Radford STA15 or SET?

[spendorman]

Stupid question, but sometimes stupid ones are the best ones:
You say the valves glow with different brightness. Were you looking at them from the same relative angles? The bases are possibly not aligned so that from a given position, you're seeing the same part of a valve on the left as on the right.

Yes, good one, this happened when fitting JJ KT77's in my STA 25 III

[The Grand Wazoo]

It's caught me out a time or two, causing a few worried moments.