Bias Pot Idea

[antiquekid3]

I bet this has been thought of before. But just in case...

So the standard bias pot in the ST-70 is 10K. But if the wiper lifts, the tubes aren't biased and start glowing. My idea is simple: take a 20K pot and tie the wiper to both sides of the pot through two 10K resistors. So if the pot is in the very center (assuming it's linear), the total resistance from one end to the other would still be 10K. However, if you move the pot to one extreme or the other, according to my calculations the resistance would drop to 9.33K. I don't think that's a problem. But when the wiper lifts, it would see half of the maximum bias voltage.

Is this a good way to eliminate the problem of failing pots? Is it the easiest?

Kyle

[Quad]

The power supply schematic which Gregg (geek) posted earlier
has failsafe bias. Details here -

http://www.classicvalve.ca/docs/ST-70_P ... FSbias.pdf

[Geek]

And the thread on it here:
http://www.diytube.com/phpBB2/viewtopic ... t=failsafe

(pic kaput, but Quad has the link w/docs)

Cheers!

[antiquekid3]

Okay, so you add a balance control to adjust the bias of all 4 tubes somewhat individually. Not perfectly individually (adjusting for one tube adjusts the other), but if I understand correctly...they can all be adjusted to get the same cathode current, right?

If I'm not looking at adjusting all four somewhat individually, will my way work just as well? And by the way, I made a miscalculation...it's not 9.33K when the pot is all of the way to one side; it's 6.67K. Dunno how that happened! :-)

I guess what I have made is kind of like a Wheatstone bridge...just with a short instead of an ammeter!

Kyle