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Old 21st Mar 2019, 4:58 pm   #1
G3PIJpeter
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Default LM13 heterodyne wavemeter

The LM 13 manual gives the VFO valve as type 77, which is (according to the R-Type website) a sharp-cutoff pentode similar to the 6J7G. However the manual shows it to be a hexode in all schematic diagrams (see thumbnail). Is this a strange US Navy convention from 1943 - or what? I find it rather confusing.

- Peter
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Old 21st Mar 2019, 7:36 pm   #2
M0SOE_Bruce
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Default Re: LM13 heterodyne wavemeter

I think it's a quirk of whoever drew the diagram. The 77 is listed as a triple grid detector in the RCA, Tung-Sol, Sylvania and Brimar manuals therefore a pentode. Looking at the thumbnail two of the grids are shown to be joined internally so only three grid connections are external.
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Old 21st Mar 2019, 8:17 pm   #3
G3PIJpeter
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Default Re: LM13 heterodyne wavemeter

Quote:
Originally Posted by M0SOE_Bruce View Post
Looking at the thumbnail two of the grids are shown to be joined internally so only three grid connections are external.
Agreed - but the thing I find confusing is the function of the grids.
Usually in the symbol for a valve, the vertical arrangement of its parts are cathode - control grid - screen grid - supressor grid - anode.
Looking at the circuit connections to the 77 valve in this diagram, we seem to have cathode - screen - control - screen - supressor.
But, as you say, perhaps it's just an aberration.
- Peter
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Old 22nd Mar 2019, 10:25 am   #4
M0SOE_Bruce
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Default Re: LM13 heterodyne wavemeter

Looking at the US Army version (SCR211 family) the early models also use a 77 valve. BC221A,C & D schematics show the same valve drawing so it appears to relate to early US military documentation.

I don't know why they did it but I don't think there is a second screen grid.
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Old 22nd Mar 2019, 12:34 pm   #5
G3PIJpeter
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Default Re: LM13 heterodyne wavemeter

Quote:
Originally Posted by M0SOE_Bruce View Post
I don't know why they did it but I don't think there is a second screen grid.
If there were two screen grids, attached to each other internally and therefore at the same positive potential, a control grid between the two would not have any effect on the electron flow from cathode to anode - or would it?

The third picture down at http://www.r-type.org/exhib/aay0034.htm shows that the control grid (connected to the top-cap) is the electrode closest to the cathode.

But then, there are times when the US just seems to like being perverse! Look at the way they draw their 'schematics' compared to the UK 'theoretical diagram' strung between parallel earth and HT rails. The HRO and the R106 diagrams are a case in point.

- Peter
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