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Tone stack

Cathode-follower treble/bass

Two knobs off a cathode follower — on two separate branches.

4 amps in the archive

This is a two-control treble-and-bass tone circuit fed from a cathode follower — but it is not a two-knob cut of the FMV stack. The published drawing splits the signal into two branches: a small capacitor into the treble pot, whose far end bleeds to ground and whose wiper is the output, and a separately coupled bass chain injected into the bass pot's wiper, rejoined through a series resistor at the next stage's grid.

Because the branches share no slope resistor and no capacitor ladder, the two controls interact far less than the FMV family's do, and no deep midrange scoop is carved between them. The cathode follower still earns its keep: it drives both branches from a low impedance, so the network can be as lossy as good control range requires without loading down the gain stage ahead of it.

For where a stage like this falls in the signal path, the guide to reading a schematic walks the coupling between stages. The plainer relatives — a single tone control and no tone stack at all — are documented alongside.

Every cathode-follower treble/bass amp

The matching circuits, with the neighbouring topology choices shown for contrast.

CircuitEraOutputTubes Phase inverterOutput-stage biasRectifier
5E4-A 1955–1956 26 W 12AY7, 12AY7, 12AX7, 6V6GT, 6V6GT, 5U4GA cathodynefixed5U4GA
5E5-A 1956–1960 30 W 12AY7, 12AY7, 12AX7, 6L6GB, 6L6GB, 5U4GA cathodynefixed5U4GA
5E6-A 1956–1957 40 W 12AY7, 12AY7, 12AX7, 6L6G, 6L6G, 5U4GA, 5U4GA cathodynefixed5U4GA
5F4 1957–1960 26 W 12AY7, 12AX7, 12AX7, 6L6G, 6L6G, 5U4G cathodynefixed5U4G

Circuits