Puget Variable-Mu

Based on the Fairchild 670 limiter. The holy grail. Originals sell for $40,000+ when they surface. Twenty tubes, eleven transformers, and the 6386 remote-cutoff triode that makes variable-mu compression possible.

How variable-mu works

The gain reduction happens by biasing push-pull tube stages — as the control voltage changes, the tube’s amplification factor (μ) changes with it. Smooth, musical, and harmonically complex in a way that VCA and FET designs can’t replicate.

We modeled the 6386 with a novel WDF triode model fitted to the original GE datasheet curves. Every tube has its own operating point. Every stage interacts.

Where WDFs really shine

The transformer coupling between stages is where WDFs excel. Transformers map naturally to WDF two-port elements, capturing the frequency-dependent saturation and phase behavior that gives this design its legendary “glue.”

• 6386 remote-cutoff triode — variable-mu compression from the actual tube model
• Push-pull Class A signal amplifier (4× parallel 6386 per channel half)
• Transformer-coupled input and output modeled as WDF two-ports
• Six time constant settings with program-dependent release
• Stereo with lateral/vertical (mid-side) operation

Use it on

The mix bus. The stereo bus. Master chain. The 670 is the reason engineers describe compression as “glue.” It ties a mix together without crushing it — the program-dependent release and tube harmonics do the work.

$199 — Perpetual license. No iLok. No dongle.

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