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

[Gtr0]

On second thought, it would probably be better to make it a box that sits on top of the amp with a footswitch, as drawn. At first I was thinking all in one floor item, but then there's lots of cable run that way.

[Pedro Vecino]

The idea is a unit located above the amplifiers (like any attenuator) and make the change remotely with a footswitch.
Grounding of the output on the inactive amplifier works as long as the amp input is also muted (indispensable condition)
.
The most delicate part in my opinion is the choice of the power relay. Once the appropriate model was selected, another one connected in parallel could be used for security purposes.

[Gtr0]

The idea is a unit located above the amplifiers (like any attenuator) and make the change remotely with a footswitch.
Grounding of the output on the inactive amplifier works as long as the amp input is also muted (indispensable condition)
.
The most delicate part in my opinion is the choice of the power relay. Once the appropriate model was selected, another one connected in parallel could be used for security purposes.

I had Finder brand relays in mind, like this --> https://www.tube-town.net/ttstore/fi...5v-2a-12v.html

I assume a pair of those would suffice?

Thanks!

[Pedro Vecino]

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

[glebert]

OK, this is just my opinion, but this seems like a lot of work to hear the difference between two almost identical amps, which you have said doesn't seem to show any difference when recorded.

[uneumann]

OK, this is just my opinion, but this seems like a lot of work to hear the difference between two almost identical amps, which you have said doesn't seem to show any difference when recorded.

Ditto - although as an engineering exercise for switching heavy power/currents, the problem/solution has some value. I can't argue with the motivation either, but I wonder if given the part tolerances of 5% and 10% - or even 20% on pots and such, never mind transformers - how realistic is it that any tone difference could be attributed to anything specific in the implementation or circuit.

Personally, I've chased after some particular tone goal and found cases where relatively minor changes in part values can make significant differences. In other cases, it's hard to hear any change with part variations of +/-50%. How would you know what's really causing the tone difference, if/when you do indeed hear one?

[Gtr0]

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

Noted, and thanks! I figure later that the finder didn't have enough to take the current. I will look into your recommendation.

On a side note - What happens in the even of a catastrophic error where one of the set of inputs/outputs were reversed? So that the amp that is supposed to be going to ground on it's output is the one also receiving signal? I have to ask :-)

OK, this is just my opinion, but this seems like a lot of work to hear the difference between two almost identical amps, which you have said doesn't seem to show any difference when recorded.

Ditto - although as an engineering exercise for switching heavy power/currents, the problem/solution has some value. I can't argue with the motivation either, but I wonder if given the part tolerances of 5% and 10% - or even 20% on pots and such, never mind transformers - how realistic is it that any tone difference could be attributed to anything specific in the implementation or circuit.

Personally, I've chased after some particular tone goal and found cases where relatively minor changes in part values can make significant differences. In other cases, it's hard to hear any change with part variations of +/-50%. How would you know what's really causing the tone difference, if/when you do indeed hear one?

First, I want to do it because I can and it's another experience with hashing out and building a useful item.

But, my usage in this type of device would be for the differences that I do hear in the room, not really what is recorded. This item seems to be unnecessary by listening to playback of two amps recorded... my point was that there are lots of things *I think* I hear in the room that do not come across in recording.... or at least seem more significant in the room than on ... disk (I almost said "tap"e - shows my age ;-) )

If I start with two amps that are in the ballpark (perhaps even using the same brand of everything)... then I will see the profound difference of swapping Xicon polypropylene caps with orange drop 716P (as an example) - and listen to it in real time in the room... not just on a recording.

As an added bonus, occasionally I may want to replicate an amp, say an old Marshall. I can catch subtleties this way rather than relying on a recording that are minutes apart. Again, in the room.

[Chuck H]

Noble efforts in my opinion. But don't complicate peripheral tools to the detriment of achieving the goal. Do you want to make the most ideal switcher, or just hear the difference between designs so you can then make the most ideal amp!?! I think Pedro has one element correct. The switching of the input is critical if only because of the additional load on the guitar when plugged into two amps. So...

