Acoustic 370 Power Amp Sim Pedal

[johnk_10]

after successfully working with Frank Appleton (aka boomertech) in designing/voicing the GK800 amp sim pedal, I decided to work on designing another power amp (simulator) that has been one of my favorite amps of all time, the vintage Acoustic 370 (power amp section).
so far, with Frank's help, I have drawn up a PCB for an exact clone of the power amp but scaled down the output to pedal level. I just need to complete a gain circuit that will adjust the AGC in the original amp, make up an input buffer for it and probably make a few adjustments.

here's what the enclosure (125B with all top jacks) will look like:

 

[nshuman]

Ooooooooohhhhhhhhhhhh This is going to be great...

 

[boomertech]

This may be really, really cool. The AGC protection circuit in the 370 has me really intrigued as to how it affects the tone.



-Frank

 

[kbakerde]

Gimme gimme gimme gimme!!!! Haha

 

[sprag]

looks sweeeeet

 

[blindrabbit]

after successfully working with Frank Appleton (aka boomertech) in designing/voicing the GK800 amp sim pedal, I decided to work on designing another power amp (simulator) that has been one of my favorite amps of all time, the vintage Acoustic 370 (power amp section).
so far, with Frank's help, I have drawn up a PCB for an exact clone of the power amp but scaled down the output to pedal level. I just need to complete a gain circuit that will adjust the AGC in the original amp, make up an input buffer for it and probably make a few adjustments.

here's what the enclosure (125B with all top jacks) will look like:

View attachment 823026

Oh my.

 

[andrew]

Very cool!

 

[johnk_10]

we're still working on it, but it may or may not have two more external controls. an input level and an AGC gain level, but they could also just be internal trimpots.
fingers crossed I hope to have a working prototype by this weekend.

 

[boomertech]

The 370's AGC protection circuit uses a FET attenuation at the power amp's input signal. The 1176 compressor uses very similar FET attenuation on its input signal for compression. Very cool!!

-Frank

 

[chris merrill]

Sub'd

 

[boomertech]

we're still working on it, but it may or may not have two more external controls. an input level and an AGC gain level, but they could also just be internal trimpots.
fingers crossed I hope to have a working prototype by this weekend.

I'm thinking that you will find a sweet setting for the AGC where you can replace its adjustment with fixed resistors. Possibly put a toggle switch in for that momentary AGC defeat switch. This way you can have two settings.

-Frank

 

[Basshappi]

Ohhh Yeahhhh!!

 

[johnk_10]

I'm thinking that you will find a sweet setting for the AGC where you can replace its adjustment with fixed resistors. Possibly put a toggle switch in for that momentary AGC defeat switch. This way you can have two settings.

-Frank

i'm putting a trimmer for the AGC gain in it for now.
I also added a mini momentary push button switch for S301 (AGC bypass) just like the original amp has. personally, I don't think that a toggle is what i'd want on the original amp or the pedal version of it.
R319 is a 680R in the original amp, and it sets the amp's overall bias ( & AC current draw). since I've rebuilt a LOT of vintage 370's, I found that putting a 1K trimmer in its place allowed me to adjust the amp to it's best sound so i'm adding this to the pedal as well. the later Acoustic 320's also have the trimmer instead of a fixed bias resistor.

 

[boomertech]

i'm putting a trimmer for the AGC gain in it for now.
I also added a mini momentary push button switch for S301 (AGC bypass) just like the original amp has. personally, I don't think that a toggle is what i'd want on the original amp or the pedal version of it.
R319 is a 680R in the original amp, and it sets the amp's overall bias ( & AC current draw). since I've rebuilt a LOT of vintage 370's, I found that putting a 1K trimmer in its place allowed me to adjust the amp to it's best sound so i'm adding this to the pedal as well. the later Acoustic 320's also have the trimmer instead of a fixed bias resistor.

Do you have the manufacturing directions for calibrating R304 with switch S301?

-Frank

 

[johnk_10]

Do you have the manufacturing directions for calibrating R304 with switch S301?

-Frank

yes. R304 is the offset adjust, you turn it so the output is exactly half the power supply voltage.
the 370 Schematic, Note 6 says: "ADJ R304 FOR SYMMETRICAL CLIPPING WITH A 2Ω LOAD AND S301 CLOSED (AGC DEFEAT)"

 

[johnk_10]

BTW, i'm etching the first prototype PCB for it right now.

 

[40Hz]

BTW, i'm etching the first prototype PCB for it right now.

Maybe I missed it somewhere. But is this something that's going to be sold, or is this just a personal project you're doing?

 

[chris merrill]

I'd buy one

 

[johnk_10]

Maybe I missed it somewhere. But is this something that's going to be sold, or is this just a personal project you're doing?

it's just a personal project at the moment, but if it sounds like I think that it will, I could probably build a few of them for others.

 

[mrwood4]

Yessssss......want want want....