Hello,
I recently rebuilt a Masco ME 18, and while it sounds really great with my guitar, it isn't working with a harmonica. This amp is supposed to be a harp monster, but I am getting feedback almost instantly. I would love some advice on where to mod this thing to make it harp friendly. The only change I made to the schematic is the removal of the phono input. Could the removal of the phono volume control be causing the issue? I also know that when this amp is modded for harp, preamp gain is lowered, but I am not really familiar with the 6sj7 tube, and the best place to start. I appreciate any insight.
Masco ME 18 Mod
Moderators: pompeiisneaks, Colossal
Masco ME 18 Mod
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Re: Masco ME 18 Mod
A good starting place is R6. Experiment with lower values. Try 220K, 100K, etc. May be all you need to do.
Re: Masco ME 18 Mod
Thank for the quick reply. After the first fire up I had some squealing, and it turned out I had used a 390R instead of a 390k.... R6 is currently 330k. I will try lowering that value. I will also try clipping in a 470k to ground where the phono volume control was... Other things I was thinking about was the input. Do you think lowering that 15M resistor, or adding a 1-4M resistor from the input jack to ground would help? I have also read that changing from gridleak bias to cathode bias will help lower the gain too, but I don't know how that would affect the mojo of the amp.
Re: Masco ME 18 Mod
Quick update. Changing R6 to 82k, and adding a 470k resistor to ground where the phono volume pot was got me another notch and a bit on the volume control. Is there anything else I can do to get some more sweep? I have been doing a lot of reading about these amps, and common mods for harmonica, but all I can seem to find are general statements about mods, and nothing specific. The consensus seems to be lowering the preamp gain, and shaving some treble. I will admit, I am a bit lost on where to go from here.