10-20-15: Charles1dad
A word to the wise is sufficient.
One would think so, Charles, but evidently not in this case.
Realdeal, in addition to the architectural differences I cited between the two tubes, with one being a directly heated triode and the other being an indirectly heated beam power tetrode (sometimes referred to as a beam power pentode), you might examine and compare the numbers in the datasheets for the two tubes. Which I did before posting.
You might consider, for example, the numbers for filament current, transconductance, plate resistance, and typical operating voltages and currents for the plates and grid(s) of each tube, among other numbers. As well as the graphs that will be included in a comprehensive datasheet.
Wild guesses? Yes, I did make a couple. In addition to guessing that the result of the substitution would likely be worse than poor sonics, I optimistically made a wild guess that depending on the design of the specific amplifier the result may very well NOT be smoke and fire.
Good luck.
-- Al