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

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by d95err View Post
    The main thing to be careful about when switching speakers is to avoid intermittent load, which can cause voltage spikes and destroy the output transformer.

    Even if the unused amp is switched to a dummy load, the relays will bounce a bit when switching. If there is no other protection, this would be very risky.

    A simple protection is to put a resistor across the amp speaker out signal (e.g. 270 ohms/5W) that is always in the circuit. This is enough load to protect the amp, without affecting the tone.

    The safety resistor can be added to the amp (I use it on all my builds), or part of the switch box (on the speaker signal input from each amp, before the relay).

    I would also recommend adding a circuit to mute the input of the unused amp.
    I like this idea ^^

    For switching tube amp outputs, wouldn't MBB power relays be the way to go as well?

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    What are MBB power relays ?

    I tried running two 5E3 type amps into a stereo wired Marshall 4x12. Caused some kind of oscillation problem, but only when a note or cord was being played above a certain volume. I even isolated the grounds on the jack plate and that didn't help.

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    Quote Originally Posted by loudthud View Post
    What are MBB power relays ?

    I tried running two 5E3 type amps into a stereo wired Marshall 4x12. Caused some kind of oscillation problem, but only when a note or cord was being played above a certain volume. I even isolated the grounds on the jack plate and that didn't help.
    Interesting.

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by loudthud View Post
    What are MBB power relays ?

    I tried running two 5E3 type amps into a stereo wired Marshall 4x12. Caused some kind of oscillation problem, but only when a note or cord was being played above a certain volume. I even isolated the grounds on the jack plate and that didn't help.
    Yeah that’s kind of wild. ‘Gotta wonder if there was some acoustic feedback getting picked up by the 2 unused speakers, and getting reflected back to the output stage and causing instability. Of course, that presumes you were switching between them and not running them both at the same time.

    MBB stands for “Make-before-break” referring to the switching action. The benefit of that kind of power relay, is you remove a lot of the risk of opening up the OT secondary in the act of switching.

    For those who might not know what that means, it means that, in the act of switching, the switching contact makes the connection to the following terminal, before breaking the connection with the terminal it is being switch from.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by SoulFetish View Post
    MBB stands for “Make-before-break”
    I'm sure, like me, no one else can believe they didn't put that together right off.

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    Lifetime Member Enzo's Avatar
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    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.

    https://forum.digikey.com/t/understa...figuration/811

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    Quote Originally Posted by loudthud View Post
    What are MBB power relays ?

    I tried running two 5E3 type amps into a stereo wired Marshall 4x12. Caused some kind of oscillation problem, but only when a note or cord was being played above a certain volume. I even isolated the grounds on the jack plate and that didn't help.
    Both 5E3 in phase to a stereo cabinet, Would it have help to reverse the phase on one 5e3? Seems a similar concept like a PI, no?

    nosaj

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    Member mhuss's Avatar
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    I didn't really have any pop problems because the amp input grid resistors keeps the input DCV at zero, and I used small-signal relays to switch the inputs. The biggest noise IIRC was the 'clack' of the two power relays on the speaker side switching, but nothing from the speaker.

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    Quote Originally Posted by SoulFetish View Post
    I like this idea ^^
    The safety resistor idea comes from Kevin O’Connor’s books. He wrote somewhere that he had never seen an amp damaged by a mismatched load (wrong speaker impedance), but he had seen dozens of amps with broken output transformers due to intermittent load (e.g. unplugging speaker while playing or glitchy speaker jack connection).

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    Quote Originally Posted by d95err View Post
    The main thing to be careful about when switching speakers is to avoid intermittent load, which can cause voltage spikes and destroy the output transformer.

    Even if the unused amp is switched to a dummy load, the relays will bounce a bit when switching. If there is no other protection, this would be very risky.

    A simple protection is to put a resistor across the amp speaker out signal (e.g. 270 ohms/5W) that is always in the circuit. This is enough load to protect the amp, without affecting the tone.

    The safety resistor can be added to the amp (I use it on all my builds), or part of the switch box (on the speaker signal input from each amp, before the relay).

