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I want to build an amp selector (two amps a/b, one cab)

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  • #46
    Originally posted by Gtr0 View Post
    Sorry, Fellas - I am not experienced enough to be comfortable with "experienced amp guy vernacular" - of course I say that in a nice way - so when you say "to put a resistor across the amp speaker out signal" can I take that to mean from the output jack's tip to ground? And is it enough to do it once for parallel outputs or should I do that per jack?

    Thanks!!!!
    Correct, the safety resistor goes between the tip and ground of the speaker jack.

    If you have multiple parallell jacks then one resistor is enough. If you have different jacks for different impedances (e.g. 4, 8, 16 ohms) though... I must confess I’m not sure if it is sufficient to hav one resistor for one of the taps (e.g. 8 ohms), or if you need separate resistors for each tap.

    If you have an impedance selector switch, you can have one resistor at the jack.

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    • #47
      One resistor (base load) permanently connected to one of the taps is sufficient. Minimum power rating depends on resistance, amp power and tap/impedance. E.g. for a 100W amp a resistor of 270 Ohm connected to the 8 Ohm tap should be rated at least at 3W (I'd use a 5W resistor to take care of increased output with clipping).
      - Own Opinions Only -

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      • #48
        Thanks very much!

        I'm splitting time between a few amps atm and when I find some time I will seek out the proper switches etc to realize this amp switcher. I could have REALLY used it today, two identical amps with differing values in the EQ, by the time I get from one to the other with swapping speaker cables, power and warm up etc, I have lost most of it.

        Much appreciated!
        "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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        • #49
          Originally posted by Enzo View Post
          I never heard it before. NC and NO are my letters for relays. MAke before break switch contacts are called "shorting" contacts.

          In my work in amusements we had pinballs FULL of relays, as well as jukebox control circuits. We used the Form A, B, or C terminology for make or break. A form C switch could be shorting or non-shorting.
          Apparently Form D is 'make before break' aka 'continuity transfer'. Form C is 'break before make', Form D is the same assignment as Form C but is 'make before break'.
          So during the switchover, all terminals are connected on form D. On Form C, during switchover, all terminals are briefly disconnected.
          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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          • #50
            Originally posted by Pedro Vecino View Post
            For switching input signals, no problem. They are the ones that Engl uses, for example.

            But for speaker switching you need something stronger since it´s a critical function. As a reference, Mesa uses in the Roadking (this amp has automated switching to different speaker outputs) a set of Nec MR301-12HSL. They are relays that can handle 10A. You would need two to complement the switching in that part. At least with that exact model.

            This is the datasheet (Page 8):

            https://www.digchip.com/datasheets/p...-12HSL-pdf.php
            Originally posted by g1 View Post
            Apparently Form D is 'make before break' aka 'continuity transfer'. Form C is 'break before make', Form D is the same assignment as Form C but is 'make before break'.
            So during the switchover, all terminals are connected on form D. On Form C, during switchover, all terminals are briefly disconnected.
            According to the datasheet that Pedro linked, this relay is a form c. And so I gather that the switch I choose MUST be of form D, make before break, correct? It makes sense as such.

            Thanks.
            "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

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            • #51
              Originally posted by Gtr0 View Post
              According to the datasheet that Pedro linked, this relay is a form c. And so I gather that the switch I choose MUST be of form D, make before break, correct? It makes sense as such.

              Thanks.
              If you use safety resistors, both types are OK.

              Make sure to have the safety resistor on the switchbox though, not just amps. Sooner or later someone will plug in an amp without safety resistors...

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              • #52
                Originally posted by Gtr0 View Post
                Sorry, Fellas - I am not experienced enough to be comfortable with "experienced amp guy vernacular" - of course I say that in a nice way - so when you say "to put a resistor across the amp speaker out signal" can I take that to mean from the output jack's tip to ground? And is it enough to do it once for parallel outputs or should I do that per jack?

                Thanks!!!!
                One resistor for each external amplifier connection.

                Comment


                • #53
                  Yep, "...non-inductive, "swamping" resistor in parallel with each amp's OTs winding input to switching circuit..."

                  as I stated in my earlier post: https://music-electronics-forum.com/...l=1#post540620
                  ...and the Devil said: "...yes, but it's a DRY heat!"

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                  • #54
                    ...non-inductive, "swamping" resistor
                    No need for a "non-inductive" resistor. The inductance of any wire-wound resistor is absolutely negligible compared to the inductance of a speaker (as well as the secondary leakage inductance of the OT which acts in series with the load and thus adds to any load inductance).
                    Last edited by Helmholtz; 10-16-2019, 06:54 PM.
                    - Own Opinions Only -

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                    • #55
                      Ted Weber has made one of these for years.

                      https://www.tedweber.com/two-head-fs

                      I've got a three head version that Ted made right before he passed. They discontinued that, but borrowed it from me to possibly reissue it. They ended up making this two head one instead which I'm sure works well.

                      Greg

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                      • #56
                        There are several commercial options available. Another is the Radial Tonebone: https://www.radialeng.com/product/headbone-vt

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                        • #57
                          Originally posted by soundmasterg View Post
                          Ted Weber has made one of these for years.

                          https://www.tedweber.com/two-head-fs

                          I've got a three head version that Ted made right before he passed. They discontinued that, but borrowed it from me to possibly reissue it. They ended up making this two head one instead which I'm sure works well.

                          Greg
                          But:

                          "The Two-Head Amp Switcher DOES NOT switch your guitar input from amp to amp. This switcher is designed for players who are already using two different amplifiers, but wish to simplify to one cabinet."

                          So you need to switch inputs and outputs separately.
                          - Own Opinions Only -

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                          • #58
                            Originally posted by Helmholtz View Post
                            So you need to switch inputs and outputs separately.
                            That's crazy talk! Hit two buttons!?! Now we're back in the stone age

                            Just kidding, of course. This was my suggestion much earlier.
                            "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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                            • #59
                              More than you want or need but out of interest...

                              I was approached by a guy with too many amps and cabs to come up with a switch for 16 amps to 8 cabs. The switching matrix used 10A DPDT relays on four identical boards used of which provided an 8x4 matrix. Both speakers wires were switched to allow for solid state amps with bridged outputs

                              It was controlled using a rotary encoder and an LCD display. You edit the connections and it gets saved in non-volatile memory. The relays are controlled over an I2C bus.

                              Here is one 8x4 board and the controller during testing:
                              Click image for larger version

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                              Building it into the box:
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                              I had intended to use dual stacked jacks on the rear of the boards to save a lot of wiring but there was a footprint problem so I had to hard wire them. I still have a couple of PCBs if anyone wants one.

                              The glamour shot:
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                              Experience is something you get, just after you really needed it.

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                              • #60
                                Originally posted by Helmholtz View Post
                                But:

                                "The Two-Head Amp Switcher DOES NOT switch your guitar input from amp to amp. This switcher is designed for players who are already using two different amplifiers, but wish to simplify to one cabinet."

                                So you need to switch inputs and outputs separately.
                                That is completely correct. Its not ideal, but to make one that switches inputs and outputs at the same time is a lot more complicated. The three head version I have I got for a specific purpose. I modified a Conn organ chassis years ago that had three separate output sections in it that shared a power transformer. So three phase inverters, three sets of power tubes, and three output transformers. I added a preamp chassis so I basically have three amps in one head, but I needed a way to switch between cabinets or I would always have to carry three cabinets around. Hence Weber's switcher.

                                Greg

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