Let's deal with your other questions about the Alpha 78.http://www.alpharadioproducts.com/legacy.asp
IF operate this amplfier PROPERLY the 8874 tubes will give years / decades of amatuer service!!
I have the Alpha 76PA, which similar to your amplifier -- but with out the "pre-set" tuning additions. This amplifier was purchased around 1980 by one of my elmers - K9EWJ(sk). It still has the original Eimac 3CX400A7 (8874) tubes -- and they are still operating at full output.
When this amplfier was built (1980s) "Legal Limit" as defined by the USA FCC was 1 KW DC INPUT (2 KW PEP INPUT).
You may have received an ampflier that was used heavily for government/military or contest usage -- in that case -- your tubes may be showing some age. The 3CX400A7 (8874) tube are used in health care/medical and comemrcial trasnmitters - so a surplus market does exist. New tubes are available from both Eimac and Svetlana.