[Harp-L] Masco ME18 cloning

[Jim Rossen <jimjimdr@xxxxxxxxx>]

The Masco ME18 is a particularly good harp amp, and has a simple circuit
design with grid lead bias input pentode, twin triode paraphase PI, 6l6 push
pull cathode bias, treble bleed tone control. The MA17 has the same basic
circuit with different preamp tubes, plus a choke.  (BTW- the Gibson Maestro
also has a similar circuit, but with a cathode biased input pentode).

I wondered if the harmonica friendliness was solely a function of the
circuit or some other feature that was unique to this particular Masco, such
as the output transformer.   I recently bought a PA amp that uses the same
preamp and power tubes as a ME18 but a different circuit, and has higher 6l6
plate voltage (420 vs 350V).  Harp tone was not very good in the original
configuration, and got great when converted to the ME18 circuit.  In this
case it mainly involved bypassing the second (triode) gain stage.  The
circuit also works well with the higher b+ voltages.  This my second
successful attempt to implement the ME18/MA17 circuit.  The first was an
all-new build that used a nine pin mini twin triode (12AY7) for the PI
instead of an octal twin triode.   So... it is not transformer magic.

The desirable Mascos are getting expensive.  The same sound can be obtained
with less expensive PA amps (mine was $45) and some circuit modifying elbow
grease.

Jim R