Yes, Glenn, I'm aware the old Alpha 78 not a 1500 watt output "brick-on-the-key" amp.

(sorry if I implied otherwise)
But, it was designed to output a lot more than 700 watts in SSB service!
Regarding its DC Input Power rating...Directly from the manual:
The ALPHA 78 is a self - contained , high frequency linear power amplifier capable of
continuous operation at input powers in excess of two kilowatts PEP/SSB and one
kilowatt average or continuous carrier, with no time limit .
Right in the manual, it states its max DC Input Power as 2.5kw CCS...PEP in SSB service.
Power Input : To 2. 5 kW PEP/SSB, 1 kW average or carrier, dc, CCS
Which of course would pegs the plate current meter on a constant single-tone or carrier...but as the plate current metering is only "average reading", it's way too slow to read any peak values in normal SSB Speech operations.
And btw, this does provide 1500+watts PEP output in SSB service....of course this is not brick-on-the-key carrier!
And, since the big Hypersil transformer (2.4kva CCS, yes?) provides > > 2kw of DC input, and along with the 1200 watts of dissipation from those three, very linear, 8874's, the Alpha 78 is certainly capable of providing a wicked-clean output of ~ 1500 watts PEP in SSB service, CCS, with no time limit...with only ~ 30 - 35ma of peak Grid Current on old (original?) Eimac 8874's!

Heck, even though it was designed back in the days of "1kw DC input power" regs, the 78's manual even makes sure the operator knew this (when discussing the plate current metering, under SSB tune-up and operation):
The actual peak or envelope-crest value of plate current under these conditions may range from roughly 0.9 to 1.5 A, depends on such factors as microphone and operator voice characteristics, the exciter ALC system (if any), and the use of speech processing.
The actual PEP (peak envelope power) plate input might range from 2100 x
(0.9 to 1.5) = (1890 to 3150 W)-. .. all the way from about 1.9 kW to over 3 kW in
extreme cases. The average case will approximate 2 to 2.5 kW PEP at 1kW average
input ... roughly equivalent to the "rule of thumb" 2 kW PEP which is often-(mistakenly) referred to as "maximum legal power."
Yep, this is a direct quote from the manual, and no those are not typos....
So, since I've not only used the 78 well for a year or two, I've also done 2-tone testing (at 1650w PEP out, ~ 825w average) and found it very clean...I feel confident saying the Alpha 78 is a beautiful 1250 - 1500 watt PEP output amp, CCS / no-time-limit, in SSB service.
{oh, and those beautiful 8874's are certainly more rugged than their newer/bigger brother, the 3cx800's.....a triplet of 8874's (with 1200w plate diss) have 15 watts of Grid diss, total....versus a pair of 3cx800's (with 1600w plate diss) having only 8 watts of Grid diss, total....just wish they weren't so expensive....last year I paid ~ $425 each, for NOS 8874's...ouch! but now I've got plenty of spares...}I know this was a bit of a long post....I started with just "yes, Glenn, I'm aware"....and, well....well, I just kinda' got on a roll. Sorry about that.
Anyway Glenn, thanks for doing what you can with Alpha!

73,
John, KA4WJA