Here’s a picture of the first assembled pre-production prototype PC-2A Photo-optival Tube Compressor. This pedal gets nice and warm even though it has just one subminiature tube inside. Everything checks out so far. The high voltage power supply is putting out a very healthy 330V, the bias voltages are good and the audio signal path is working beautifully. I have some adjustments to make on the photocell for the attenuator circuit. The photocell in the prototype was a silonex SR3, which didn’t produce the correct characteristic slope, being too steep over a narrow range. These are the audio gold standard, but the resistance is too low for good control over compression ratio. I have some specials on order from Silonex, however am dipping into some of my old batch of Clairex photocells in the meantime. These have the right spec but are very inconsistent, like old germainium transistors. Finally, just some small adjustments to make on the limit/compress switch to get the ratio transition just right. Sounding good though and it won’t belong before the first of these beautiful little tube compressors are available.
Originally posted 12th November 2009.