This is a great choice for the .50 cal + if you want an aggressive thick sound. I had 5 pairs of the STR 420's, and I tried them all in my amp, and I found that the Blue rated tubes sound the thickest and most aggressive out of Yellow, Gray, and red. The yellow rated 420's just sounded terrible in it, the red were better than the yellow, but They didn't sound great. Then when I tried the gray they sounded thick and Nice, but not very aggressive, Next were the blue. These were the best in it. They sounded great! 
They were very thick, tight and aggressive, They made My amp come to life and Roar :twisted: . The best it ever has sounded in the 1 and 1/2 years I've owned it, If you want to make your .50 cal sound great, I highly recommend these for High gain rhythms. I am playing the .50 cal through a marshall 4x12 with G12T-75's.
Here's My setup
They were very thick, tight and aggressive, They made My amp come to life and Roar :twisted: . The best it ever has sounded in the 1 and 1/2 years I've owned it, If you want to make your .50 cal sound great, I highly recommend these for High gain rhythms. I am playing the .50 cal through a marshall 4x12 with G12T-75's. Here's My setup
