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Old 21st Aug 2018, 6:22 pm   #1
Rogers-Lodger
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Default Avo MK4 and RCA Type 50

Hello People

Im trying to find some settings for my Avo MK4 to test an american output valve. They are Type 50 RCA Cunningham :
https://frank.pocnet.net/sheets/049/5/50.pdf
I can find them in my avo book. I guess I can work it out from the pdf . Perhaps someone could assist me.

I understand that this type of valve isnt the Avo's forte as it uses AC instead of dc for some part of its measurement.
http://www.emissionlabs.com/diversen/FAQ.htm

I have a PX 25 that draws too much current and trips the relay mechanism. Im not sure if thats just a dead valve or a problem with the AVO not being able to provide enough current....

Any advice much appreciated.

Regards

Dave
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Old 22nd Aug 2018, 10:32 am   #2
David Simpson
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Default Re: Avo MK4 and RCA Type 50

Dave, regards PX25 - If your Mk 4 has passed the valve wrt the Leakage & Insulation tests, set -ve Vg to -40V, and Va to 300V, instead of -31 & 400 V. If the c/o doesn't trip & the valve conducts 5 to 15mA, then slowly lower Vg to -31V & watch Ia rise. Maybe to 25 - 30mA. If all is well - switch from Test to Gas - should read nigh-on zero or + or - just a few uA. Then back to Test & raise Va to 350V & note Ia, then finally to the book's 400V. The book says 40mA, but it could be more. I once had an NOS CV1040 equivalent which drew 50mA at -31V Vg. I actually got it to draw 80mA at -12V, as a quick test.
If your Mk 4 still trips at any of the above Ia tests then perhaps there is a parasitic oscillation problem, damage to the valve's electrodes which weren't revealed in the basic 'C' C/H' tests, or a dodgy cut-out relay.
What is the PX25's history ? Found in a gash box ? Bought off the internet ? Or what? How does your Mk 4 cope with an EL37, or a KT66 or similar powerful beam tetrode ?

Regards, David
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Old 23rd Aug 2018, 6:04 pm   #3
David Simpson
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Default Re: Avo MK4 and RCA Type 50

Dave,
Re your American RCA 50 - my ancient Bernards Valve Manual's American Section gives the same data as my AVO VD Manual for a '50'. I.e. Pins one & four = Filament, Pin 2 = Anode, and Pin 3 = Grid. Thus the code would be 264300000. Vf = 7.5V(at1.25A), Vg = -70V, and Va = 400V. Should give an Ia of 55mA, and a Gm of 2.1mA/V if its a decent valve. More powerful than a PX25 mA-wise, but a lower Gm, therefore less of an Ia/Vg slope.
Two great thumpers of pre-war triode valves. I really hope they are both OK. Rocking horse manure status.

Regards, David
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Old 9th Sep 2018, 6:29 pm   #4
Rogers-Lodger
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Default Re: Avo MK4 and RCA Type 50

David

Thanks very much for your comprehensive reply. Ive been distracted from valves by people and work ... such is life. But I intend to return to them soon. I'll post the results of my further probing . Thanks again.

Dave
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Old 14th Sep 2018, 3:50 pm   #5
Rogers-Lodger
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Default Re: Avo MK4 and RCA Type 50

David

I set the avo as you advised and the valve drew nothing . I upper the voltage to 350v and it would happily draw 40ma it then began to rise after 50 seconds so I backed of to -31 and it held for a while perhaps a few minutes. Then it began to ride and the avo reset.
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Old 14th Sep 2018, 3:51 pm   #6
Rogers-Lodger
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Default Re: Avo MK4 and RCA Type 50

I tried to switch to gas when the valve was pulling 20ma and it trips the avo.
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Old 14th Sep 2018, 3:55 pm   #7
pmmunro
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Default Re: Avo MK4 and RCA Type 50

I hope you will not find my point too pedantic, but it might help in knowing the subject of a thread if the title made clear that the reference is to an AVO Mk 4 VCM (Valve Characterist Meter) rather than the Mk 4 version of one of the AVO multimeters.

PMM
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