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Old 24th October 2007, 09:54 PM   #1
Sherman is offline Sherman  United States
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Question One guitar, multiple amps at once?

Not strictly a tube question but since I respect the depth of knowledge here I thought this would be the place to ask.

My son has asked me if I can build a cabinet for him that would house 2 12" speakers and hold four of his small guitar amps (combos). I will modify them to add external speaker jacks (actually I modified one a few years ago so he could drive a 4x12).

He would ideally like a single 1/4" jack on the front that would feed all the amps simultaneously. All four amps would then be connected to the 2 speakers which would be wired in series.

I think a series of push button switches would be nice so he could select any or all of the amps.

So...
1- Would driving four amps simultaneously from a guitar require a preamp? (I assume it would but you know about assumptions.)

2- What issues (if any) would I run into driving the speakers from multiple amps simultaneously?

Though I've built a couple guitar amps I've never thought about this scenario.
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Old 24th October 2007, 10:14 PM   #2
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Set it up with a big multipole switch or relays so that, when input A is selected, B, C and D all have their inputs shorted, and A output goes to the speaker(s) and B,C and D go to a power resistor each. No power should be dissipated as their inputs are shorted, but it's a worthwhile safety precaution for the output transformers.

The only real downside on the input is a bit of extra capacitance from the switching. Probably not significant with good layout.
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Old 24th October 2007, 10:35 PM   #3
KSTR is offline KSTR  Germany
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You will have some problems with a setup like this. One cannot directly connect multiple amp's outputs to one speaker simultaneously, you can only select one at a time, and the inputs of the others must be muted also (it's not recommended to run tube amps without load and signal applied -- output tubes and/or output xformer may be damaged by arcing). Passive summing of the outputs with resistor dividers would be possible at cost of loosing output level and sonic quality also. An actual circuit for this will sure require extra care and thought.

Paralleling amp inputs also has its share of problems, if the grounds of the amps are hardwired to safety ground (ususal with guitar amps) one often ends up with hum when there is a second ground connection via the paralleled inputs. This can be handled with an active driver circuit with individual ground compensated or balanced servo/xformer outputs which can decouple the grounds.

When both outputs and inputs are switched, using multiple amps one at at time with one guitar and one cabinet can be done without effort. The best way to do this would be an arrangement that shorts the inputs of the unused amps to the individual gnd, while both input "hot" and ground are routed only to the selected amp. Only the output of the selected amp connects to the speaker, but again both ends might have to be switched (if it works with output gnds connected must be checked. It may, or it may not). A rotary switch for this looks to become quite complex, maybe it's easier to drive some relays with a decoder matrix fed from a simple rotary switch, or a mixed approach.

- Klaus
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Old 25th October 2007, 01:52 AM   #4
Sherman is offline Sherman  United States
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Brett and Klaus,

Thanks so much for the info and ideas!

Quote:
Originally posted by Brett
... so that, when input A is selected, B, C and D all have their inputs shorted, and A output goes to the speaker(s) and B,C and D go to a power resistor each. ...

Quote:
Originally posted by KSTR
...When both outputs and inputs are switched, using multiple amps one at at time with one guitar and one cabinet can be done without effort...
- Klaus
The consensus seems to be the easiest route would be to switch both the input and the output such that only one amp at a time is receiving the guitar's input and that amp is the only one feeding the speakers. That should be easy enough to do with a two pole, 6 position rotary switch (which I have in the junk box).

For a single guitar feeding multiple amps I guess I need something like an A/B/Y switch except A/B/C/D switch!

I found a schematic for a "hum-free" A/B/Y switch but nothing for more than 2 outputs. I know that Morley makes a three-way switch but find no info for DIY three-ways.

As for feeding the speakers from all the amps, I think he can live without that, using the speakers in each combo and just switching one at a time to the dual 12 inchers.
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Old 25th October 2007, 04:05 AM   #5
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Setting the way back mackine to the 1960's I find myself playing in a surf music band. We used a common method called daisy chaining. You plug your guitar into amp number 1. You plug a guitar cord into the second input of amp number 1, and plug the other end of this cord into amp number 2. I have seen this method used with as many as 4 amps, although the last amp in the chain gets a slightly reduced signal.

One of the guitar players in this band had well to do parents, and he did own 4 amps, while I was playing my guitar through an old record player. He didn't seem to care if anything blew up so many very strange experiments were tried including plugging the speaker output of one amp into the guitar input of another amp (don't try this). This resulted in some extreme distortion. One of the more unique connections included daisy chaining two amps then feeding the daisy chained cord from amp number two through a fuzz box or wah - wah back into the second channel of amp number 1. I remember amp number 1 being a black face Bandmaster, but I don't remember what amp number 2 was. We used this setup until the band dissolved.

Now 40 years later I would just build a simple low gain preamp to drive all of the amps. You want something like a line stage.
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Old 25th October 2007, 02:55 PM   #6
Sherman is offline Sherman  United States
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Quote:
Originally posted by tubelab.com
Setting the way back mackine to the 1960's I find myself playing in a surf music band. We used a common method called daisy chaining. You plug your guitar into amp number 1. You plug a guitar cord into the second input of amp number 1, and plug the other end of this cord into amp number 2. I have seen this method used with as many as 4 amps...

Now 40 years later I would just build a simple low gain preamp to drive all of the amps. You want something like a line stage.
I like the daisy chain idea except that not all the amps he wants in this box have two inputs.

A line stage does seem to be the long-term answer and should be relatively easy to implement. I'm envisioning a clear panel at the top of this (massive) cabinet behind which the glow of tubes in the line stage can be seen.

It should be an interesting project to say the least.
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Old 26th November 2007, 11:45 PM   #7
Sherman is offline Sherman  United States
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Thumbs up UPDATE

Just thought I'd post an update for this thread.

We ended up wanting to drive 6 amps from one guitar and we found an easy (though not inexpensive) solution. My son already had an A/B switch to run his guitar to either or both of two amps. It turns out that Morley (the A/B switch maker) makes a product called "The Tripler". It takes a single input and can drive any combination of three amps. Its circuit requires 9V and provides adjustable gain so the signal can actually drive three amps.

Well, connect guitar to effects pedal, connect pedal to A/B switch, connect each output to a Tripler and then to the six amps. Any combination of amps can be selected with foot switches and the sounds available are pretty amazing. The downside is that the Triplers cost about $75 USD each.

Now to build the cabinet to house the six amps!
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