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Old 2nd June 2013, 12:26 AM   #1
SandyG is offline SandyG  United States
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Default DIY Bias Buddy

Waiting on PCB's to be fabricated for the Quad KT88 (EZ125 Quad KT88 Project) I decided to keep forging ahead and build my own 'Bias Buddy'

Was going to pick up off of Ebay one of the Bias Setting Tools but they all use the tube socket style connection which I don't really need. Also they only display one tubes bias at a time.

So instead I picked up some small EBay special's 5 Digit LED voltmeters. I picked up 4 Each in a different colors. They were about 10 bucks each shipped from China. The rance is 0-33.333 vdc

Each color which will match the Bias Measurement plugs and jacks to keeps things straight.

Picked up a small 4.67" x 3.68" x 1.18" Hammond die cast box (P-H1590BBCE-BK) and made a simple CAD template for the meters and did some drilling and dremel work with a final file to fit.

Recycled an old AC wall wart and wired it up.

Cost for the project was about 50 bucks and an hour or so of time.

Enjoy

Sandy
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Last edited by SandyG; 2nd June 2013 at 12:30 AM.
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Old 2nd June 2013, 12:29 AM   #2
SandyG is offline SandyG  United States
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Here are the rest of the pics...
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Last edited by SandyG; 2nd June 2013 at 12:32 AM.
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Old 2nd June 2013, 04:10 AM   #3
dgta is offline dgta  United States
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Not sure how exactly you are planning to use those, but I have a couple of words of caution.

1. Check the meters. I bought those before and they were both individually inaccurate and different between units.
2. Not sure if you're measuring the bias voltages to each tube or the actual currents, but the 33V range would suggest the former. I would want to measure small voltages across a shunt in each cathode instead.
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Old 2nd June 2013, 06:03 AM   #4
SandyG is offline SandyG  United States
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These are measuring the bias across the 10 ohm cathode resistor, so all good on that front with the lower voltage, I also checked them at under 1 volt for accuracy and all seems good, they all track the same.

The meters were all within one number (.001) or exact at any measurement I took so they seem to be a good batch. That was the first thing I did. I also verified against my Fluke and it was accurate to the number of digits on the display of the LED to the same digits on the Fluke so I think I'm good.

Thanks for the heads up, I think it's a very good thing to check some of parts before putting them into service.

Sandy
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Last edited by SandyG; 2nd June 2013 at 06:07 AM.
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