BadCat HotCat 30
This model of BatCat amp was a bit of a challenge. That was until I stopped having a brain fart and said "oh yeah.....MATCHLESS amps"
If you have never had a peak inside one of these amps, think military air craft wiring. The point to point construction and numerous bundled wires are all very tidy, but still quite overwhelming. While most things are easy to access, it is a bit of a puzzle trying to make any changes without a schematic.
My journey of reverse engineering this amp was very short lived, as I somehow forgot (then remembered) that these amps are essentially Matchless amps. After I pulled out some of my old Matchless schematics, what I had set out to do here became much simpler.
The Crossline Master Volume
Our task was somewhat of a simple one on this amp. These amps come equip with a Master Volume already, but in the opinion of many (and of my own) it is not very good. The Master Volume used in these amps are what's called a "Cross Line" master. It is TECHNICALLY still considered a post phase inverter master (type 3 I believe), but I do not like to call it that because it is far inferior. When you open an amplifier and see that the Master volume only has 2 conductors soldered to it, it is a big indication that it is utilizing a cross line. That is exactly what I had here, but to sort through ALL of these neatly bundled wires was going to be a pain. I am glad I had the Matchless DC 30 power amp schematic to kind of help me find a few things. In addition to my meter.....thanks continuity.
PPIM
Out with the old. In with the new. One of my biggest problems with the original cross line is that the way these function are they simply take the two outputs of the phase inverter and blend them together with a pot wired as a rheostat. These two signals are 180 degrees out of phase, so the more they are blended together, the more they cancel each other out, thus the drop in volume. Its an amazing idea its just too bad it doesn't function very well. This type of MV doesn't have much of a sweep. Set the pot at zero and it kills it, go to 1 and its 75% volume while anything higher than 1 is max, completely defeating the purpose of a good master volume.
The newly installed Master has a much more controlled sweep, which allows you to play the amp at lower volumes and still get all the amp has to offer while not offending the neighbors. Additionally, this new Master is completely isolated and does not affect the bias of the power tubes.