express bias pot question

[pinkmarkos]

hello everybody- i just started my express build and i have a question about how the bias pot should be wired. is the the bias pot grounded to anything? if so, where is the ground connection? also- the bias pot from the BOM has three prongs, one in the center and two on the edge. it is unclear to me from the layout which prong should go where. from the francesca pics (different type of bias pot I know) it seems the center prong is attached to the 47K resistor and the other lead is attached to the 10uF bias cap. i can't tell where the 3rd lead is going. if someone could help, i would appreciate it- thanks

[stubbyfex]

You are correct, the center does go to the 47k resistor. One of the outside legs will go to ground, and the other outside leg will go to the 220k resistors which in turn feed the power tubes and their grid resistors. IF you hook the outside terminals of the bias pot up wrong, when you turn it clockwise, the voltage will decrease instead of increasing. If that is the case, then just swap the outer two wires.
Keep soldering!

Oh, I'm using the "Original Kelly '90" schematic.

[gahult]

Try this.
The center and 1 leg to ground.
These layout diagrams are in the files section of the forum.

Gary

[pinkmarkos]

thanks guys- i am using the layout in the files section. i think my problem was that the lugs on my board weren't quite the same as they appear in the layout. however, i do understand now. sometimes you just need another point of view (from someone more experienced) to help you when you're stuck- mark

[Ron Worley]

You are correct, the center does go to the 47k resistor. One of the outside legs will go to ground, and the other outside leg will go to the 220k resistors which in turn feed the power tubes and their grid resistors. IF you hook the outside terminals of the bias pot up wrong, when you turn it clockwise, the voltage will decrease instead of increasing. If that is the case, then just swap the outer two wires.
Keep soldering!

Oh, I'm using the "Original Kelly '90" schematic.

Um no....

As noted by Gary, the original layout diagram and Schematic did not have the 3rd leg of the pot grounded- the new versions do.

The 1st lug goes to the 47K, the Wiper and the 10uf (not a "220K resistor) are grounded as is the 3rd lug.... check the layout in the files section

[pinkmarkos]

so then, the center leg of the bias pot is the same as the wiper? in that case, like Ron said- we have one leg to 47K, the center leg/wiper to 10uF then to ground, and the third leg also to ground- right? this is how i have wired it.

[gahult]

You are correct.
Gary

[Ron Worley]

Ya got it!!

[M Fowler]

Good thing this question came up because I have a Joebob express board that had been shipped with the trimmer not in place as so it did not get damaged in shipping. I had figured it was wired this way but Joebob's layout is slightly different but correct.

Thanks guys.

Gary,

I will work on that air brake tonight, thanks for the phone call. Got a few hours of sleep, up at 6:30. I am working 10 hour days this week to get veteran's claims out.

Mark

[Ron Worley]

I am working 10 hour days this week to get veteran's claims out.

Mark

And we creaky old Vets really appreciate you doing it!!

[drhulsey]

... we creaky old Vets really appreciate you doing it!!

You didn't get your portion of the TARP money, yet

[Ron Worley]

I would not take it if offered.... there's too much pork getting stuffed into these supposed "stimulus" bills as it is....

[Noel Grassy]

hello everybody- i just started my express build and i have a question about how the bias pot should be wired. is the the bias pot grounded to anything? if so, where is the ground connection? also- the bias pot from the BOM has three prongs, one in the center and two on the edge. it is unclear to me from the layout which prong should go where. from the francesca pics (different type of bias pot I know) it seems the center prong is attached to the 47K resistor and the other lead is attached to the 10uF bias cap. i can't tell where the 3rd lead is going. if someone could help, i would appreciate it- thanks

Using the search function would have brought up the threads where this has been sorted out already. The Amp Garage Forum has innumerable
wise and willing souls on it, just don't wear it out.

[pinkmarkos]

in response to noel grassy's comment- i did use the search function and I came up with in excess of 2400 matches for "bias pot wiring"- hardly narrowing it down. i don't plan on "bothering" the wise old souls with every little question, and I believe that I can troubleshoot most things without resorting to forum help. again, I thank the people who gave me assistance with the understanding that I tried as hard as I could to figure it out before asking for help, but if there is an unspoken rule about asking too many questions, let me know so i can use 'em wisely.

[smokin' tone]

Using the search function would have brought up the threads where this has been sorted out already. The Amp Garage Forum has innumerable wise and willing souls on it, just don't wear it out.

No offense and not being trying to be mean but I get tired of hearing this. The search function on these boards suck. Sometimes you get great hits and sometimes not. Occasionally I've had trouble finding an old thread when I already remember half of what it said. Some guy's may not know how to narrow the search fields properly or use the correct terms. For example searching for "bias pot wiring" under Trainwreck discussion kicks back 18 pages. One of the subject titles are "Rocket w/ EF86 channel worthy, or just a DC30". Try to narrow it by using "Express bias pot wiring" and you get 38 pages. Nine of which are blank. It may be more polite to just post a link to the thread instead. That way it won't have to be answered again.

Edit: Looks like you posted while I was typing Pink.