Why does the RM20 & RM22 lack the punch of the bigger am

While doing testing this morning on my RM22 to determine if the Foglifter would be a worthwhile upgrade for this amp, something of interest crossed my mind. I often get inquiries about why the RM20 and RM22 don't sound the same as the bigger MTS amps with any given module. Sure, the EL84's are part of it. But the power supply is a bigger part of the equation that you may have never considered.

The RM22 provides 245V of B+ to the module when it's at rest. Palm muted chugging calls for an amp to deliver more than most other techniques. With a stock Mr Scary module in my RM22 the B+ drops from 245V down to 212V when palm muted chugging was run through it. That's a 14% drop in available voltage.

Now let's do a comparison with an RM4. The RM4 provides 310V at rest, as does the RM50 and RM100. Chugging caused 0% voltage drop.

MOD 50? The module was seeing 372V for a difference of 34-48%!

So the RM4 at rest provides 21% more voltage at rest and as much as 35% more under stress to the module. There is a huge variable in the equation with regards to the lack of punch and sounding as big as the other amps. When cranking the volume up on the Mr Scary to 3:00 or higher the amp can actually be pushed to cut off, which sounds terrible.

So there you have it: there's more to the tone difference than the small bottles alone.

Question is, how extensive would modifications have to be to supply better plate voltage to the modules?

wesarvin said:

Question is, how extensive would modifications have to be to supply better plate voltage to the modules?

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I tried to look into this for you over breakfast this morning, but it wasn't a simple thing to tell from the schematics. My gut feeling is it might be quite extensive though.

The schematics do not show any of the voltage readings in needed to determine the scope of what all would need to be done. There are quite a few lower voltage caps downstream that could be at risk with higher voltage provided, so there could be a trickle down effect regarding the size of the job. The fact that the preamp sags as much as it does indicates it's more of a global supply thing than just "juicing up the preamp".

Thanks for the effort!

It makes sense though, and in all actuality, I prefer that bit of sag and bark that the 20 provides. It couples quite well with Plexi and Vox type modules. I have the rm50 for the other tones :wink:

That's kind of my take on it as well. There is a charm to that sag and feel for sure. Not ideal for metal, but very cool for vintage and roots stuff. Of course it's not much of an issue if your volume is below 10:00 and you could always run to an external power amp if need be.