Monday, July 20, 2015

Re-imagining the 10LS/1LB Preamp, Part 2

I built the 10LS/1LB preamp in 2008, and modified it a few times over the years. It has mostly been serving as a direct injection (DI) box for guitars. I recently undertook a major rework. See Part 1 for more background.

In-situ Modifications

Previous configuration.
For the first round of modifications, I kept everything in its original enclosure and on the turretboard.  For the first experiment, I removed the 12BZ7 dual triode and associated circuits, as well as the level control.

In-situ modifications
To replace the cathode follower stage, I put in a source follower with a LND150 MOSFET.  This might not be an ideal component for source follower service due to its relatively high RDS /low transconductance, but I had some on hand and it has a sufficient voltage rating (500V) for the application. I used the solder lugs from the 12BZ7 socket as a terminal strip for the MOSFET. The bias network on the 5879 was adjusted somewhat to allow the source follower to be center biased with DC coupling to the pentode stage.  Power supply and heater elevation circuits are omitted in the schematics for clarity.


There 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 steps

Next round of planned modifications.
The gain can be raised by increasing the anode resistor, and adjust the operating point to center bias it again. Of course there's a compromise: Increasing the anode resistor means reducing the idle current, which means lower transconductance and lower gain.  A 330k resistor seems to be about right.  The circuit will be something like the one here.

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.

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