    I would also recommend adding a circuit to mute the input of the unused amp.
    Quote Originally Posted by mhuss View Post
    I built a two-relay box similar to the one shown above, but used full-time 200 ohm 5w resistors across the amp jacks (to keep a minimum load). 200 ohms because... that's what I had laying around! I used it for a bit with no problems, but haven't used it in a while. (I was using it for two different amps/sounds, not for A/B testing).
    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!!!!

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    Quote 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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    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).

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    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!

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    Quote 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.

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    Quote 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
    Quote 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.

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    Quote 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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    Quote 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.

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    Senior Member Old Tele man's Avatar
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    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

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    ...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).

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    Last edited by Helmholtz; 10-16-2019 at 08:54 PM.
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    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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    There are several commercial options available. Another is the Radial Tonebone: https://www.radialeng.com/product/headbone-vt

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    Quote 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.

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    Bent Member Chuck H's Avatar
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    Quote 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.

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    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:
    Click image for larger version. 

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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:
    Click image for larger version. 

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    Quote 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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    Still have this on the back burner.... but still want to do it, so I have been looking around in spare time for the correct relays that are available.

    How about these ones??
    Finder F36119-009A

    10A 9V SPDT

    I looked up a datasheet and couldn't find the switching type - though I may have missed it, my daughter woke me up early to play Minecraft so I am a bit hazy still.

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    Quote Originally Posted by d95err View Post
    The main thing to be careful about when switching speakers is to avoid intermittent load, which can cause voltage spikes and destroy the output transformer.

    Even if the unused amp is switched to a dummy load, the relays will bounce a bit when switching. If there is no other protection, this would be very risky.

    A simple protection is to put a resistor across the amp speaker out signal (e.g. 270 ohms/5W) that is always in the circuit. This is enough load to protect the amp, without affecting the tone.

    The safety resistor can be added to the amp (I use it on all my builds), or part of the switch box (on the speaker signal input from each amp, before the relay).

    I would also recommend adding a circuit to mute the input of the unused amp.
    Also I believe an 625MOV 200J Varistor will sufficently quelch any big spikes to the output transformer, and eliminate any worries, including a total speaker meltdown and open condition. I was told by another engineer at Digikey that transformers should be safe well above a 625v clamping voltage for short spikes, and above the clamping voltage there is an effective loop to ground with virtually no resistance, saving the transformer.

    Here's the TDK ones I use as per Rob Robinette's recommendation. I also called Mouser directly to see if the below was good for the intended applicaiton, and they seconded the selection based on typical amp voltages, and capacity for current absorption.

    https://www.mouser.com/ProductDetail...3Xfj0G66%2Fw==

    Easy to install between the center tap and each tube plate for a push pull amp, and gives piece of mind, not perfect but better.

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    Last edited by HaroldBrooks; 11-23-2019 at 01:21 PM.

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    I missed the first part of this one.

    You need to not leave the amps unloaded. Some, but not all, tube amps oscillate and destroy themselves with no speaker load. You can fix this issue by using a "safety resistor" on the amp itself and a dummy load resistor to eat the excess power is the unused amp is ever accidentally fed signal somehow. The safety resistor sustains the non-oscillation during any tiny not-connected periods while output switches are open.

    Unless you've done some fancy grounding and/or signal isolation work, you need to leave the amp grounds NOT connected, especially through the speaker "ground". Speaker ground may not be ground on all amps, although it is on most. The easier solution is to arrange the grounds not to be connected. This means you need to switch both signal and return on the speaker output. You'll still have the common signal ground problem to solve with the inputs, but that's a low current version of the same problem.

    Then you need relays. The classical solution is electromagnetic hard switches, which introduce the problem of bounce. The heretical new wave way is with MOSFET-output solid state relays. MOSFETs "saturate" to a resistance, and you can have sub-milliohm MOSFET resistances if you want to pay for them. What's probably going through your mind right now is "but wait - signal going through MOSFETs to speakers will ruin the tone".

    That's a solid maybe. Bipolar transistors are fancy variations of diodes. MOSFETs are modulated resistors. They go into pinchoff, but when "saturated" with a big gate voltage, they are just the channel resistance. There is some nonlinearity of the channel resistance, but it can only have an effect to the extent that the change in resistance is significant compared to the resistors/impedance it's connected to. To be audible, this is the speaker. With a channel resistance in milliohms, a variation in channel resistance in the tenths of milliohms (this is downright pessimistic - it's more like tens of micro-ohms), any MOSFET distortion will be 60-70db down, probably more. I've tried this. By actual testing, I can't hear the difference between hard-contact and MOSFET relays on speakers. Your ears may be better, but it might be worth a try.

