420DCV B+ is not low for a 50 watt Marshall amp. My 1972 Marshall 50 watt super lead is 400DCV B+. The average 100 watt Marshall is 450-485DCV with some early 80's JCM800's clocking in at 500DCV-510DCV for whatever reason.
Higher B+ would give you slightly more headroom and wear the tubes faster. Anything above 450DCV really stresses the new production tubes especially EL34 types. You can use 2.2K screen grid resistors instead of the traditional 1K value which gives less screen grid current giving more reliability using modern EL-34's. It does reduce the overall output 1.5 db. George Metropolous is using this in his builds now.
The difference from 420 to 450DCV would be that the amps response would get tighter with less sag kinda like Eds' brown sound in reverse. If you like the super tight sound then you may enjoy that. I think Wizard amps are in the 500DCV range.
http://forum.metropoulos.net/viewtopic.php?f=51&t=46942
Post by
VelvetGeorge » Mon Sep 12, 2022 11:08 am
Those Welwyn 1k are actually 7 watts. Surprisingly. I stopped using them because the working voltage ratings are too low.
FWIW I now use 700V rated 2.2K Less screen current results in about 1.5db less overall output, but far better reliability with modern EL34's.