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

1. Two boxes may make more sense than one. It depends entirely on what the person can deal with in terms of running cables and not getting speaker cables and coaxial shielded cables mixed up. It is possible to get into difficult situations routing high impedance input cables through a box that contains speaker cables as well.

Switching with microcontrollers can be just as reliable as hard switches - if you know what you're doing.

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2. Originally Posted by vintagekiki
Correction
I'm wrong expressed. Amplifiers are connected in parallel, not in series.
[...]
The Y cable is very short, so not capacitively affected on to amplifier
Ultimately, the guitar can be switched with a relay.
This is the issue I was referring to. If you use a Y cable to connect the inputs of two amps in parallel, you guarantee that any issues with ground leakages in the two amps will cause hum problems if the grounding isn't immaculate on both amps. On setups with two-wire amps (no third-wire safety ground) hum is almost guaranteed.

Third wire grounding of both amps is really, really helpful.

Y cables on inputs is its own separate problem, which complicates amp and cab switching as well.

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3. The Y cable is very short, so not capacitively affected on to amplifier
It is often overlooked that amplifiers not only have input resistance but also input capacitance. A typical value for tube amp inputs is around 150pF.
So paralleling amp inputs will not only load the guitar by the parallel input resistances but also by the adding input capacitances.

Being an experimental physicist I am always keen to verify theory by experiment. So to see the influence of the Miller effect, I measured the input capacitance of my 80s Super Champ with the amp turned off and on. Results: Coff = 30pF, Con = 130pF. The difference of 100pF can be attributed to the Miller effect.

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4. Originally Posted by Helmholtz
It is often overlooked that amplifiers not only have input resistance but also input capacitance. A typical value for tube amp inputs is around 150pF.
So paralleling amp inputs will not only load the guitar by the parallel input resistances but also by the adding input capacitances.

Being an experimental physicist I am always keen to verify theory by experiment. So to see the influence of the Miller effect, I measured the input capacitance of my 80s Super Champ with the amp turned off and on. Results: Coff = 30pF, Con = 130pF. The difference of 100pF can be attributed to the Miller effect.
So really it's a small capacitance that would be easily countered by using shorter cables or some fancy low capacitance cables. The Y connector has some capacitance of it's own too. If the player typically uses a 10' cord then it's going to be tricky. But if the player typically uses a standard 20' cord then three 10' \$\$\$ low capacitance cables would probably be alright.

And if it's YOU then there's no trouble at all because you use something like a 2000pf cable, right?

Thanks for the real world investigation on the matter

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5. And if it's YOU then there's no trouble at all because you use something like a 2000pf cable, right?
No, for me the optimal cable capacitance with my vintage type PUs is 1000pF +/- 10% . Not mainstream, I know.

My point was that paralleling amp inputs will add extra capacitance, like 200pF/amp including short extra cable. So 4 amps in parallel would add some 600pF and reduce effective input resistance to 250k.
But with a buffered pedal this won't matter.

Paralleling inputs may cause ground loop problems - even more so if additionally the amp output grounds are connected.

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6. Originally Posted by Helmholtz
... and reduce effective input resistance to 250k.
Right. I knew there was something I was forgetting to mention. And this CANNOT be worked out with low capacitance cables.

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7. That Leo Fender and Jim Marshall had used computer emulation, there wouldn't be Rock & Roll, nor Jimi Hendrix.

Twisted question
How created the first amp for an electric guitar?

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8. "How created the first amp for an electric guitar? "

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9. Copied the RCA Receiver manual?
Or maybe an AT&T/Western Electric circuit?

Would love to see the original patent.

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10. Originally Posted by Jazz P Bass
"How created the first amp for an electric guitar? "
No

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11. Originally Posted by Helmholtz
Or maybe an AT&T/Western Electric circuit?

Would love to see the original patent.
In the time of Jimi Hendrix, that book did not exist

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http://www.gibson-prewar.com/gibson-1936-e-100-amp/

http://www.gibson-prewar.com/gibson-1940-eh-185-amplifier/

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12. In the time of Jimi Hendrix, that book did not exist
...and that proves what?

BTW, how does this Fender history stuff relate to the amp selector topic?

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13. The first guitar amp was created by soldered several capacitors, resistors and 2 tubes.
That's history, everything else is technology.
At that time there was no nightmare (computer emulation), handbooks were a rarity, schematics created in the head became a reality. Only 2 controls (volume, and cut) 2 tubes and 12 ”

This is real vintage sound which we now unsuccessfully try to process and emulate in various ways.
It is up to the musician to make the most from it.
The first serious schematic is audio amp from the legendary All American Five.

https://frrl.files.wordpress.com/2009/01/aa5_genericschematic.jpg

http://www.gibson-prewar.com/gibson-prewar-amplifiers/

https://www.vintageguitar.com/1804/antique-guitar-amps-1928-1934/

Although early prototypes exist, the first commercially produced guitar amplifier was made by Fender in 1947

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15. ## Miller Effect

Originally Posted by vintagekiki
... ...
http://www.r-type.org/articles/art-129h.htm
Miller Effect

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