    As for sequencing, relay timing and bounce for EM relays will always be a problem. MOSFET relays, less so. They don't bounce at all unless their control signal bounces. They switch in microseconds after their control signal does.

    One interesting approach to a pro setup is to use both MOSFET and EM relays in parallel. It will need some care in timing, but the MOSFET makes and un-makes on a sub-millisecond scale. Hard relays make and break in ten of milliseconds or more and commonly bounce for 20mS to 100mS. Do the controls with a "make" to the MOSFETs and EM contacts at the same time. The MOSFET makes, conducts audio, and then some time later the EM relay makes and bounces for a fraction of a second. The MOSFET "shorts" the open condition of the bounce, and the hard relay "shorts" the constant signal of the MOSFET, removing any possibility of MOSFET signal poisoning. For break, you open the hard relay, wait until its laggardly masses have moved, then drop the MOSFET control signal and it does a clean break.

    None of this is particularly susceptible to typical "um... use a diode and a ... ah... cap to delay the.. um" kind of circuits work, unfortunately.

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    Senior Member vintagekiki's Avatar
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    Late vk come to thread. Chuck H was faster than me.

    R = 8 Ohm / 100 - 200W depends on the power output from the amplifier. Resistor is desirable cooling by PC fan.
    The relay contacts must withstand the rated current from the audio amplifier.
    For 8 Ohm / 200W this is minimum 5 A. It is best contacts for 10 A, to prevent any possible spark when switching.
    The relay must be for strong mounting. (Not in socket, not in PCB)

    Voltage for the relay perform through an external 12 VDC adapter.
    To avoid transient occurrences when guitar switching, inputs amplifiers connect in series (from amp1 - to amp 2)
    Click image for larger version. 

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  30. #65
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by R.G. View Post
    I missed the first part of this one.

    You need to not leave the amps unloaded. Some, but not all, tube amps oscillate and destroy themselves with no speaker load. You can fix this issue by using a "safety resistor" on the amp itself and a dummy load resistor to eat the excess power is the unused amp is ever accidentally fed signal somehow. The safety resistor sustains the non-oscillation during any tiny not-connected periods while output switches are open.

    Unless you've done some fancy grounding and/or signal isolation work, you need to leave the amp grounds NOT connected, especially through the speaker "ground". Speaker ground may not be ground on all amps, although it is on most. The easier solution is to arrange the grounds not to be connected. This means you need to switch both signal and return on the speaker output. You'll still have the common signal ground problem to solve with the inputs, but that's a low current version of the same problem.

    Then you need relays. The classical solution is electromagnetic hard switches, which introduce the problem of bounce. The heretical new wave way is with MOSFET-output solid state relays. MOSFETs "saturate" to a resistance, and you can have sub-milliohm MOSFET resistances if you want to pay for them. What's probably going through your mind right now is "but wait - signal going through MOSFETs to speakers will ruin the tone".

    That's a solid maybe. Bipolar transistors are fancy variations of diodes. MOSFETs are modulated resistors. They go into pinchoff, but when "saturated" with a big gate voltage, they are just the channel resistance. There is some nonlinearity of the channel resistance, but it can only have an effect to the extent that the change in resistance is significant compared to the resistors/impedance it's connected to. To be audible, this is the speaker. With a channel resistance in milliohms, a variation in channel resistance in the tenths of milliohms (this is downright pessimistic - it's more like tens of micro-ohms), any MOSFET distortion will be 60-70db down, probably more. I've tried this. By actual testing, I can't hear the difference between hard-contact and MOSFET relays on speakers. Your ears may be better, but it might be worth a try.

    As for sequencing, relay timing and bounce for EM relays will always be a problem. MOSFET relays, less so. They don't bounce at all unless their control signal bounces. They switch in microseconds after their control signal does.

