Amp to Speaker Selector

jp_nyc

jp_nyc

Kick Henry Jackassowski
I don’t really want to buy another cabinet right now. I want to hook multiple amps up to the one I have. There are switches for this but many of them are complex beasts for people who want to switch between huge stables of guitars and amps and cabs which isn’t what I need. I just need a switch between multiple amps and one cabinet. KHE gear looks good but it’s more complex and expensive than I really need. I think that this selector by de Lisle would work, or this one. Help me do this without blowing up an amp!
 
Either of the de Lisle switchers should work with some caveats. You have to observe proper impedance matching between amp and speaker cab, or you can damage an amp. Also, you can’t simply turn on all your tube amps, ready to output to the cab, and use a passive switcher to switch between them, because that will damage an amp. You have to observe a specific procedure, doing things in the correct order, to keep from blowing up your amps.

(edit) Kudos to @Bob411 for helping me fix a few errors about impedance, solid state vs tube amps. :thu:

Tube amps need to have a speaker-type load attached or they get damaged. Therefore, with a passive switching box, amps have to be put in bypass, or off, before you change the switcher to a different amp.

If you get mixed up with the switcher, select the wrong tube amp, it’s pretty easy to blow up an amp. If you forget to switch a tube amp to bypass, or off, before using the selector to switch amps, you will likely blow up an amp.

You have to be careful about impedance matching between amp and cab. With solid state amps, you should not attach them to speaker cabs that have ohm ratings below the amp minimum ohm rating. With tube amps, you can damage them hooking them to cabs well below their minimum ohm rating, or by attaching them to cabs above their maximum ohm rating.

Tube amps are very sensitive to impedance mismatches. Matching exact 1:1 ohm rating (ex: 8 ohm output to 8 ohm cab) is best. Anything else could risk damage. However, in practical terms, most amps made by Marshall, Fender, Vox, Mesa, can handle 4 ohm, 8 ohm, or 16 ohm cabs.

I thought about getting a passive switcher to make it convenient to switch between different amps and different cabs. However, the convenience also makes it easier to blow up an amp by making a simple mistake. The active boxes, with protections built into them, are safer, but more expensive.

I opted for adding labels with cab ohm ratings to my wires. I have to turn amps to bypass, or off, manually unplug, and plug in wires, to change amps, and cabs, but it forces me to pay attention.

I have not blown up an amp since labeling the wires. Years ago, I blew the output transformer of an Epiphone Valve Junior with an impedance mismatch. Thank goodness I learned that lesson on that cheap POS amp.
 
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Solid state amps do not typically have output transformers, so they do not require a speaker load the way a tube amp does. It’s easy to blow up a tube amp if there is no speaker hooked to the amp. By contrast, you can turn on a SS amp, turn it up without any speaker cab attached, and it will be fine.

If you wanted to use a passive switcher and all the amps were typical SS without output transformers, you could leave them all powered up and switch between amps to your hearts content. No need to put the SS amps in bypass, or turn them off before changing the switcher.
 
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With a SS amp, you can go from 4 ohm output to 8 ohm or 16 ohm speaker cab and it’s safe. It may or may not sound great, but it’s safe.

With tube amps 1:1 ohm ratings is safe. Generally, with tube amp impedance mismatches, it is considered safer going higher to lower, 8 ohm to 4 ohm (2:1), than going lower to higher, 8 ohm to 16 ohm (2:1). That's opposite to SS where high side mismatches are completely safe (4 to 8, 8 to 16, 4 to 16). With a tube amp, going from a 4 ohm output to a 16 ohm cabinet is high risk of damage. 1 step (8 to 4 or 8 to 16) is safer than 2 steps (16 to 4 or 4 to 16).

Most tube amps are simple circuits. Impedance mismatches can blow them fast. But, the amp will often start to sputter and choke out, almost like a gated fuzz, before something blows up. So, if you act quickly to shut it down, you can save the amp.
 
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Detailed information about tube amp impedance mismatches….

“With tube amps there are two different kinds of risk

If your amp’s output transformer is designed for an 8 Ω load:
  • 8 Ω tap → 4 Ω speaker = low impedance mismatch (2:1 mismatch)
  • 8 Ω tap → 16 Ω speaker = high impedance mismatch (2:1 mismatch)
The mismatch ratio is the same, but the stress on the amp is different.

8 Ω output into 4 Ω speaker (too low a load)

The tubes must deliver more current than intended. This can:
  • Heat the output tubes more.
  • Stress the output transformer.
  • Increase power supply strain.
8 Ω output into 16 Ω speaker (too high a load)

The tubes deliver less current, but the output transformer can develop higher flyback voltages when the speaker’s impedance causes energy to reflect back into the transformer. This can:
  • Stress the output transformer insulation.
  • Cause arcing at tube sockets or tube elements.
  • Damage transformers in severe cases.
Historically, transformer damage from excessive voltage spikes is why many amp techs consider a high-side mismatch (too many ohms) potentially more dangerous than a low-side mismatch.”
 
I edited my previous posts to remove misinformation. I got some things wrong and I don’t want anyone messing up their amps on my account.
 
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