Brain Teaser: 2 Amps, 2 Impedences, 1 Cab, 1 Switch...?

[new2twang]

I have two amps I really like. Amp 1 is a Fender Stage Lead II 2-12, and amp 2 is a Jet City JCA20H. Amp 1 has sparkling Fender-ific cleans with lush, deep reverb, and Amp 2 has gritty, grindy, Soldano-tastic dirt.

So I gets me this idear... What if I could somehow wire it all up so that both amps use the same cab? Ideally I hit a foot switch, and not only A/B the amps, but send the correct amp signal to the Fender's twin 8 ohm speakers with the correct impedence (either 4 ohms for the Fender or 16 for the Jet City), without any loud humming, wisps of smoke, 911 calls, etc...

Can it be done?
Would it be expensive and/or dangerous?
Am I nuts?
All the Above?

[Rich_S]

Yes, you can do it. All you need is a 5PDT footswitch.

6PDT if you want LEDs.

[printer2]

From what I found the Fender speaker is 8 ohms and the Jet City is happy with 8 ohms.

[new2twang]

Thanks guys!

Rich_S: Hmm... That looks to be a rare and expensive animal. Better off with a relay, perhaps?

printer2: I believe I was mistaken . The Fender has two 8 ohm speakers, but looking at the wiring diagram, which is supposedly for both the 1-12 and 2-12, it says "8 ohm min." It's a few miles away at the moment, but I'm guessing that it's actually wired in series for 16ohm, which simplifies the impedance issue. Assuming that's the case, would it be OK to have both amps running to the same two speakers at the same time? I mean, if I just were to A/B switch the input, they're still both hot to the speakers, right? Would that be like crossing the beams in Ghostbusters? Or at least break my wisp of smoke or loud buzzing rule?

[printer2]

It is a little more tricky than just a simple switch. The tube amp needs a load on the output all the time, you do not want it to go open circuit. What you can do is put a higher value resistor (say 100 ohm 2W) across the amp output permanently and then it would be safe to switch between the two amps as long as you are not sending a signal through them.

You can not have both amps power the speakers at the same time. Both amps have a low output impedance and when hooked up across each other they can look like a short to the other. A little more complicated than that as they also put out a voltage but just be content with the idea that you can not do that.

[bwacke]

new2twang,

I'd be careful switching two amps too simply. You are correct in wanting a relay to switch low voltage, high current reliably, but you don't want either amp's output transformer to be without a load for even a millisecond because the inductive voltage spike generated by opening the secondary will be destructive to the tube(s), eventually. You want a 100 ohm 10 watt resistor across each amp output at all times and a double pole double throw relay to switch the respective amp outputs to the speaker. The 100 ohm resistors will affect the tone somewhat and could be switched out a few milliseconds after the speaker is connected with another relay, but now you're getting complicated. The cost for a good circuit might exceed another speaker system if your know-how is limited.

Two amps feeding the same speaker simultaneously creates impedance and phasing issues the are also complex to deal with and require some intricate understanding of the electrical principals involved. One amp would be driving the transformer of the other, which would be acting as a power-sucking load at best and a short circuit at worst. It would also affect the tone of the whole system. I can't say if it would be better or worse (my guess...not so good).

Bob

[bwacke]

Ooops, printer2 beat me to it while I was on the phone...oh well, consensus reality says be careful.

[new2twang]

Yeah, that makes sense. I thought the idea of sharing the circuit sounded not quite right, but completely forgot about needing a load. Having to switch to and/or from the dummy load definitely makes it an unworthy project.

Guess I'll schlep the little 112 around for the JCA, and maybe spend the time/money on a good A/B/Y rather than the altoids A/B I was planning. Who knows, they might sound great together!

Thanks for sharing the wisdom.