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  • Dual Power Supplies?

    Hi All,

    Making an amp with 2 OTs from an old tone cabinet chassis. This is gonna be a head, so I may not brme able to run 2 cabs/separate speakers at all times.

    Thinking to put 2 Standby switches, one each to cut the B+ to the CT of each OT. Would also have each respective following power supply node downstream of that amp's Standby switch. Basically, two separate & complete parallel supplies.

    I'm trying to make it so I don't have to worry about dummy loads or always having a speaker attached to whichever amp I'm not using.

    Alsothinking to include heater switching for each side? That way any side I'm not using can basically be totally shut off...

    I have a roomy chassis & radial caps are tiny & cheap. Rectifier is a 5U4GB. I'll see if I can attach a pic from my phone; values can be determined later. This amp is also for me, as I need something a bit less powerful
    2xEL84, 2x12AX7 per side.

    Thanks,

    Justin
    "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 -

  • #2
    Here's what I was thinking...
    Attached Files
    "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


    • #3
      Originally posted by Justin Thomas View Post
      Here's what I was thinking...
      Justin, this doesn't answer any questions directly, but I have recently come into possession of a very similar stereo Zenith amp (2 x EL84 per side, 12ax7s and 5U4GB). It uses a rather large common PT and I've made some changes so I could use it as a guitar amp. The chassis I have only contains one 12AX7 used as a second stage and a PI. The schematic lists the 12AX7 as a 6FQ7, and that was changed at one time or another.

      The amp sounds great by the way, and I am amazed that I missed out on EL84 tubes for all these years. I had some irrational bias towards skinny tubes, but no more !

      The Zenith stereo output transformer configuration is an odd one, but I will leave it to you extract anything that might be helpful or [B]useful in some other way.[/B

      Oh, and there is another one RIGHT NOW for sale on ebay for what I consider inexpensive, (if you don't mind dealing with the odd output transformer arrangement). The PT alone should be worth the purchase IMHO : https://www.ebay.com/i/223704672737?...yABEgKpLvD_BwE

      Click image for larger version

Name:	Zenith 7D31 Schematic (Stereo EL84).jpg
Views:	2
Size:	96.6 KB
ID:	857423
      Last edited by HaroldBrooks; 04-12-2020, 08:42 PM.
      " Things change, not always for the better. " - Leo_Gnardo

      Comment


      • #4
        Originally posted by Justin Thomas View Post
        Here's what I was thinking...
        You need the PT from an Evil twin I've been hanging on to, to run that?

        nosaj
        Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

        Comment


        • #5
          Originally posted by HaroldBrooks View Post
          Justin, this doesn't answer any questions directly, but I have recently come into possession of a very similar stereo Zenith amp (2 x EL84 per side, 12ax7s and 5U4GB). It uses a rather large common PT and I've made some changes so I could use it as a guitar amp. The chassis I have only contains one 12AX7 used as a second stage and a PI. The schematic lists the 12AX7 as a 6FQ7, and that was changed at one time or another. ] The amp sounds great by the way, and I am amazed that I missed out on EL84 tubes for all these years. I had some irrational bias towards skinny tubes, but no more !

          The Zenith stereo output transformer configuration is an odd one, but I will leave it to you extract anything that might be helpful or [B]useful in some other way.[/B

          [ATTACH=CONFIG]58002[/ATTACH]
          Silvertone 1485 is another dual OT config
          Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

          Comment


          • #6
            Harold,

            I was just wondering if having the dual Standby switches would be okay to just "shut down" one side of the amp if I don't have a stereo cab or two cabs. If I only run one side, I'd like to keep the other side "safe" without having to plug in a dummy loa or some other switching thing. I could add a heater switch for each side also if that would help keep the tubes alive longer.

            The only thing that would be "on" all the time would be the negative bias supply for the EL84s.

            So I was just asking whether the switching arrangement would work.

