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TSE Coarse/Fine Bias Adjustments

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Greetings!

I have a successful TubeLab SE build (completed around 2012) that I would like to modify. I have vintage panel meters on the amp to monitor each of the 300b tubes bias. It appears that one of the tubes is aging differently than the other, and so my bias is drifting (albeit very slowly & only a few ma).

This is where my issues lies. Adjusting the bias is a huge ordeal. I have to remove the tubes, flip the amp over, remove the bottom plate, tape spacers to the transformers (so that the tubes can fit while the amp is upside down), remount the tubes, and then start the bias procedure. My PCB is mounted upside down, with most of the components on the bottom and the tube sockets on the top. If I had thought of it, I would have mounted the trim-pots on the socket side and drilled access holes to stick a screwdriver in. As it is, it's waaaay to much work to remove the PCB to do so.

So, I'd like to mount 2 new pots on the back of the chassis. I'd like them to work in tandem with the existing PCB mounted pots (in a kind of Coarse/Fine adjustment arrangement). I've looked around to try and find how to do this, and it seems that the proper way is to use a dual gang pot (with one half on each side of the original pot). I've included a portion of the orignal schematic, and the proposed changes.


Q1: Is the schematic shown correct and will it work as expected?

Q2: Using the values specified (the original 100K pot and the new 20K dual gang pot) what kind of adjustment range can I expect? I'd like to have 5~7ma of adjustment, once the main 100K pot is set.

Q3: Will there be any oscillation or sound issues by running wiring from the PCB to the new pots at the back of the chassis?

Q4: Any safety issues with having these pots easily accessible (looks like -150V could be present?) I will be using plastic knobs (held on with set screws).







Thanks for any support/advice!
 
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It appears that one of the tubes is aging differently than the other
Were they both same age (new?) output tubes?

and so my bias is drifting (albeit very slowly & only a few ma).
Were resistors surrounding trim pot circuit good condition when you built it? If not, you may want to check their conditions to see if any of them are going bad.

I'd like them to work in tandem with the existing PCB mounted pots (in a kind of Coarse/Fine adjustment arrangement).
I don't have problem adjusting bias to make both side equal with "original" trim pot. Your proposed method may be an overkill. Personally, I would just put chassis mount 100K trim pot near by where you can reach from the top of chassis and solder wires to PC where original pot is removed.

Something like this maybe.
An externally hosted image should be here but it was not working when we last tested it.
 
Were they both same age (new?) output tubes?
It's been quite a while since I got this set of tubes, but I think they may have been used. So no way to really tell their actual age.

Were resistors surrounding trim pot circuit good condition when you built it? If not, you may want to check their conditions to see if any of them are going bad.
Yes, both trim-pots and resistors were purchased new from Digi-key. I haven't checked them recently; I will do so the next time I have the chassis open.

I don't have problem adjusting bias to make both side equal with "original" trim pot. Your proposed method may be an overkill. Personally, I would just put chassis mount 100K trim pot near by where you can reach from the top of chassis and solder wires to PC where original pot is removed.

Something like this maybe.
I really like this idea! I guess there are no issues with oscillations or noise by doing this? I guess I could just try it, and see. Thanks for the suggestion!

I think you've given me some great advice!
Thank You!
 
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