I had them all - except the impossible to find U getters. Rankings from Tube World
English Description:
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CCa was a E88CC specially selected for "Post Germany"; special low noise, low microphonic, long-life 6922.
Philips had similar selections for the "Dutch Post", some tubes are selected had "PTT" etched.
The "German and Dutch Post" = telephone, telegraphie, telex, and the postal system.
Much of the telephone centrals used tube equipment and for telephone equipment you want a "noise-free" enviroment.
The letters "CC" probably equates to "double triode", and/or a for special design.
The Best Sounding CCa=6922 ever made
TOP 5:
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1) CCa Siemens & Halske 1950's "U" getter halo and 1960's "O" getter halo
"Rarest and most sought after CCa, has "gray shield" between plates,
Most realistic sounding holographic soundstage, pure seductive sonic joy,
complex symphonic images emerge effortlessly"
2) CCa Telefunken West Germany 1960's
"excellent neutral holographic soundstage, vast vocabulary of tone
establishes remarkable layers of harmonics, very rare"
3) CCa Siemens & Halske A-FRAME construction late 1960's - early 1970's
"beautiful open air holographic images, low microphonic tube construction, rare"
4) CCa LORENZ West Germany early 1960's
"beautiful open air holographic images, very rare"
5) CCa VALVO Heerlen Holland 1960's
"real sonic holography, extremely rare"
6) CCa Siemens Rohre A-Frame early 1973-1974 (in stock, silver shield)
6922 with "GRAY SHIELD" 1950's - 1965 / "SILVER/CHROME SHIELD" 1966 - 1980's
The "shield" is the "gray" or "shiny silver" plate (electrostatic shield) that is situated between
the 2 gray dual triode plates inside a framed grid tube, such as a 6922, 6DJ8 and 7308.
A "framed grid" tube is a high amplification(mu) VHF triode with a slotted frame around
the grid to shield part of the grid structure and allow a controlled field of electrons
to flow through to the plate.
