Pultec MB-1 Valve Microphone Preamp Clone

This is a picture of my prototype clone of a Pultec MB-1 tube microphone preamplifier.  I built it from this schematic  posted by Rafael Texiera.  It uses two triode gain stages in a 12AX7 direct coupled to a cathode follower consisting of paralled sections of a 12AU7. This circuit is notable because it uses variable negative feedback to vary the gain between 28 dB and 48 dB.  Most other preamp designs I've seen control the level with an attenuator on the input, a pot between gain stages, or an attenuator on the output.  All of these approaches waste signal.  The true variable gain feature maintains maximum signal to noise ratio for any input level.  The negative feedback also lowers the output impedance of the cathode follower to about 50 Ohms. It can drive low impedance unbalanced lines great without an output transformer.  I have also used a 600:600 line matching transformer to provide balanced output.   Drives +20 dBm  into 600 ohms no problem.   A 14 dB pad on the output is usually necessary to avoid overdriving -10 dBv consumer-level inputs.  I'm using a UTC A-11 microphone to grid input transformer with excellent results on dynamic and ribbon microphones.  I built it with a Triad R-4A (250-0-250 VAC)  power transformer and a 6X4 tube rectifier.  My B+ is about 320 VDC, 25V shy of the 345 nominal from the Triad R-7A (300-0-300 VAC) used in the original.  Pultec spec'd the power supply very conservatively, as the circuit draws less than 15 mA from the B+.  I designed my own turret layout and built it on a 6.5" x 2.5" x 0.0625" scrap of G10-FR4 board.

I am so satisfied with my results that I've ordered two 2U rack chassis and four input tranformers.  I plan to make two dual channel preamps - one to keep and one for my friend Arpad's studio.  I'll publish the results as construction progresses.
Pultec MB-1 microphone preamp prototype
The third ceramic socket isn't wired for this ciruit - this is my "preamp prototyping" testbed.