Could someone verify that this bias pot is wired correctly please ?

[gtrplayr1976]

I would like to make sure this is wired correctly. I copied the wiring from another post. This is an X100B RevK. 100K pot , 27k resistor ,wiper ,and 1 terminal wired together (black wire) to grid resistors (R127) , and 27K resistor and red wire wired to remaining lug grounded to back of pot.

[Gtr_tech]

I wouldn't do it that way. Its hard to tell what's what since its a pcb amp and you can't tell where the connections are without pokin' around with a meter.

There's 2 ways to do it. Either do it the way the mfgr suggests and put a pot in place of R127 to dial it in, or (the way I do it) replace R126 with a pot/resistor combo and change the voltage via the gnd reference. That way, if the pot goes open or intermittant, you get max available bias....not the other way around.

Amps that use a pot in series with the bias feed is always a crapshoot. Pots get flaky...and you don't wanna lose bias. Ever. I will always convert that type to a variable gnd ref type for reliability.

[gtrplayr1976]

So leave R127 , and replace R126 (220K) with same value in pot/resistor form ? ,and how would that be wired ?

[Gtr_tech]

Just a straight series connection. A 100k pot in series with a 150k 1/2w should give decent range. I forget the exact values I used. Its been 15 years since I did that on my main X amp.....and that sucker has never let me down once in that time gigged out regularly.

[gtrplayr1976]

That's great. What is your definition of series ? Use the wiper ,and one lug with the resistor , leave the other unused ?

[Gtr_tech]

Yep. Or you can short the unused outside lug to the wiper. Doesn't matter much either way. All yer doin' is making that fixed 220k into a variable 150k-250k.

[gtrplayr1976]

Cool, that's the way I have it now except on R127. Does it make any difference which lead goes where ?

[Gtr_tech]

Since its a series connection, lead orientation doesn't matter.....it does the same thing either way. But, as I stated before, you can run into bias loss issues if that pot ever goes open. You've been warned.

[gtrplayr1976]

I totally understand the possible consequences. I will probably change it to R126. Thanks