Bias adjustment -Meters and scredrivers

[moggi1964]

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.

 

[PDL]

That klein looks like it may be too big.

 

[moggi1964]

PDL: Thanks; I need to check the dimensions just to be sure. Those trimmers are tiny

 

[rknize]

You should be able to find a plastic tweak tool at Radio Shack or similar.

 

[moggi1964]

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

 

[tomchr]

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

 

[moggi1964]

Thanks Tom, I was thinking of a very similar setup so I am pleased to see you have something that works.

 

[moggi1964]

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.

 

[Evenharmonics]

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).

 

[moggi1964]

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

 

[Evenharmonics]

You can download a temp free version from Virtins.

I live far away from NJ. I think there a few TSE owners living near you.

Here's recommended bias for 300B tube (scroll down). Here's step by step process.

When I set it from 62mA to 68mA, I couldn't hear the difference. It should sound good if you have it somewhere in the mid range.