Guitar Amp Mic Input Mod

[CJ]

we are going to add a mic input to a DIY Fender Princeton Reverb amp,

that way you can play and sing or play and use a harmonica and not have to haul a PA around,

some acoustic amps have this already, so we need to catch up with these guys,

step 1 is to rip out the 1/4 inch phone for Instrument #2 and punch a hole for an XLR,

then we install the XLR jack and wind a small input transformer,

why not use a small Neve input transformer and convert it to tube use,

here is the XLR installed and a pri being wound on a Neve bobbin>

 

[CJ]

now we need to figure out a way to mix the guitar and mic signals without screwing them up,

we tried a cap to isolate the transformer secondary from the guitar input,

but the guitar signal is AC so it travels right down the cap and shorts out through the transformer secondary,

then we try a 470 resistor to see if that works, - it works great!

the mic signal does not drop much and neither does the guitar signal,

plus, we lose a little high end off the mic which works perfectly to EQ the signal for the guitar tone controls, and the guitar and mic levels match each other very well as the volume control is turned up,

and we also lose a little top end off the acoustic guitar which required that the treble control be turned down most of the way to get a good sound, a win-win for using the resistor, we isolate the signals, get the levels right, and EQ them also, 

here we are testing the cap and resistor methods before mounting the transformer>

 

[CJ]

we are going to find a way to  mount the transformer inside the chassis and bolt it up.

then we ar going to irratate the neighbors with some Neil Young music, 

here is the schematic with transformer details>

 

[PRR]

If you did Donovan you'd *really* want another half-12AX7 for the mike channel, mix later. (See any 2-input Fender.)

You have a severe noise penalty and some EQ effect versus the guitar (or pedalboard).

But doing Neil, this will work.