair of 6V heaters to be placed in series are matched. So the pair within any 12A*7 tube should be fine, but when you are putting a couple 6V heater tubes in series (ex. 2 x EL34), they should be the same brand for best match of heater voltage.
                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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                • #9
                  Is the difference between "best" & "works" the same as "lightning bug" & "lightbing"? or is it not quite that dramatic?

                  Okay, I'll look into my stash & see what I can come up with. Thanks All!

                  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 -

                  Comment

                  • #10
                    If you meant lightning, not that dramatic.
                    But the 12V would not be shared equally with tubes that spec same heater current but are off tolerance. So you could end up with one at 5.5V and the other at 6.5V for example.
                    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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                    • #11
                      Cool, I can live with that. Yes, lightning. As long as nothing legit burns my house down. The joy of tubes - FAFO to your heart's content.

                      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 -

                      Comment

                      • #12
                        57 preamp tubes? Someone raided an organ!

                        My beloved Peavey Bravo feeds its first (2) 12AX7s heaters in series with the 27VDC tap used for its bias/fan supply.

                        It then uses a 6.3VAC tap for the last 12AX7/PI and its EL84s.

                        Its a gainy little amp and might need the DC heaters to tame noise even with its diode coring "pseudo noise gate" and omitted 3rd stage cathode bypass cap (it still has a pad!)
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                        • #13
                          This Baldwin seems similar... 6.3V for 2x12AX7 & 2x6L6GB, 12.6V for the other 57 12A_7s that I unfortunately did not get...

                          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 -

                          Comment

                          • #14
                            I have a PT for one of those Cordovox CG units that also has ballpark 60 preamp tubes, so probably similar heater capability. Anyone that wants to pay shipping on it, let me know.
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            ​
                            • Likes 1

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                            • #15
                              Originally posted by Justin Thomas View Post
                              This Baldwin seems similar... 6.3V for 2x12AX7 & 2x6L6GB, 12.6V for the other 57 12A_7s that I unfortunately did not get...

                              Jusrin
                              I did get an old Conn organ with some huge number of preamp tubes. I didn't even save the power transformer because:

                              1) Lots of rust

                              2) Enormous for a two power tube guitar amp.

                              I, on the other hand, did get all the keyboard tubes. This thing had been left in a yard outdoors for a while. I had to clean and deox all the pins. Which was labor intensive. And they were all 12au7's. So not especially useful for most designs. Then I had to test them all. Not having a tube tester I plugged them into the first position of an amp one at a time to observe gain and microphonics. About half were not worth saving. The other twenty some tubes went into my stash. I still have a ten or twelve of them after almost thirty years. I've considered using them as power tubes for a little amp. I'm thinking something like a Marshall type preamp with a push pull circuit using four 12au7's. Probably around six or seven watts?

                              But I have several other projects I'm also not getting to in front of this idea

                              I guess my point is that you didn't miss much not getting a bucket of marginally useful tubes that probably needed to be fastidiously cleaned.
                              "Take two placebos, works twice as well." Enzo

                              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                              "If you're not interested in opinions and the experience of others, why even start a thread?
                              You can't just expect consent." Helmholtz
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