Phase Inverter Tubes

12AU7 - ECC82 12AX7A - ECC83 6GW8 6U8A - 6GH8 - 6BL8 7199


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Phase inverters symmetrically "split" the incoming (unbalanced) preamp-signal into two "balanced" signals out of phase by 180 degrees to drive the push-pull output tubes to maximum efficiency.  Various phase inverter designs include: the  "long-tailed" pair,  "Isodyne,"  transformer,  "cross-coupled,"  "Schimdt,"  "floating paraphase,"  and  "split-load" or "Cathodyne."

In all Vintage H.H. Scott products these tubes are configured, as one of two inverter types:

the "floating" paraphase inverter (also known as the "see-saw" ) circuit type,
all Scott amps feature an AC balance adjustment

the later "split-load" phase inverter (also known as the "Cathodyne")
type which eliminated the AC balance adjustment.

During manufacturing production runs (of particular Scott types -- the factory often substituted the presently more expensive and harder-to-find, 7199 for the older (and more common) 6U8 -- 6GH8 -- 6BL8. The two tube types are not pin-out interchangeable, as their triode sections are pinned differently. 

HOW TO: Convert from 7199 to 6U8 Phase Splitters

However, with a simple re-wiring of the 7199 tube sockets the stock circuits are identical. WARNING: DO NOT SUBSTITUTE a 6U8-family tube for a 7199 without first performing this MODIFICATION!
Use the table below as a guide:

 Pentode Function

7199 6U8A -- 6GH8 -- 6BL8
Anode/Plate Pin 2 Pin 6
Control Grid Pin 7 Pin 2
Cathode Pin 6 Pin 7

Typical Scott Amplifier (with "Split-Load" Phase Inverter -- 6U8) Schematic -- Type 208


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