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Old 18th Feb 2018, 12:29 pm   #1
cathoderay57
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Default X79 valve Spec Verification

Hello, I'm finishing off restoration of a McMichael 151. Well pleased with the performance - pair of N78 triode-connected pentodes in push-pull sounds very good. I've tested the valves using a VCM163 but the test settings for the X79 seem to vary depending where you look; even allowing for the variations in Grid1 voltages the specs don't seem consistent:

Avo VCM163 manual:
X79 (Osc) Va=100; Vg=-2; Ia=5.0mA; gm=2.2
X79 (Mix) Va=250; Vg1=-3; Vg2=75; Ia=10.0mA; gm=3.0

AVO valve data errors (corrections by Philip Taylor BVWS Journal):
X79 (Osc) Va=100; Vg=-2; Ia=5.0mA; gm=2.2
X79 (Mix) Va=250; Vg1=-2; Vg2=75; Ia=4.5mA; gm=2.2

A.M. Ball (9th Ed):
X79 (Osc) Va=100; Vg=0; Ia=10.0mA; gm=2.8
X79 (Mix) Va=250; Vg1=0; Vg2=75; Ia=4.5mA; gm=0.78

Bernards Radio Manual No.100:
X79 (Osc) Va=100; Vg=0; Ia=4.5mA; gm=0.78
X79 (Mix) Va=250; Vg1=0; Vg2=75; Ia=4.5mA; gm=0.78

Which one to use??
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Old 18th Feb 2018, 1:17 pm   #2
Boater Sam
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Default Re: X79 valve Spec Verification

Well they all look plausible.
The differences in Ia are due to the different Vg1 voltages, bias and possibly the tester in use.
The best valve tester is the set itself.
Do your results with your VCM163 match that published set of figures? They are the hardest drive settings.
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Old 18th Feb 2018, 2:49 pm   #3
Mr Moose
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Default Re: X79 valve Spec Verification

Hello,
The Marconi data sheet is here https://frank.pocnet.net/sheets/126/x/X79.pdf.
The gm (0.78) figures given in the Ball and Bernards data are actually the conversion conductance (gc) figure not gm. (The hexode has two control grids, its own and the grid connected to the triode grid
.)
My Bernards book says "gm or gc" above the relevant column.
The Marconi data says Ia=4.5ma for typical operation which might suggest using the corrected AVO data.
Yours, Richard

Last edited by Mr Moose; 18th Feb 2018 at 3:04 pm.
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Old 18th Feb 2018, 4:45 pm   #4
FERNSEH
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Default Re: X79 valve Spec Verification

X79: https://www.radiomuseum.org/tubes/tube_x79.html
"The gm (0.78) figures given in the Ball and Bernards data are actually the conversion conductance (gc) figure not gm."

The conversion conductance of the X79 triode-hexode compares well with the ECH81:
https://www.radiomuseum.org/tubes/tube_ech81.html

DFWB.
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Old 18th Feb 2018, 10:29 pm   #5
cathoderay57
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Default Re: X79 valve Spec Verification

Thanks all; I guess the Marconi Sheet ought to be correct as they made the valve. The figures in it agree with Ball so I'll go with that. When I tested mine previously I used -2v on the grid for both triode and hexode tests: measured values for triode were Ia=3.8mA; gm/gc=0.9 and for the hexode Ia=1.0mA; gm/gc=0.4. I will try again with the Marconi/Ball settings but it's looking like this X79 might be past its best. It does still work though. Cheers, Jerry.
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Old 18th Feb 2018, 11:11 pm   #6
turretslug
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Default Re: X79 valve Spec Verification

Valve-testers are relatively simple devices, whatever the latter-day hype around them, and they test mixer sections as if they were straight amplifiers, hence the conductance figure quoted being gm, rather than the expected gc and resulting in somewhat higher conductance figures than if operating as multiplicative mixers with varying signals on both controlling grids. The maths says that gm can approach 4x gc, but that assumes optimum operating conditions for both cases of operation (amplifying gm and mixing gc). A device to measure gc would be a rather more complex (though not impossibly so) device with more variables to set up and not really worth doing when the everyday valve-tester can give a guidance figure as to the related gm.
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Old 20th Feb 2018, 9:53 pm   #7
cathoderay57
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Default Re: X79 valve Spec Verification

Hello Moderators, I'm happy with the answers and links so this thread can be closed. Kind regards, Jerry
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