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Since getting my old Hammond amp, I've been using it exclusively because it sounds a lot better than my SS amps. Could I benefit from replacing any or all of the tubes? Right now there is one 5U4, one 6BA6 (preamp) and 2 6V6's
The 5U4 is a GE, made in USA and is marked quot;5U4GA [over] 5U4GBquot; on the base
The 6BA6 is marked quot;Realistic Lifetimequot; quot;Germanyquot;
One 6V6 is marked quot;6V6GTAquot; quot;Realistic Lifetimequot; quot;Britainquot;
One 6V6 is marked quot;Sylvania Made in USAquot; and has a faded quot;6V6GTquot; on top.
All the tubes appear to be reasonably old (the printing on the glass has faded a lot) and there is a spot on the 5U4 where someone seems to have touched the tube because there is a fingerprint-sized burn mark on the glass. Any ideas/help?
Thanks

It doesn't sound like you'll need to bias it, so you'd just have to buy them and pop'em in. The 5U4 and 6V6's will be easy to aquire. Email Todd at He's the cheapest, and carries all the brands.
The only issue will be the 6BA6, since it may not be common. Don't put a 12AX7 in there, since you don't know if the pins do the same thing. Honestly, it's probably still a good tube, so if you can't locate one easily, just replace the other 3.
I just checked for you, and revolution has the GT 5U4 for $17.60 and JJ 6V6's for $31 for the set of two. Yours aren't matched, but these will be. Also, it sounds like your rectifier tube (5U4) is old and worn out if there's a white/gray burn spot on the side.
They'll soon be toast.

your call as far as new tubes ... I certainly wouldn't tell you it won't improve the tone, and maybe power output of your amp. Most of the good old tubes lasted years, though.
One good argument for replacement is that sometimes, depending on the circuit, power tubes can fail in such a way as to damage the transformer.

I'd suggest KCA NOS if you want to go the NOS route, or Antique Electronic Supply. There are several current versions you could also use.

As usual, I'll say you can also quot;Ask Mylesquot; over on the expert forums of Guitar Player. He always has excellent advice.