Stupid question

[jestaudio]

I suspect I know the answer and stand ready for incoming mick takes, Output transformer ratings, I have seen OT,s rated at 40 watt with 2 6l6 type tubes, some rated at 30 watt and then some rated at 50 watt, what makes the difference in ratings?

[martin manning]

The size of the core to get the necessary bandwidth before saturation, and the gauge of the wire to control resistive heating and temperature rise.

[Stevem]

Two 6l6gc tubes can do a max of about 64 watts RMS in a class AB1 circuit and at there max rated plate and screen voltage, and with a OT with a inpeadance of some 4600.
If your circuit ( plate and screen voltage) and OT are set up for only 30 watts you will of course have tubes that last twice as long as the maxed out 64 watt output stage.

[LOUDthud]

Not all "6L6" tubes are created equal. The 6L6 {nothing}, 6L6G and 6L6GB are rated for only 360V on the plate, 270V on the screen and 19W plate dissipation. These early versions are what 50's Tweed amps were designed for. Those amps rarely exceed 30W. The 6L6GC is rated at 500V on the plate, 450V on the screen and 30W plate dissipation. The GC version can do 60W in class AB1 push pull but earlier versions won't last long in that circuit