This months tube is one of my favorite directly heated triodes, the 10Y:
The close up shows that the A-version has a plate which is about 50% larger! Quite a significant difference. This would indicate a higher maximum plate dissipation. This was probably done to ruggedize the tube.
Another significant difference is the change from a bright emitter thoriated tungsten filament in the VT25 to a dull oxide coated filament in the VT25-A. Similar to the filaments of 45, 2A3 or 300B. Quite a different appearance when they are operated in the dark:
A collection of a variety of different 10s and 10Ys:
I'm about to build a new linestage with the 10Y using the line output transformer with the secondary as tapped volume selector from Tribute. This linestage will use the same chassis style as the modular preamp series presented in recent post. The making of that preamp will be shown in an upcoming article.