u73b quick start manual (public edition (preview)) - schematics and calibration and pages removed - available in client copy)
drip server hard copy : https://ws.onehub.com/files/i2nasubu
the drip design of the classic and out of reach u73b compressor limiter is an amazing feat of layout achitecure and electrical engineering . drip brings you the 1:1 design after a year of work taking on the challenge to bring this rare bird
back to life, it was drips intention to keep the entire design as accurate as possible , we contacted SOWTER transformers to design us the new 1505E transformer a modern replacement for the T188 1:1 transformer.
the plate choke and output transformer are also designed by sowter (V72 plate and output)
the u73b is one of the very few all tube feed forward Mu designs with a theoretical ratio of 100:1 . a push pull 3 stage design. (E99F :input amplifier(MU) / E88CC second stage and output amplifier) E80CF (6BL8) / E80CF : control amplifier)
it is one of the fastest tube compressors on the planet due to its feed forward design.
the circuit uses two E99F tubes as the audio amplifier input Mu stage and an e88cc as the output amplifier stage.
gain reduction is achieved with the E80cf tube . 5 signal transformers per channel compliment this beauty (input/signal amp interstage/signal amp output/signal amp plate choke/control amplifier output.
using the drip pcb you may wire the board as the original , or add input and output attenuators and a more modern time constant . ( the drip design does not include the input pass filter circuit - of course you can add this simple passive circuit if you wish)
drip has spared no expense in the quality of the design and the attention to detail in preserving the historic qualities of this historic compressor limiter.
all calibration controls are on the pcb as trimmers and optionally can be hard wired to the front panel using PEC mil spec locking pots.
we made a video of the drip U73b in action , this was our first test prototype and loosely calibrated - we attached an oscilloscope to show the control voltage attack and recovery imposed over the input audio signal. for the time constants of this video we used the FAIRCHILD 670 time constants which seem more suitable for modern music applications - of course keeping the original time constants is up to you .