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swordholder · 2nd Jan 2010, 9:33 am

Rob,
I'll try to answer your points. The anode current meter is indeed a shunted voltmeter, a resitor of 1 ohm will generate 1mV when 1mA is passed through it. The 200mV module (actually 199.9 mV) will actually read to a resolution of 0.1Mv , current can be read to 0.1Ma . Tested an ECC83 and read anode current of 1.8Ma.
I use approx 100mV of audio on the grid, Similarly, the AC Mv meter reads to 0.1Mv and the ECC83 reads 1.7 mA/V.
The modlules I used will operate from 6v AC so admittedly the transformer winding is at HT, but so far no peoblems, and I dont Anticipate any. The modules are well insulated from the panel. Go to the website and have a look at the pictures.
I did think about a transformer in the anode like AVO do with the VCM163, but decided repeatability would be a problem, and I'm not too sure how a small transformer woiuld react to constant DC. Would it saturate the core? plus of course a resistor is cheaper.
Do you run your tester from DC like mine? It may be possible with your design that the transformer is picking up hum from the mains transformer giving false readings.
In my design, the GM meter which is running all the time reads zero for 99% of the time with an occasional flick to 000.1, which may be the zeroing of the meter itself. Nothing to get worked up over.
Hope this answers your questions
Regards
Mike

2nd Jan 2010, 9:33 am	#30
swordholder Rest in Peace Join Date: Aug 2009 Location: East Preston, Sussex Posts: 162	Re: Homebrew Valve Tester Rob, I'll try to answer your points. The anode current meter is indeed a shunted voltmeter, a resitor of 1 ohm will generate 1mV when 1mA is passed through it. The 200mV module (actually 199.9 mV) will actually read to a resolution of 0.1Mv , current can be read to 0.1Ma . Tested an ECC83 and read anode current of 1.8Ma. I use approx 100mV of audio on the grid, Similarly, the AC Mv meter reads to 0.1Mv and the ECC83 reads 1.7 mA/V. The modlules I used will operate from 6v AC so admittedly the transformer winding is at HT, but so far no peoblems, and I dont Anticipate any. The modules are well insulated from the panel. Go to the website and have a look at the pictures. I did think about a transformer in the anode like AVO do with the VCM163, but decided repeatability would be a problem, and I'm not too sure how a small transformer woiuld react to constant DC. Would it saturate the core? plus of course a resistor is cheaper. Do you run your tester from DC like mine? It may be possible with your design that the transformer is picking up hum from the mains transformer giving false readings. In my design, the GM meter which is running all the time reads zero for 99% of the time with an occasional flick to 000.1, which may be the zeroing of the meter itself. Nothing to get worked up over. Hope this answers your questions Regards Mike