    One interesting approach to a pro setup is to use both MOSFET and EM relays in parallel. It will need some care in timing, but the MOSFET makes and un-makes on a sub-millisecond scale. Hard relays make and break in ten of milliseconds or more and commonly bounce for 20mS to 100mS. Do the controls with a "make" to the MOSFETs and EM contacts at the same time. The MOSFET makes, conducts audio, and then some time later the EM relay makes and bounces for a fraction of a second. The MOSFET "shorts" the open condition of the bounce, and the hard relay "shorts" the constant signal of the MOSFET, removing any possibility of MOSFET signal poisoning. For break, you open the hard relay, wait until its laggardly masses have moved, then drop the MOSFET control signal and it does a clean break.

    None of this is particularly susceptible to typical "um... use a diode and a ... ah... cap to delay the.. um" kind of circuits work, unfortunately.
    I've been thinking about these as well, and I like the idea of SSRs for all the benefits you mentioned.
    The downside is because they would be switching reactive loads, they would need to be protected from Back EMF when switching, or they could be destroyed by overvoltages. In addition to switching, transients could be present during normal operation when the output stage is overdriven (I'm not sure how sensitive SSRs might be in this case, I'm only going on material I'm reading) In addition, any short at the output terminals can damage it and cause it to fail.
    Not a deal breaker by any means, but it does add to the parts cost and complexity. There are plenty of protections schemes offered in applications notes, and I would think some of the examples for switching motor loads would be a good place get some ideas. That's probably where I would start, anyway.
    RG, in the examples you mentioned having experimented in the past, were you using SSRs switching speaker loads? or using mosfets for protection in line between the speaker and amp?

    ...Then again, I suppose I'm taking for granted that we would be adding some kind of clamp on the coil of a relay to protect the driver as well.

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  31. #66
    Bent Member Chuck H's Avatar
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    Ok... Only vaguely addressed so far is that there's some proximity considerations WRT handling the input switching and the output switching with the same device. I think it should be obvious that having the inputs and outputs managed in the same enclosure would come with instability risks for the same reason we manage layouts in amp chassis to keep both ends of the amp separated as much as practical. You'd probably be best off to have two enclosures, localized with their respective switching functions, and then the foot switch should only power the relays in each switcher. That said...

    It's getting more complicated and I can't help thinking that A/B switches are readily available and the circuit shown by myself and vintagekiki could be operated with a Carling snap switch. This would require pressing a whole two buttons though

    EDIT: Another consideration might be to use the snap switch to manage the output switching function AND trigger power to a relayed input switcher. That gets us down to one button again, keeps the two circuits remote, removes the stringency of high power relays and avoids any potential for the amplifier currents to hinky any relays (as noted by SoulFetish) if that's a real concern.

    I don't know if such a snap switch is available though. It would need to be a 3PDT foot button. Otherwise one might need to fabricate a toggle within a self supporting treadle. Still doable.

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    Last edited by Chuck H; 11-24-2019 at 01:41 PM.
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by SoulFetish View Post
    The downside is because they would be switching reactive loads, they would need to be protected from Back EMF when switching, or they could be destroyed by overvoltages. In addition to switching, transients could be present during normal operation when the output stage is overdriven (I'm not sure how sensitive SSRs might be in this case, I'm only going on material I'm reading) In addition, any short at the output terminals can damage it and cause it to fail.
    Not a deal breaker by any means, but it does add to the parts cost and complexity. There are plenty of protections schemes offered in applications notes, and I would think some of the examples for switching motor loads would be a good place get some ideas. That's probably where I would start, anyway.
    Anything that can switch an AC line motor load probably won't even get its hair mussed up with speaker loads from nominal tube amps. It ought to be fairly easy to protect this. You're right about looking at the schemes for SSRs and motors as a start.

    RG, in the examples you mentioned having experimented in the past, were you using SSRs switching speaker loads? or using mosfets for protection in line between the speaker and amp?

    ...Then again, I suppose I'm taking for granted that we would be adding some kind of clamp on the coil of a relay to protect the driver as well.
    I first tried it with signal voltages, and it worked so well that I tinkered with speaker loads. This was just my tinkering, and it's not by any means exhaustive - I didn't go hunt down a Marshall Major for testing or anything. But mostly it just worked, which is always both a good sign and cause for suspicion (Dang! it just worked! ... Wait a minute - that was toooo easy. What does that mean?)

