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McMartin MA-20 mods
McMartin MA-20 mods
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Old 16th April 2008, 06:45 PM   #1
mawalley is offline mawalley  United States
diyAudio Member
Join Date: Apr 2008
Question McMartin MA-20 mods

I seem to have a problem posting. I am going to try a shorter post this time.

Attached is a schematic for the amplifier. I am looking to make some changes, and I am open to suggestions. I am interested in turning this into a guitar amplifier.

Already planning and ready for:

1. Change AC power cable

2. Change to 1 input jack (1/4")

3. Change to 1 output jack (1/4")

4. Standby switch (is this really necessary?)

Need assistance with:

1. Preamp redesign

2. Addition of Presence Control

3. Effects loop

4. Bias adjustment pot(s)

Any assistance would be greatly appreciated. I have specific ideas I'll post once I have determined that I can ACTUALLY POST!!!

Attached Files
File Type: pdf attachment.pdf (43.2 KB, 138 views)
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Old 16th April 2008, 07:53 PM   #2
mawalley is offline mawalley  United States
diyAudio Member
Join Date: Apr 2008
It appears I can now post. GREAT!

In reference to my original post, I have an idea in my head for the preamp, but I need some technical advice on how to make it happen. Here's what I had in mind:

input---1st gain stage---volume---dpdt bypass switch---2nd gain stage (switched)---dpdt bypass switch---3rd gain stage (switched)---tone stack---4th gain stage---PI and power amp

The idea is that I'd like to have an amplifier that can do clean/crunchy/wild/full-tilt with the flip of a couple of switches. I think it would be really cool to have the 2nd and 3rd stages "voiced" differently, so that I could effectively have 4 distinct sounds (1+4, 1+2+4, 1+3+4, and 1+2+3+4) in the same chassis. All this, of course, before I ever even hit the power section!!!

I don't fully understand impedance matching, so I have no idea what resistor/capacitor values I should use in the stages or in the coupling sections, or what affect different values will have on the sound characteristics of the different stages.

So, am I totally nuts, or can this be done? Should I consider a different order (like the tone stack after the last stage just before the power section, for instance)?

If it works, would this be footswitchable with the addition of shorting jacks and a custom footswitch?
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