Switching 2 amps into 1 cab

[Gregg]

Hi,

I'm sure this has been asked before but I couldn't find anything.
I need to footswitch between two 50 Watt amps hooked to one cab. The non active input will be connected to ground. For the outputs is it enough to just get a relay that will handle the current? What other safety measures should be considered?
Any help will be appreciated.

[bob p]

I guess that one of the most important questiosn to ask is whether or not your application requires switching while playing. That complicates things a bit.

What I would recommend is switching the amp back and forth between the speaker load and a suitable power resistor. The power resistor will reflect an appropriate impedance back through the output transformer to the power tubes when the amp is not in use. I think that to do this right, you might need a pair of DPDT relays.

You are talking about a tube amp, right?

[Gregg]

Yes, I'm talking about switching between two tube amps while playing.
I pretty much have the idea how to build it but it seemed too easy so I thought it would be better to check with the more experienced of you guys.
Has anyone actually built anything similar?

[bob p]

No, I haven't acutally built one. If I had to do this sort of switching, I would prefer to do it with the amp on standby. Maybe someone else could offer some help about how to do it "live".

[Mark Black]

These guys make exactly what you are talking about:

http://www.tonebone.com/tb-headbone-vt.htm

Apparently they thought it important to include amp input signal switching as part of the process - the description yields some good clues for a possible DIY version.

FWIW I've heard nothing but good about everything Radial makes...

[hasserl]

I wouldn't be switching speaker loads on any tube amp that is running under a load, I don't care who makes the damn switching device. That is just asking for trouble with arcing of tubes and/or sockets and piercing of output trans winding insulation.

[Gregg]

OK, maybe I wasn't clear enough.
I'm talking about something like this one:

http://www.randallamplifiers.com/pro...ts/mtsrack.asp

My idea is to run single input and switch simultaneously both amps' inputs and outputs /single cabinet/. The non active amp input will be switched to ground /relays/. The non active amp output will be switched to a dummy load - for example a 10W power resistor. Anyone of the amps will be off load /and with muted input/ only for miliseconds.

[KevinOConnor]

Hi Guys!

It is entirely safe to switch the outputs of two or more tube amps into one cabinet PROVIDED you also switch the inputs simultaneously.

When you remove the input signal, there is no risk to the output stage of disconnecting the load. You can connect/disconnect the load all you want as long as there is no signal going through the amp.

We built many units to do this very thing and the description of that piece gave the Tonebone guys their information. Our new amps include part of the amp switching system, with the external accessory unit handling just control voltages - no audio. Exclusive selection of up to four amps is possible with our unit, but one could easily expand such a system for any number.

Have fun
Kevin O'Connor

[Gregg]

Actually it's already done and working. I used a PIC12F629 creating the following sequence:
1/ When the switch is pressed the input of channel 1 /active/ connects to ground /meanwhile Channel 2 input is connected to ground/
2/ While channel 1 input is connected to ground a conservatively rated industrial grade relay switches the amps outputs.
3/ Channel 2 input is lifted from ground /now active/.

All this happens very fast - the latency is 30 ms and not audible /except if you're looking for it/. Everything works OK without clicks, pops or any other noises. This is how the finished amp looks like: