I want to build an amp selector (two amps a/b, one cab)

[d95err]

Sorry, Fellas - I am not experienced enough to be comfortable with "experienced amp guy vernacular" - of course I say that in a nice way - so when you say "to put a resistor across the amp speaker out signal" can I take that to mean from the output jack's tip to ground? And is it enough to do it once for parallel outputs or should I do that per jack?

Thanks!!!!

Correct, the safety resistor goes between the tip and ground of the speaker jack.

If you have multiple parallell jacks then one resistor is enough. If you have different jacks for different impedances (e.g. 4, 8, 16 ohms) though... I must confess I’m not sure if it is sufficient to hav one resistor for one of the taps (e.g. 8 ohms), or if you need separate resistors for each tap.

If you have an impedance selector switch, you can have one resistor at the jack.

[Helmholtz]

One resistor (base load) permanently connected to one of the taps is sufficient. Minimum power rating depends on resistance, amp power and tap/impedance. E.g. for a 100W amp a resistor of 270 Ohm connected to the 8 Ohm tap should be rated at least at 3W (I'd use a 5W resistor to take care of increased output with clipping).

[Gtr0]

Thanks very much!

I'm splitting time between a few amps atm and when I find some time I will seek out the proper switches etc to realize this amp switcher. I could have REALLY used it today, two identical amps with differing values in the EQ, by the time I get from one to the other with swapping speaker cables, power and warm up etc, I have lost most of it.

Much appreciated!

[g1]

I never heard it before. NC and NO are my letters for relays. MAke before break switch contacts are called "shorting" contacts.

In my work in amusements we had pinballs FULL of relays, as well as jukebox control circuits. We used the Form A, B, or C terminology for make or break. A form C switch could be shorting or non-shorting.

Apparently Form D is 'make before break' aka 'continuity transfer'. Form C is 'break before make', Form D is the same assignment as Form C but is 'make before break'.
So during the switchover, all terminals are connected on form D. On Form C, during switchover, all terminals are briefly disconnected.

[Gtr0]

For switching input signals, no problem. They are the ones that Engl uses, for example.

But for speaker switching you need something stronger since it´s a critical function. As a reference, Mesa uses in the Roadking (this amp has automated switching to different speaker outputs) a set of Nec MR301-12HSL. They are relays that can handle 10A. You would need two to complement the switching in that part. At least with that exact model.

This is the datasheet (Page 8):

https://www.digchip.com/datasheets/p...-12HSL-pdf.php

Apparently Form D is 'make before break' aka 'continuity transfer'. Form C is 'break before make', Form D is the same assignment as Form C but is 'make before break'.
So during the switchover, all terminals are connected on form D. On Form C, during switchover, all terminals are briefly disconnected.

According to the datasheet that Pedro linked, this relay is a form c. And so I gather that the switch I choose MUST be of form D, make before break, correct? It makes sense as such.

Thanks.

[d95err]

According to the datasheet that Pedro linked, this relay is a form c. And so I gather that the switch I choose MUST be of form D, make before break, correct? It makes sense as such.

Thanks.

If you use safety resistors, both types are OK.

Make sure to have the safety resistor on the switchbox though, not just amps. Sooner or later someone will plug in an amp without safety resistors...

[mhuss]

Sorry, Fellas - I am not experienced enough to be comfortable with "experienced amp guy vernacular" - of course I say that in a nice way - so when you say "to put a resistor across the amp speaker out signal" can I take that to mean from the output jack's tip to ground? And is it enough to do it once for parallel outputs or should I do that per jack?

Thanks!!!!

One resistor for each external amplifier connection.

[Old Tele man]

Yep, "...non-inductive, "swamping" resistor in parallel with each amp's OTs winding input to switching circuit..."

as I stated in my earlier post: https://music-electronics-forum.com/...l=1#post540620

[Helmholtz]

...non-inductive, "swamping" resistor

No need for a "non-inductive" resistor. The inductance of any wire-wound resistor is absolutely negligible compared to the inductance of a speaker (as well as the secondary leakage inductance of the OT which acts in series with the load and thus adds to any load inductance).

[soundmasterg]

Ted Weber has made one of these for years.

https://www.tedweber.com/two-head-fs

I've got a three head version that Ted made right before he passed. They discontinued that, but borrowed it from me to possibly reissue it. They ended up making this two head one instead which I'm sure works well.

Greg

[d95err]

There are several commercial options available. Another is the Radial Tonebone: https://www.radialeng.com/product/headbone-vt

[Helmholtz]

Ted Weber has made one of these for years.

https://www.tedweber.com/two-head-fs

I've got a three head version that Ted made right before he passed. They discontinued that, but borrowed it from me to possibly reissue it. They ended up making this two head one instead which I'm sure works well.

Greg

But:

"The Two-Head Amp Switcher DOES NOT switch your guitar input from amp to amp. This switcher is designed for players who are already using two different amplifiers, but wish to simplify to one cabinet."

So you need to switch inputs and outputs separately.

[Chuck H]

So you need to switch inputs and outputs separately.

That's crazy talk! Hit two buttons!?! Now we're back in the stone age

Just kidding, of course. This was my suggestion much earlier.

[nickb]

More than you want or need but out of interest...

I was approached by a guy with too many amps and cabs to come up with a switch for 16 amps to 8 cabs. The switching matrix used 10A DPDT relays on four identical boards used of which provided an 8x4 matrix. Both speakers wires were switched to allow for solid state amps with bridged outputs

It was controlled using a rotary encoder and an LCD display. You edit the connections and it gets saved in non-volatile memory. The relays are controlled over an I2C bus.

Here is one 8x4 board and the controller during testing:


Building it into the box:


I had intended to use dual stacked jacks on the rear of the boards to save a lot of wiring but there was a footprint problem so I had to hard wire them. I still have a couple of PCBs if anyone wants one.

The glamour shot:

[soundmasterg]

But:

"The Two-Head Amp Switcher DOES NOT switch your guitar input from amp to amp. This switcher is designed for players who are already using two different amplifiers, but wish to simplify to one cabinet."

So you need to switch inputs and outputs separately.

That is completely correct. Its not ideal, but to make one that switches inputs and outputs at the same time is a lot more complicated. The three head version I have I got for a specific purpose. I modified a Conn organ chassis years ago that had three separate output sections in it that shared a power transformer. So three phase inverters, three sets of power tubes, and three output transformers. I added a preamp chassis so I basically have three amps in one head, but I needed a way to switch between cabinets or I would always have to carry three cabinets around. Hence Weber's switcher.

Greg