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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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21st Aug 2018, 6:22 pm | #1 |
Pentode
Join Date: Aug 2011
Location: Brighton, East Sussex, UK.
Posts: 149
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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 |
22nd Aug 2018, 10:32 am | #2 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,858
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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 |
23rd Aug 2018, 6:04 pm | #3 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,858
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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 |
9th Sep 2018, 6:29 pm | #4 |
Pentode
Join Date: Aug 2011
Location: Brighton, East Sussex, UK.
Posts: 149
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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 |
14th Sep 2018, 3:50 pm | #5 |
Pentode
Join Date: Aug 2011
Location: Brighton, East Sussex, UK.
Posts: 149
|
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. |
14th Sep 2018, 3:51 pm | #6 |
Pentode
Join Date: Aug 2011
Location: Brighton, East Sussex, UK.
Posts: 149
|
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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14th Sep 2018, 3:55 pm | #7 |
Octode
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,813
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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 |