Can I put 6550s in my TSL602?

[MDV]

Rather than EL34s?

[HJM]

Probably.....how it sounds depends on the output transformer, and whether there's enough -bias o get the valves working. New transformers are a bit poo in Marshalls.

If it works you'll have less of the Marshall mids and more low end punch.

[Skybone]

Can't see why not, although I would recommend that you take it to your local amp tech to get this done. If you were swapping brands of EL34, biasing the TSL/DSL is supposed to be relatively easy, but changing valve types is a whole different kettle of fish.

I quite like 6550's (used to have an Ampeg VL-503 with them in), and it sounded quite good, although I did have to muck around with the EQ for a while to "get in the zone". The advantage of them is that they have a lot more headroom than the EL34, but don't sound as sweet when cranked, if you're going for power valve distortion, then an EL34 will wipe the floor sonically with a 6550. If you're just after more power ( :D ) then the 6550 should give you plenty.