Bias adjustment -Meters and scredrivers - diyAudio
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Old 19th February 2014, 01:28 AM   #1
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Default Bias adjustment -Meters and scredrivers

So I wish to simply bias adjustment as much as possible whilst maintaining a safe approach.

I would like to install VU meters that measure the requisite voltages to help me establish if a tube has drifted from an acceptable position.

I also plan on having access holes in the chassis to allow me to turn the trimmers to make the necessary changes. I figure these screwdrivers Klein 2-Piece Insulated Screwdriver Set would be a safe option for digging into a live chassis or would I be better saving money and going for a ceramic screwdriver? Eclipse Ceramic Screwdrivers - Philips Blade Tip - at the Test Equipment Depot

Thanks.
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Old 19th February 2014, 04:44 AM   #2
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That klein looks like it may be too big.
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Old 19th February 2014, 11:09 AM   #3
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PDL: Thanks; I need to check the dimensions just to be sure. Those trimmers are tiny
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Old 20th February 2014, 04:41 PM   #4
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You should be able to find a plastic tweak tool at Radio Shack or similar.
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Old 20th February 2014, 05:25 PM   #5
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Thanks Russ, I'll take a look next time I am in.

I'm still in two minds about the panel meters; I would like to be able to adjust the trimmers and see the effects on the meters but it's not part of the build so I would be flying blind a little. Google isn't turning up much but I am still working my way through Morgans 3rd edition so maybe I'll grasp what is needed by the time I finish it
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Old 22nd February 2014, 06:18 AM   #6
tomchr is offline tomchr  United States
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In my Damn Good 300B amp, I drilled holes in the top plate for trimpot access. The holes are 1/8" (3.2 mm) in diameter as I recall. Maybe 4 mm. They're centered over the trimpots. To prevent accidents from a slipping screwdriver, I fitted a "cone of shame" made from heat shrink tubing over the trimmers. I trimmed the height of the cone of shame so it just touched the inside of the top plate when the circuit board was mounted. It seems to work pretty well...

I'd still use a screwdriver with a plastic handle, though. Also, the slot in the trimpots I use is made from plastic.

~Tom
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Old 22nd February 2014, 03:47 PM   #7
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Thanks Tom, I was thinking of a very similar setup so I am pleased to see you have something that works.
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Old 8th March 2014, 02:19 AM   #8
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SO who uses meters to help them set bias and how did you configure them?

I want to be able to do as much as possible to keep the amp running as sweet as possible without having to drop the PCB from the top panel.

Thanks.
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Old 8th March 2014, 03:59 PM   #9
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I do. I happened to have FFT program which shows distortion level and the characteristics of distortion. I set mine to minimize it which is around 65mA. Other tubes and OPT have different point. You can just tune it by ears (not easy but doable).
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Old 8th March 2014, 04:58 PM   #10
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Quote:
Originally Posted by Evenharmonics View Post
I do. I happened to have FFT program which shows distortion level and the characteristics of distortion. I set mine to minimize it which is around 65mA. Other tubes and OPT have different point. You can just tune it by ears (not easy but doable).
Yet ANOTHER reason for me to come visit
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