Help understanding bias resistors, please!
Posted: Mon Feb 10, 2020 4:19 pm
Hello all...
I have a Super Lead replica that I built from a kit I got from Mojotone. (Not sure what the general consensus is around this forum about the Mojotone kits. I really like the SLP replica and the JTM45/JTM50 replica I have built from their kits, though.)
The amp has been operational and working well since completing it for a good few years now. I *thought* I understood pretty well about the bias resistor and changing the value of the resistor in order to adjust the "window" that the bias adjustment pot sweeps. If you see the pic I've attached, I'm referring to the 47k resistor circled in red. One end of that resistor goes to an outer lug on my 10k linear bias adjustment pot, and the other end goes to ground. I'm not quite sure if this resistor is actually acting as a voltage divider, like I've read about in some places, or if it's just working with the 10k resistance of the pot to add overall resistance with the pot acting as a variable resistor. So that's my first question--how exactly is my bias pot operating in this case. (For reference, the middle lug of my 10k bias pot is connected to "G", circled in blue, at the base of the two 220k resistors, and the other outer lug of the 10k bias pot is connected to "H", also circled in blue, on one side of the 15k resistor between the 8uf caps.)
I've understood that if my "bias window" is too cold or too hot for me to get a good idle bias on my tubes, then I can adjust the 47k resistor in value up or down a little bit to "move my window".
But then recently, it was explained to me that the 22k resistor (circled in red in my picture) is the primary resistor that adjusts my bias voltage. It connects to my 103v tap on my PT (not in the picture), and then goes thru the 1N4007 diode before connecting to that same 15k resistor.
So now I'm wondering if I've been doing it "wrong"...? I've acheived the results I needed by adjusting the value of the pictured (red-circled) 47k resistor in order to adjust my bias pot's "window"... But I'm wondering if I should have been adjusting the 22k resistor instead? I'm seeing the 22k resistor referred to as the "bias range resistor" in places.
If anyone can help me get educated on the detailed workings of this bias circuit, the roll of each (red-circled) resistor in question, and some detailed insight on the way my bias pot is functioning (voltage divider vs variable resistor), I'd greatly appreciate it. Maybe the key question is, if adjusting the 47k resistor-to-ground value successfully changes my bias window, and if adjusting the 22k resistor *also* has influence on my bias window, why should/shouldn't I change one versus the other?
Sorry for the lengthy post. Hope the pic and details are clear. Appreciate any help. Thanks in advance.
I have a Super Lead replica that I built from a kit I got from Mojotone. (Not sure what the general consensus is around this forum about the Mojotone kits. I really like the SLP replica and the JTM45/JTM50 replica I have built from their kits, though.)
The amp has been operational and working well since completing it for a good few years now. I *thought* I understood pretty well about the bias resistor and changing the value of the resistor in order to adjust the "window" that the bias adjustment pot sweeps. If you see the pic I've attached, I'm referring to the 47k resistor circled in red. One end of that resistor goes to an outer lug on my 10k linear bias adjustment pot, and the other end goes to ground. I'm not quite sure if this resistor is actually acting as a voltage divider, like I've read about in some places, or if it's just working with the 10k resistance of the pot to add overall resistance with the pot acting as a variable resistor. So that's my first question--how exactly is my bias pot operating in this case. (For reference, the middle lug of my 10k bias pot is connected to "G", circled in blue, at the base of the two 220k resistors, and the other outer lug of the 10k bias pot is connected to "H", also circled in blue, on one side of the 15k resistor between the 8uf caps.)
I've understood that if my "bias window" is too cold or too hot for me to get a good idle bias on my tubes, then I can adjust the 47k resistor in value up or down a little bit to "move my window".
But then recently, it was explained to me that the 22k resistor (circled in red in my picture) is the primary resistor that adjusts my bias voltage. It connects to my 103v tap on my PT (not in the picture), and then goes thru the 1N4007 diode before connecting to that same 15k resistor.
So now I'm wondering if I've been doing it "wrong"...? I've acheived the results I needed by adjusting the value of the pictured (red-circled) 47k resistor in order to adjust my bias pot's "window"... But I'm wondering if I should have been adjusting the 22k resistor instead? I'm seeing the 22k resistor referred to as the "bias range resistor" in places.
If anyone can help me get educated on the detailed workings of this bias circuit, the roll of each (red-circled) resistor in question, and some detailed insight on the way my bias pot is functioning (voltage divider vs variable resistor), I'd greatly appreciate it. Maybe the key question is, if adjusting the 47k resistor-to-ground value successfully changes my bias window, and if adjusting the 22k resistor *also* has influence on my bias window, why should/shouldn't I change one versus the other?
Sorry for the lengthy post. Hope the pic and details are clear. Appreciate any help. Thanks in advance.