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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
Maybe it means metal but not sure.
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
gt= grid triode
pd1=plate diode 1
pd2 =plate diode 2
pt= plate triode
hope this helps. Gord
Thanks to Gordon Dann, John Greenland and Sterling Spurrell for the advise on the pin designations. Much appreciated.
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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