    I used an off-the-shelf MOSFET output SSR rated for 240Vac. I wouldn't use the same device in a real setup as it was too expensive (~$15 each) but the principles are portable. If I were doing a similar design as a working unit, I'd probably pick my own MOSFETs and drivers. An AC switch with MOSFETs is just two MOSFETs connected source to source to get the body diodes opposed and some kind of floating gate drive to turn them both on at the same time. A dual LED to photovoltaic works fine, even though it slows the MOSFET switching down a lot.

    Quote Originally Posted by Chuck H View Post
    Ok... Only vaguely addressed so far is that there's some proximity considerations WRT handling the input switching and the output switching with the same device. I think it should be obvious that having the inputs and outputs managed in the same enclosure would come with instability risks for the same reason we manage layouts in amp chassis to keep both ends of the amp separated as much as practical. You'd probably be best off to have two enclosures, localized with their respective switching functions, and then the foot switch should only power the relays in each switcher. That said...

    It's getting more complicated and I can't help thinking that A/B switches are readily available and the circuit shown by myself and vintagekiki could be operated with a Carling snap switch. This would require pressing a whole two buttons though

    EDIT: Another consideration might be to use the snap switch to manage the output switching function AND trigger power to a relayed input switcher. That gets us down to one button again, keeps the two circuits remote, removes the stringency of high power relays and avoids any potential for the amplifier currents to hinky any relays (as noted by SoulFetish) if that's a real concern.

    I don't know if such a snap switch is available though. It would need to be a 3PDT foot button. Otherwise one might need to fabricate a toggle within a self supporting treadle. Still doable.
    Good thought. It would make sense if only for the cabling to make this all connect up well to have it be two units. An input unit to switch inputs, an output unit to switch outputs, and a signal cable to make them both switch at the same time.

    The solder-fumes-and-wires version is to run the coils of the relays in the two boxes in series. The modern signaling version is to run the same logic signal to both units. The hyper-fancy version is to put a microcontroller in each unit that understands MIDI.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

  33. #68
    Senior Member vintagekiki's Avatar
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    Good thought. It would make sense if only for the cabling to make this all connect up well to have it be two units. An input unit to switch inputs, an output unit to switch outputs, and a signal cable to make them both switch at the same time.
    The solder-fumes-and-wires version is to run the coils of the relays in the two boxes in series.
    No two boxes. Everything is in one box, located near the cab.
    In the box is only one relay and four female 6.3 mm jack.
    Switching the amplifier (amp1 / amp2) is by with ordinary foot sw. Voltage for relay is any 12 VDC cheap adapter.

    No switch inputs. Guitar connect to amplifier so inputs from amplifiers connect in series (from amp1 - to amp 2) or connect via guitar Y-cable.

    Everything is simple and efficient without processors and microcontrollers, and most importantly it works reliably.

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  34. #69
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by vintagekiki View Post
    No switch inputs. Guitar connect to amplifier so inputs from amplifiers connect in series (from amp1 - to amp 2)...
    ??? I'm having trouble picturing how to plug two amplifier inputs in series.

    Quote Originally Posted by vintagekiki View Post
    ...or connect via guitar Y-cable.
    Ick, for the added capacitance on the guitar signal. And I don't think it's best to have continuous amplifier operation when switching to a dummy load. We sure don't want two amplifier outputs connected together, so the switches have to be non shorting. There's a moment of no load. In that circumstance it would be best to minimize any possibility of drive at the amps input.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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

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  35. #70
    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by Chuck H View Post
    ??? I'm having trouble picturing how to plug two amplifier inputs in series.
    Correction
    I'm wrong expressed. Amplifiers are connected in parallel, not in series.

    Click image for larger version. 

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    Quote Originally Posted by Chuck H View Post
    Ick, for the added capacitance on the guitar signal. And I don't think it's best to have continuous amplifier operation when switching to a dummy load. We sure don't want two amplifier outputs connected together, so the switches have to be non shorting. There's a moment of no load. In that circumstance it would be best to minimize any possibility of drive at the amps input.
    Click image for larger version. 

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    The Y cable is very short, so not capacitively affected on to amplifier
    Ultimately, the guitar can be switched with a relay. Schematics follows soon.1)

    1)
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