Clint,
You have gotten good advice ... but a couple more thoughts
Almost any amplifier is a good starting amp but be prepared to make
some changes and learn from the experience.
1) The MLA-2500 is to be avoided - as others have said - because of
the fragile and out of production 8875. Yes, it can be converted
to other tubes but the small chassis is a significant handicap to
a clean conversion.
2) home built amplifiers using ex-USSR tubes are probably not a good
vehicle for learning. Replacing the ex-USSR tubes can be a problem
as many are not being made and surplus sources are not reliable.
3) External anode tubes (8874/3CX400, 3CX800, 8877/3CX1500) are
expensive and have fragile grids. The 3CX800 and 3CX1500 are
great tubes in the hands of an experienced designer but are
probably not a good place for the beginner to start.
Recommendations:
A solidly built single 3-500Z or pair of 3-500Z amplifier ...
SB-1000, AL-80, SB-220, L4B, L7, AL-82, etc. You can learn
much but be prepared to eliminate the "floating grids", add
the proper glitch resistor, improve the cooling of any external
power supply, fix the bias and metering circuits, and convert
the T/R switching to low voltage and (possibly vacuum relays).
Converting any of the 3-500Z amplifiers to one of GM3SEK's
"triode board" controllers would be worthwhile from both a
reliability and learning prospect.
A clean amplifier with FOUR 572Bs is not to be avoided. With
the proper voltage 4 x 572B will produce nearly the legal
limit and the tube cost will not break the bank. Again, be
prepared to fix many of the "floating grid," bias and T/R
switching issues. I would avoid the two tube 572B amps if
only because they are generally too small, undercooled have
poor layout are difficult to service and cost as much or more
than a single 3-500Z amplifier.
73,
... Joe, W4TV
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