Randall RM20 - Tube Layout - What tube does what?

[Big Steve]

I just got a new RM20. It has 3 12ax7 preamp tubes. Please help me to understand what each one of them does. I can't believe that Randall didn't put this information in the owners manual. Thanks!

 

[speed2dirt]

My guess is that one's pre-modules, one's for the effects loop, and the other is the PI tube. One closest to the input jack is pre-modules, one closest to the power tubes or transformer is PI tube, and the other is for the effects loop.

My question is this:
The RM20 doesn't have bias points like the bigger RM's. Does the RM20 need to be biased or is it a fixed bias design?

 

[Jaded Faith]

You are correct about the layout on the 12AX7's. However the RM20 does have bias points.

 

[speed2dirt]

You are correct about the layout on the 12AX7's. However the RM20 does have bias points.

I'll be darned. I've never noticed them before. The JJ EL84's that came loaded in my RM20 have yet to be replaced. Good to know for future reference.

 

[Big Steve]

Had a gig tonight and ran the RM20 with the JF Vybe Deluxe module. I switched the input tube to a 12at7 and the phase inverter to a 5751. The tube change made the module sound so much better, much more true to the nature of the module. The extra gain of the stock 12ax7's along with the EL84 were just providing too much grind for me. But with the lower gain tubes, cleans were able to be achieved at higher gig volumes. The EL84's still added their characteristic tone to the mix, but they didn't overwhelm it. Much closer to the tones coming from my RM50 now.