ResultsThere was a decrease in hum when compared to the old circuit (-48dBu) and the tone seemed at least similar to the old circuit, but there were some new problems:
- The gain was not quite enough. E.g.: Playing my Danelectro '63 baritone through it, I was peaking at about -15dBFS on my DAW.
- Removing the 12BZ7's heater load sent the heater voltage excessively high-- about 8.4V AC! Remember that the heater transformer has a voltage regulation of 30% and was rated for 115V (i.e. Canadian) input, so a very high open circuit voltage happens with 120V on the primary.
|Next round of planned modifications.|
For the purposes of reducing hum, I was already contemplating a DC heater supply. After seeing the overvoltage condition, it's obvious I need to do it regardless of the hum issue. (Incidentally, I never measured the heater circuit voltage when the 12BZ7 was still in. It was probably somewhat high, but not quite this high.)
I will probably do a by-the-book supply with a capacitor filter and linear regulator. No reason to get fancy. A low-dropout regulator is not even going to be necessary with input voltage this high.
Since I'll need to add a heater power supply PCB, it's probably time to remove the turretboard and rewire the amplifier circuit. The circuit is so simple, with so many components connected to the pentode socket, it makes sense to go point-to-point, on old fashioned terminal strips. I might even be able to move everything (pentode, reservoir capacitor, and choke) inside the enclosure.