            Justin
            "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


            • #7
              Originally posted by nosaj View Post
              You need the PT from an Evil twin I've been hanging on to, to run that?

              nosaj
              Nah, I don't THINK so! I've already got the PT from the original amp. If I hafta end up adding a filament tranny, I will.

              And I've got a Fender Twin Reverb tranny sitting at home anyway.

              Thanks, though!

              Justin
              "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


              • #8
                Originally posted by Justin Thomas View Post
                Nah, I don't THINK so! I've already got the PT from the original amp. If I hafta end up adding a filament tranny, I will.

                And I've got a Fender Twin Reverb tranny sitting at home anyway.

                Thanks, though!

                Justin
                You know if push ever comes to shove i think you can workout with the PT and the OT.
                nosaj
                Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

                Comment


                • #9
                  They're not that heavy. Of allmy amps the Bassman 100 has the record - the OT is 2x the size of the PT.

                  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
                    There's a sort of good idea. You would have a bit of a rise in B+ for the output tube pair left running. I'll leave it to you whether you make a more complicated compensation for that when the amp's running one channel only. I'd say don't sweat it, chances are pretty good the bias current won't go way out of whack, but it is bound to rise a chunk, when you go single channel.

                    I can see shutting off filament current to the "sleeping" output tubes too, if you're worried about cathode poisoning which may take place in tubes left filament on with no B+ if left that way for extended time.
                    Enjoy. Every. Sandwich.

                    Comment


                    • #11
                      Originally posted by Leo_Gnardo View Post
                      There's a sort of good idea. You would have a bit of a rise in B+ for the output tube pair left running. I'll leave it to you whether you make a more complicated compensation for that when the amp's running one channel only. I'd say don't sweat it, chances are pretty good the bias current won't go way out of whack, but it is bound to rise a chunk, when you go single channel.

                      I can see shutting off filament current to the "sleeping" output tubes too, if you're worried about cathode poisoning which may take place in tubes left filament on with no B+ if left that way for extended time.
                      Are you looking at some type of Stereo Tremelo or something similar?

                      nosaj
                      Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

                      Comment


                      • #12
                        Originally posted by Justin Thomas View Post
                        They're not that heavy. Of allmy amps the Bassman 100 has the record - the OT is 2x the size of the PT.

                        Jusrin
                        I bet you've never dropped one on your foot then

                        Heavy enough I think the PT an OT from the evil twin are probably the biggest ones I have sitting around.
                        nosaj
                        Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

                        Comment


                        • #13
                          Thanks Leo,

                          I was thinking about the B+ rise... This thing might end upgetting a pair of 6V6s in one or both sides to compensate for that... It originally had 6973s in it. So the. 6V6s would be less heater draw anyway. I'm just trying to use junk I have on hand, one of which is a matched quad of JJ EL84s to blow...

                          For the heaters, I'll have the CT winding grounded. Would a simple SPST switch in half the winding be cool to cut the heaters? I'm not great with all that current theory stuff...

                          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
                            It's my thread & I'll derail it if I want to...

                            Here's the BM100 OT beside the PT with a JJ 6L6GC for comparison:
                            Attached Files
                            "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


                            • #15
                              Originally posted by Justin Thomas View Post
                              For the heaters, I'll have the CT winding grounded. Would a simple SPST switch in half the winding be cool to cut the heaters? I'm not great with all that current theory stuff...
                              Your proposal would definitely reduce filament current to the inactive tubes to vanishingly small. BUT - I'm worried it would induce hum into your circuit instead of having the hum cut to nearly nil as it is with current draw equal on both sides of the virtual center tap. Only one way to find out... if it does induce hum then just let the filaments light up & don't sweat cathode poisoning. If you were were using some big bucks NOS or exotic output tubes, it's worth a worry. Also if you were planning to run the amp thousands of hours in mono mode. JJ EL84 or 6V6? Cheap and expendable.
                              Enjoy. Every. Sandwich.

                              Comment

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