Saturday, April 26, 2014

Mike Matthews Dirt Road Special - Speaker Upgrade

Several months ago, I posted about a repair on the Mike Matthews Dirt Road Special.  A few days ago, I had the good fortune to run across an exact replacement, electronically and acoustically, for the Celestion 12 Inch original speaker for this amplifier.  For example, the new Fender 12 Inch replacement speaker has almost exactly the same free-air resonance frequency.  The two sound nearly identical under a "tap" test.  By removing the GE (antique) temporary replacement and installing the new Fender, it is now possible to re-install the original power supply transformer to get the original 70 Volt DC power supply level.  The small stone phase shift effect is almost infinitely adjustable with the full 70 Volt supply.  Here is the Mike Matthews Dirt Road Special with new Fender replacement speaker:


Mike Matthews Dirt Road Special with Fender Speaker
Mike Matthews Dirt Road Special with Fender Speaker



Monday, April 7, 2014

Foam Core Midrange Horn

I made a foam core horn using the same mold from the two plywood horns.


Foam Core Midrange Horn
Foam Core Midrange Horn
This is not as rigid as the plywood versions.  The sides resonate a little.  I believe this could be corrected by spraying foam on the horn exterior to stiffen the construction.  This would not add significantly to the weight.  As pictured above, this horn including driver weighs less than one pound.

This type of construction might be applied to the bass horn to reduce the weight.  The basic horn interior horn shape could be built up from foam core and spray foam used to fill in behind the sides to stiffen them.  While still quite large, the bass horn would be light enough to be carried by one or two people.  Maximum dimensions should be less than the standard exterior door opening, about three by seven feet.