I am not going to get into the perennial discussion of the sonic properties of the yellow print valves over the stock boring white print ones but instead discuss A DIFFERENT advantage the yellow has over the prosaic white.
Some of our eagle eyed readers will have noticed that the Yellow Print Old Shield Mullard valves have underneath the shield typically the valve notation in Mullard - Philips (ECC83) or even RETMA (12AX7) then underneath this is the elliptical BVA logo - just like for the white print valves.......
But here's the rub - they yellow valves have a letter code immediately to the right of the BVA logo and immediately to the left too. Any idea what they mean?...... were they placed there by Dr Steve Strange (Marvel not New Romantic!) to tap into chakra energies from alternate universes to imbue special sonic qualities? Actually not, for these were a stamping code signifying the date the yellow print was applied for as well all know, labelling was not part of a continuous process and often different batches of the same valve were labelled in a production run sometimes many months hence from manufacture completion.
The left number signifies the year of stamping
The right number signifies the month of stamping
So, for my example here (Stock No D335)- a yellow print ECC83, Blackburn January 1965 we have the stamping codes MB which equate to 1965, February. By simple extrapolation you can now derive stamping dates for any other year/month combination on your favourite yellow prints.