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As my Triwatt has the newer output transformer with the second primary tapping for running EL34s, is there any reason I shouldn't fit a double pole switch to allow switching to bigger bottles without needing to solder anything? I can't currently envisage needing more power than I get with the 6V6s, but have in he past run both a 50W Marshall 1987 (with a PPIMV mod) and even a 100W JCM2000. In the good old days when it was easier to clear up at then end of the night if you'd laminated to crowd to the back wall

Going from memory, I believe for running EL34s, more is required than just changing the OT secondaries... Not 100% sure, but I think there are some changes required for the bias circuit... in which case, making it switchable would be more involved. Again, that's from memory, so consult the build guide to be sure.

This is discussed on page 51 of the builder's guide...the bias supply needs tweaking, replacing the 620K resistor (R44) that is switched in on the KT66 setting with a 1M resistor as a starting point. You would still need another switch for the OT primaries. This 2-switch approach would be analogous (but not identical) to the approach used in the Tramp to run a variety of octal tubes....one switch to select the proper cathode resistor(s), and another to select the OT primary impedance. FWIW, running the high DC voltage of the OT primaries through a switch has been known to cause arcing across the switch, at least in the Tramp. If you take this approach in the Triwatt, it might be a good idea to use a heavy duty 5A Carling switch and also some of that black paint-on electrical insulator goop on the switch terminals.