I can help, but my thread hasnn't been posted
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I don't know if this will help with your query but here goes.
6SN7 tubes have different voltages as you know. I would ask Supratek to recommend a "specific" 6SN7 tube that matches the design of your pre-amp or check the specs.
A 6SN7GT / 6SN7WGT is 300 volts max & 3.5 watts max per section.
A 6SN7GTA / 6SN7GTB is 450 volts max & 5.0 watts max per section with this tube being more efficient than the later.
You didn't specify the Raytheon 6SN7 tube type, ie. GT / GTA / GTB, etc...
Hope this helps.
Your EML 45 failure was probably a coincidence. By the way, a friend of mine had two pairs of EML mesh 45s fail within ten hours (total for both pairs), so you got a lot more mileage. The replacement pair of solid plate 45s have been going strong for more than a year. If you are concerned about reliability, you could change the bias to run a lower current through the tube; that increases life dramatically (though at some sacrifice in sound quality).
I run EML meshplate 2a3s. I like the sound very much. So far, they have been holding up fine.
I am not familiar with the Supratek gear. From your description, it does not sound like it is possible to tweak the bias. Amps that allow for such bias change would have a knob and either a meter or a place to plug in a meter to make the adjustment.
I think it's a pretty nice feature to have a switch that allows for use of different tubes. My guess is that more than just bias is changed for the different tubes. Probably, the plate voltage is changed as well. I doubt that any changes are made to the transformer winding ratios, but the requirements of these tubes are close enough that tubes could be interchanged by just changing bias and plate voltage.
Good luck with the replacement EML tubes.
By the way, I am a bit confused. Are you saying that a PREAMP runs 45's and 300B's for its output? If so, I bet they are run very conservatively; who needs that kind of output from a preamp.