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Old 1st Jan 2010, 11:15 pm   #29
glowinganode
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Join Date: Apr 2009
Location: Aylesbury, Buckinghamshire, UK.
Posts: 1,522
Default Re: Homebrew Valve Tester

Mike, thanks for taking the time to publish this design.
I wonder if you might clarify / confirm a few points, as I'm not sure if I understand this circuit totally.
The Anode current meter is a DC Voltmeter, with a 1 ohm shunt the display reads directly in in Amps. This seems slightly insensitive.
The GM meter is an AC Voltmeter, with a 10 ohm shunt. Each displayed unit equates to 100mA of superimposed AC. If a 5mA/V valve were being tested, then to get a reading of 5.0 on the display would require 500mA's of superimposed AC, and 100 Vac signal on the grid. This seems too high. Also should the voltage across the shunt be AC coupled to the meter as it has DC component too.
The DVM modules I've used in the past require a DC auxilary supply of around 12 Vdc, assuming yours are the same, where do you obtain this from as both the above meters are at Anode potential.
I'd be inclined to decouple the Screen and Anode supplies after the S/R's to reduce any tendancy of parasitic oscillations, and to reduce the output impedance of the Anode supply appearing as additional Anode load which might cause inaccurate GM readings.
I am currently building a Valve tester and would like to incorperate parts of your design as you have obviously put a lot of thought and effort into yours.
I've used a small audio output transformer in reverse to measure the Anode AC component, as this gives a reasonable signal level from a low apparent impedance, removes the DC component, and isolates it from the HT so the metering circuit can be at ground level. I've experimented with a band pass filter (2KHz - GM test frequency), to reduce the effect of ripple and the amount of HT smoothing required.
Many thanks in advance, hopefully your advice will help resume a stalled project.
Rob.

P.S. I've just read your reply above which you must have posted at the same time as me. I assume the meters are scale 0 - 200mA, and 0 -20mA/V, with 1 Vac on the grid (you might find this is a little high for some valves). It's quite easy to modify these DVM modules to get the required D.P. to illuminate with a suitably placed resistor.
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Last edited by glowinganode; 1st Jan 2010 at 11:25 pm.
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