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    Les Dickson
    CVRS Member

    When looking at schematics for specific tubes there are letters beside the pins that indicate what the connection is. For example heaters are sometimes shown as h and other times as f. Cathodes seem to usually be k but there are some that I just can’t figure out.
    In the attached images

    • what does m mean in 6J5?
    • what are the different pins that start with p (pins 4, 5, 6) in 6SQ7?
    • why is pin 2 in 6SQ7 marked as GT as opposed to G, or G1, G2 etc?
    Sterling Spurrell
    CVRS Member

    Maybe it means metal but not sure.

    John Greenland
    CVRS Member

    Evening Les’

    In a 6J5 with no letters , the pin marked “m” is connected to the metal case. (check it with an ohmmeter if you have a metal 6J5. In some tubes S or IS is a connection to an internal shield.

    A 6SQ7 has in effect, 3 plates. 2 act as diodes and are labelled PD1 and PD2. The other P is the plate for the triode and is labelled PT.

    I can’t help you with “GT” other than to suggest it is leftover from something else.
    In a tube with a triode and a pentode such as a 6AU8, the grid of the triode is labelled GT and the grids of the pentode as g#P.
    There is no real requirement for it to be labelled GT as opposed to just G. in a single triode tube such as a 6SQ7.

    Just my 2 cents. Perhaps there is a better reason out there .

    John G. VO1 CAT

    Gordon Dann
    CVRS Member

    gt= grid triode
    pd1=plate diode 1
    pd2 =plate diode 2
    pt= plate triode
    gt=glass tube
    hope this helps. Gord

    Les Dickson
    CVRS Member

    Thanks to Gordon Dann, John Greenland and Sterling Spurrell for the advise on the pin designations. Much appreciated.

    Jerry Proc
    CVRS Member

    From the book Tube Lore:

    G suffix means large , spherical tubular (ST) bulb
    GT suffix means small tubular (T9) bulb
    GT/G is a GT size bulb which is a replacement for either G or GT bulbs.

    The is no definition for 6J5M, however M/G is listed as metal-glass construction.
    Hope that helps.

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