[Harp-L] Re: Masco ME18 cloning

[Jim Rossen <jimjimdr@xxxxxxxxx>]

On Sun, Dec 27, 2009 at 2:04 AM, Ray Beltran <raybeltran@xxxxxxxxxxxxxxx>wrote:

>
> Which PA amps specifically are good candidates for ME18 cloning?
>
>

Ray-  I don't know specific amps for this.   The one I used, a Knight
Standard 30, took a lot of work.

The closer the donor amp is to the desired circuit, the easier the clone.
ME18 and MA17 schematics can be downloaded from-
http://groups.google.com/group/jdramps

The key characteristics are-
-6l6 x2, cathode biased (most 40-60's PA amps are cathode biased)
-Input pentode preamp tube, at least one- usually an octal base 6sj7.  Some
Masco MA17 amps used a loctal base 7c7.  7c7 tubes are cheap, but I don't
like the loctal tube sockets I have worked with.
-Best if the input pentode is the first gain stage.  Some Mascos use a
triode for the first gain stage and a pentode for a subsequent stage.
-Best if the input pentode first gain stage is grid leak biased- that is a
cap leads from the input jack to the tube grid.
-Twin triode paraphase style phase inverter.  Typically this tube is an
octal base 6sc7.  Best is probably a 6sl7 or 6sn7.  A paraphase inverter can
be identified by the presence of a resistor with value in the range of 100k
to 500k going from a 6L6 grid (pin 5) to one of the a grids of the twin
triode. The other 6l6 grid will have the same value resistor to ground.  The
triode grid receiving the resistor from the 6l6 grid will have a second
resistor in the range of 2k to 10k to ground.
-A choke.  This looks like a transformer but only has two wires.  The MA17
has a choke, the ME18 does not.   A choke is preferred but not essential.
New ones cost about $15.
An amp with multiple input pentodes and twin triodes is OK, but will need
more work to simplify it to a ME18/MA17 style circuit.

The conversion could involve a lot of work, and might be prohibitively
expensive.  It is fairly demanding and requires the ability to translate
schematic data to a circuit layout,  work in a congested and chaotic
appearing point to point circuit, knowledge of tube pin configurations and
use of tube socket terminals as tie points, and lead dress.  I am not sure
that one could get ones money out of a project like this given the low
prices commanded by fully serviced generic PA amps I occasionally sell.

One advantage of a clone is that you can start and end up with a more
powerful amp than a genuine ME18/MA17.

The point is the Masco sound can be obtained without a Masco branded amp.

Jim R