Use the very simple amp switching design I provided above, which is a safe and the quickest way to the goal, and then use an A/B pedal (readily available) for the amp inputs. Yes, it's two whole buttons to press!!! Big deal. I don't think the amount of time it takes to press two buttons instead of one is a deal breaker for this. At least it shouldn't be. Like I said, keep the real goal in mind. You want to get to the best amp design. NOT the ideal switcher. Putting time into developing the ideal switcher, which is eminently more than is needed for the project only takes time away from getting to the design of the ideal amp. JM2C on that.

And yes, to use the above schematic you'll need to make sure your speaker cables are wired correctly. While your at it you will also want to make sure your AC plugs are wired correctly, your speakers are in correct polarity and your amps bias is adjusted, etc. So really, what the hell sort of a nit pick is that? Commercially wired, and therefor 99.9999% likely to be correct speaker cables can be purchased. If you're wiring your own then you have only yourself to be accountable to.

Find the quickest way to your end goal and don't muddle in the process steps where it isn't necessary. Unless the process, rather than results that achieve is the original goal is the point. Which I guess it may be for some. It's all choices.

[Gtr0]

Noble efforts in my opinion. But don't complicate peripheral tools to the detriment of achieving the goal. Do you want to make the most ideal switcher, or just hear the difference between designs so you can then make the most ideal amp!?! I think Pedro has one element correct. The switching of the input is critical if only because of the additional load on the guitar when plugged into two amps. So...

Use the very simple amp switching design I provided above, which is a safe and the quickest way to the goal, and then use an A/B pedal (readily available) for the amp inputs. Yes, it's two whole buttons to press!!! Big deal. I don't think the amount of time it takes to press two buttons instead of one is a deal breaker for this. At least it shouldn't be. Like I said, keep the real goal in mind. You want to get to the best amp design. NOT the ideal switcher. Putting time into developing the ideal switcher, which is eminently more than is needed for the project only takes time away from getting to the design of the ideal amp. JM2C on that.

And yes, to use the above schematic you'll need to make sure your speaker cables are wired correctly. While your at it you will also want to make sure your AC plugs are wired correctly, your speakers are in correct polarity and your amps bias is adjusted, etc. So really, what the hell sort of a nit pick is that? Commercially wired, and therefor 99.9999% likely to be correct speaker cables can be purchased. If you're wiring your own then you have only yourself to be accountable to.

Find the quickest way to your end goal and don't muddle in the process steps where it isn't necessary. Unless the process, rather than results that achieve is the original goal is the point. Which I guess it may be for some. It's all choices.

Well said.

Yes, you'd be correct - I am, at this moment, more interested in the best amp design. Two switches is not an issue. However I wouldn't mind having the more "involved" plot for later use for when I do have some free time, which occasionally does happen.

My first go at finding the appropriate relay was not fruitful... though I haven't spent long so far. My concern is that I haven't seen a DPDT with these parameters. More intense search later this afternoon.

Thanks!!

[Enzo]

DPDT relays are not hard to find.. Mouser has EXTENSIVE options. I looked at Jameco, because they are often a handy source for the hobbyist. Here is a selector guide of their line:
https://www.jameco.com/Jameco/catalogs/c181/P72.pdf

[Helmholtz]

On a side note - What happens in the even of a catastrophic error where one of the set of inputs/outputs were reversed? So that the amp that is supposed to be going to ground on it's output is the one also receiving signal? I have to ask :-)

Feeding the amp which has its output shorted must be avoided. It would cause redplating and very short tube life.
In case of doubt it would be better to switch the outputs to dummy resistors instead of grounding them.

[loudthud]

My reasons for doing something like this are slightly different. I want to set up an experiment to see if people can tell the difference between two amps, one tube and one solid state. The problem I see is making adjustments to each amp without cluing the test subject as to which is which.

[Helmholtz]

How would you know what's really causing the tone difference, if/when you do indeed hear one?

I think the procedure will be changing components/circuits based on educated guess or simply trial and error until the tone difference between the amp under test and the reference amp is minimized.
(Care should be taken to not compare amp tones of different sound levels.)

[d95err]

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.

[Helmholtz]

^^^Good point!