How do you know when to replace tubes

I had the same tubes in my '60 Fender Deluxe for over three decades.

Wolf, Why did you go for the KT 150's instead of just replacing the KT 120's? Double the $$ double the pleasure?

They became more affordable and now cost about the same as 120s on a weight basis (it might take 5 120s to equal 4 150s), they could last a ling time, and they're larger than any other tube that will fit in my amp (!)...I was listening to a great live jazz CD (Fred Hersch) tonight and the 150s sort of woke up and now sound even better than yesterday...they sound large.

So da big question is should I go with 120's or 150's when I replace the original Tungsol 6550 on my Jolida 502 p? I have about 1500 hours on them I would guess.

The cheapest price I see on ebay for 4 kt150 is about $130 more for a set of 4 than kt 120's. You got a hell of a deal.

LONG time...not "ling" time...my retired 120s were about 100 bucks less than the 150s I just bought (they're still available by the way), and when you adjust for inflation, "present value" of money, Russian raw goods indices, jo mama, gross weight, groceries, Albert Grossman, the savings on heating your hut, the cost of cooling your hut (!), and labor used in wiping fingerprints from the glass, it's not too bad a deal...