speaker patch bay question

[Sensei Tim]

Question for you guys:

I am lazy. i hate having to go in behind my amp heads, find the speaker cable, unplug it, and then go to amp head #2, find the speaker out jack, and then mash around back there blindly trying to plug the cable in.

i want to make some kind of patch bay.  The left side will have 3 inputs, which are the speaker outputs from my amps.  the left side will have 3 outputs which will go to 3 different speaker cabs.

I would then want to have two rotary switches - one to select which input is active and a second that will select which cabinet is active.

(yes, i know all about running tube amps with no load and the dangers it poses. i only run one amp at a time and won't be switching anything while any single amp is on.)

ok. so the question is... most of these amps are 100watt, though i rarely ever run them past 2 or 3 (thank you, two notes captor!)... what kind of rotary switch should i use that can take that kind of signal?  am i good with the typical alpha-style 4PDT rotary switches that we use with pedals or will i need something beefier?  


T

[reddesert]

Let's suppose your peak signal is 100 W into an 8 ohm speaker.  Power is P = V^2 / R = I^2 * R.

Solving for V and I, we find that V = 28.3 volts and I = 3.5 amps.  (This is consistent with Ohm's law: I = V/R.) This gives you an idea of what we are up against, since the output voltage isn't incredibly high, but the output current is really large.

For example, a normal Alpha rotary switch like this: https://www.mouser.com/search/refine.aspx?Ntk=P_MarCom&Ntt=146305454 is rated to 125 VAC and 0.3 amps. So it would handle the voltage just fine but the current is way over the switch rating.

In fact, the power for 0.3 amps into an 8 ohm speaker is just 0.7 Watts! Realistically the switch rating is probably conservative, and amps are rarely run at maximum power, so you might not fry the switch with a 1 watt amp, but you could probably fry it with a 100 W amp even if it was only turned up to 2 or 3.

You could look for a rotary switch that's rated at 3-4 amps or better. Mouser seems to have some that only cost several bucks each (and some that are really expensive). I would buy a non-shorting, break-before-make switch for the amp outputs, so that you never connect an output to another output.

Another simple solution could be a regular patch bay: run the inputs / outputs to a box that has a series of 1/4" jacks on the front, and to use a short section of speaker cable with 1/4" plugs to manually patch the desired amp to speaker.  You have to be careful to not patch an output to an output, of course.

[balazs.bencs]

I don't know where you are located, but if you are in Europe, here you go: https://www.tube-town.net/ttstore/Switch/Rotary-Switch/Rotary-Switch-1-pol-3-pos-6-A-Impedance-Voltage-Selector::4080.html

or for example Tubedepot in the US.. https://www.tubedepot.com/products/three-position-rotary-switch

This rotary switch is typically used in amps as an impedance selector switch, so it's exactly what you need. These are very robust and hard to switch (won't move accidentally). I used them in two of my amps that I built and I'm very happy with them, these are monster switches. I know this switch is not cheap, but once you get one, you'll understand why :)