RM20 power amp mods

[CrunchBunch]

Have done a few mods to my RM20 combos and heads that others may be interested in.

Master volume mod:
- lift the right side (looking from the front) of C9 from the board;
- solder to pin 1 of an A1M pot (1M log / audio taper) pot;
- solder an 0.1uF cap from pin 2 of the pot back to the vacant hole from C9;
- ground pin 3 of the pot (a convenient ground point is the ground pin at R20;
- drill a hole in the back of the head for the master volume pot (somewhere around the serial number is a good location). Alternatively, on one of my amps I've relocated the boost pot up a little and put the master volume below it so its on the front of the amp. Relocating the boost pot does involve cutting the PCB though.
Explanation: we're basically bleeding signal to ground before it goes to the power amp.

Switchable density:
The RM20's come with different values on the density PCB to whats in the RM100s. Changing to the RM100 values makes for a noticeable improvement. Basically change C3 out to a 100uF 100V electrolytic cap. While you're doing this, you can make the density control switchable by fitting a switch to the + lead of C3. With the switch open, the density control is disabled and the amp is far less stiff (you're basically turning off the NFB) but the power section will distort alot easier and its response becomes far less linear. I like both settings depending on what I'm playing. I'd also recommend fitting a small value cap across C3 and its + on the PCB to block DC and provide some attenuation for high frequencies. Something around the 500pF is good. I used a 470pf cap.

 

[Spencer4hire]

I wish I was capable of doing this mod. A master volume pot on my RM20 head and on my one off RM22 combo would be such an awesome improvement as you would be able to crank the module's volume up to get it in the sweet spot!Could you post some pictures of the amp with the volume pot please.

 

[CrunchBunch]

I've added some pictures here that should help out - http://faqload.com/faqs/music/randall-rm20-master-volume-mod

 

[Spencer4hire]

Thank you for the link. Maybe I can find an amp tech to do the mod for me.
I like the idea of having a master volume for this amp.

 

[stalksdalls]

Hi! I'm intrigued about the possibility of adding a Master Volume pot to my RM20, but I find your instructions confusing. The written instructions seem to conflict with the photos.

"- lift the right side (looking from the front) of C9 from the board;
- solder to pin 1 of an A1M pot (1M log / audio taper) pot; "

If I understand that correctly, C9 is the larger of the two capacitors (the one on the left), labeled 0.1uF. I can see that the right side has been moved up to the top pin of the volume pot.

"- solder an 0.1uF cap from pin 2 of the pot back to the vacant hole from C9;"

In the photo, it appears that the capacitor that is soldered from pin 2 of the pot back to the vacant hole (on the right) of C9 is 1nF, not 0.1uF.

The rest of the written instructions seem to make sense, but the caption under the photo says that the 0.1uF cap goes to pin 3 of the pot instead of pin 1. In the photo it is soldered to the top pin. Is that pin 1 or pin 3?

Thanks,
Jim

 

[CrunchBunch]

Thanks for pointing out the errors. All fixed now.

 

[ricky]

Any pix of what it looks like after? I am interested where the master volume knob is on the amp.

 

[Spencer4hire]

Here you go Ricky.

One of the best mods to do to the RM20 and RM22 amps in my opinion.

 

[ricky]

For some reason it says something about third party hosting on photobucket and I can't figure out where to change my settings to allow it. So your picture didn't show up. I have an RM20 and I am curious bout this mod. I use the foot switch boost to bring the volume down but I think your period would sound better.

 

[Spencer4hire]

Ricky I sent you PM.

 

[shortlife]

Have done a few mods to my RM20 combos and heads that others may be interested in.

Master volume mod:
- lift the right side (looking from the front) of C9 from the board;
- solder to pin 1 of an A1M pot (1M log / audio taper) pot;
- solder an 0.1uF cap from pin 2 of the pot back to the vacant hole from C9;
- ground pin 3 of the pot (a convenient ground point is the ground pin at R20;
- drill a hole in the back of the head for the master volume pot (somewhere around the serial number is a good location). Alternatively, on one of my amps I've relocated the boost pot up a little and put the master volume below it so its on the front of the amp. Relocating the boost pot does involve cutting the PCB though.
Explanation: we're basically bleeding signal to ground before it goes to the power amp.

Switchable density:
The RM20's come with different values on the density PCB to whats in the RM100s. Changing to the RM100 values makes for a noticeable improvement. Basically change C3 out to a 100uF 100V electrolytic cap. While you're doing this, you can make the density control switchable by fitting a switch to the + lead of C3. With the switch open, the density control is disabled and the amp is far less stiff (you're basically turning off the NFB) but the power section will distort alot easier and its response becomes far less linear. I like both settings depending on what I'm playing. I'd also recommend fitting a small value cap across C3 and its + on the PCB to block DC and provide some attenuation for high frequencies. Something around the 500pF is good. I used a 470pf cap.

I am thinking to do the master volume mod on an RM22.
After adding the Master, will I be able to use the power section of the amp as a slave by feeding a line into the rm22 effects return and control the volume?

I want to go Marshall JCM 800 - Shur reactive load - Rm22 power amp.