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Old 23rd Nov 2021, 6:25 pm   #21
David Simpson
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Default Re: How to use this valve tester?

Haan Anwash,
AVO & many other makers use either valve or semi-conductor rectifiers. The AVO MK3 uses D77 small double diode valves, for example. But their main use is to just half-wave rectify sinewaves for 10mS pulses of voltages on the screen(if used) or grid. The AVO's have full P-P sinewaves on a valve's anode(& hence the meter, so take care). The meter balancing(backing-off) network also requires a half-wave pulse. No capacitive input filters with a big choke & large electrolytic capacitors.
I'll attach a "loosely pictorial" circuit representation of an AVO CT160's( a MK3 in a military clamshell case, more or less) operation. AVO's full circuit diagrams are similar to Calcutta's or Mumbai's Railway Station network, so difficult to follow.

Regards, David
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Old 23rd Nov 2021, 8:30 pm   #22
ms660
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Default Re: How to use this valve tester?

There was a modification for the Avo VCM MKIII to convert from valve rectifiers to silicon rectifiers, the anode supply is full sinusoidal AC, the screen grid supply is raw unfiltered half wave, and the control grid supply is raw unfiltered full wave.

I did a drawing some moons ago of a simplified part of the Avo VCM MKIII schematic which shows the control grid waveforms (see attached drawing) showing half wave for calibrating the voltage supply to the control grid circuit and full wave for normal operation...'scooze the scrawl.

Lawrence.
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Old 24th Nov 2021, 12:07 pm   #23
David Simpson
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Default Re: How to use this valve tester?

Quote:
Originally Posted by Radio Wrangler View Post
The valve testing circuits may be AC, but the meter movement is DC, so there has to be rectification of some form to suit the meter. Sometimes the valve under test does the job, but some DC is needed as well.

David
Sorry David, not the case. Full Anode P-P sinewave can be measured on both meter terminals. Admittedly, just a few mV apart, thus never hopefully exceeding the 30uA limit. However, if one had good enough eyesight, the meter's needle could be seen vibrating with 10mS pulses every 20mS. This phenominon is actually evident with the huge meter in the old 2 Panel VT's, if allowed to whang to full fsd. AVO in their wisdom, with their VCM's, decided to let the meter & it's resistive network network be responsible for rudimentry damping.

Regards, David
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