The conversion was very simple; replacing the 1.1K output tube cathode resistor with a 1.5K resistor, and installing a 1.5K resistor in place of the 270 ohm power supply choke. While those two changes were all that were actually required for the changeover, I took the opportunity to do some ‘rearranging’ under the hood….
Since I had to replace the cathode resistor anyway, I chose to swap it with a chassis mount style resistor from Vishay-Dale. This also necessitated replacing the bypass capacitor, as the leads on the original bypass cap were not long enough.
I replaced the original 10 ohm hum balance potentiometer (and its associated 22 ohm resistors) with a 50 ohm precision ten-turn pot. Also, I relocated the power switch and reconfigured the interstage coupling cap scheme to eliminate some wiring.
Lastly, I relocated the plate choke and output transformer to free up some additional chassis plate real estate. I stacked the two pieces of iron (as in the S.E.X. amp) to make room for future modifications…..stay tuned!
The result? A smoother and more natural sounding amplifier! To my ears, the 45 seems more ‘distinguished’ than the 2A3, although slightly lacking in ultimate power output. In my system this is of no consequence, as the amps are directly driving a pair of 110dB compression drivers. Listening to these amps has become even more enjoyable and engaging! I’ll turn on my system with the intention of listening as background music, but get drawn into my listening chair for hours…..one LP (or CD) leads into the next, and before I know it, it’s midnight!
If you’ve got the high-efficiency speakers to take advantage of this modification, I urge you to try it! I don’t think you’ll be disappointed.
Shunt regulators are on deck….
