If I used a single 1625 in the final, it would be good for about 25
watts output without straining. Then if I needed more power, I'd build
a linear amp using a 2 or 3 811A's (that's 340-410 Watts PEP output).


But will 25 watts drive a two- or three-hole 811A amp in grounded
grid? All of the data I've seen says you need about 15 watts per 811A
in GG. Grid Driven requires a lot less (about 5W) but then you need a
grid tank, etc.

The 15 watt figure is close, I think the offical handbook rating was 12
watts per tube in GG. The higher figure was with an UNTUNED input
without any impedance matching network (just the way most 811 gg amps
were built, since the average exciter provided about 100 watts output
who cared?).

TWO 811A's should easily be driven by a 25w out exciter with a good
matching network between the amp and the exciter, it might be pushing it
to drive three (But you can get 35w pep out of a single 1625, or use two
of them in the final for 50w output in AB1).

BTW the 1625 used to be the last cheap final left. Fair Radio had new
ones for $4.50 up till last year. Now they only have used ones left for
$3.00. Everyone has discovered that 6159's are really 25v heater 6146's
and the price has jumped from a few bucks to the $18 range. The US made
811A's are drying up (and you have to be carefull of those Chinese
tubes, some are OK, others don't last long). I remember back in the
60's when BA was selling NOS surplus 1625's for $0.25 each! (Should
have bought several gross and put them in storage, but not on a
teenagers budget).



Not a full gallon, but the extra few db ain't worth the cost! I have
some 813's in the junkbox, but the sockets for them are costly, and they
have such HIGH output capacitance that making a good tank circuit in a
single ended amp is rather a pain. Still a grounded grid tetrode
circuit looks interresting! (cathode driven with normal g1 and g2 voltages).


I always thought the big advantage of GG was elimination of the need
for screen and bias supplies.


The 813 isn't such a great tube in pure GG. Most of the spec's I've
seen show it as only 200 watts or so output in GG. Problem is the tube
just doesn't have much power gain in GG. If you bias it as a tetrode
the power gain goes way up, you can still run it 'cathode driven' to
avoid neutralization though.