Help understanding bias resistors, please!

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ESR
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Help understanding bias resistors, please!

Post by ESR » 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.
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ESR
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Re: Help understanding bias resistors, please!

Post by ESR » Tue Feb 11, 2020 4:19 pm

Anybody?

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wdelaney72
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Re: Help understanding bias resistors, please!

Post by wdelaney72 » Tue Feb 11, 2020 4:28 pm

There are better technical experts than me on here, but I can tell you how I've seen it work.

The 47K resistor that works in series with the pot impacts the bias voltage range. I will say, I typically use a 25K pot, not a 10K pot. With the pot and resistor you have, the bias range should be -30vdc through -50vdc (based on how you adjust the pot). If you lower the 47K resistor, you will shrink the bias range. lowering it to 33K might give you -30vdc through -40 vdc. I don't recommend changing this resistor.

the 22K is used to lower the AC voltage coming from the PT bias tap of approx 100VAC. Power tubes draw different bias voltage and I have encountered situations where -30vdc is still too cold and I need it lower than -30vdc. THIS is the resitor you'd want to adjust if that's the case. Raise the 22K resistor to 33K and you'll be able to get the bias range more like -15vdc through -35vdc.

Hope this helps.
Walter

"There's no great thing in being a soloist. I think the hardest thing is to play together with a lot of people, and do it right." - Angus Young, 1984

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Re: Help understanding bias resistors, please!

Post by ESR » Tue Feb 11, 2020 5:33 pm

wdelaney72 wrote:
Tue Feb 11, 2020 4:28 pm
There are better technical experts than me on here, but I can tell you how I've seen it work.

The 47K resistor that works in series with the pot impacts the bias voltage range. I will say, I typically use a 25K pot, not a 10K pot. With the pot and resistor you have, the bias range should be -30vdc through -50vdc (based on how you adjust the pot). If you lower the 47K resistor, you will shrink the bias range. lowering it to 33K might give you -30vdc through -40 vdc. I don't recommend changing this resistor.

the 22K is used to lower the AC voltage coming from the PT bias tap of approx 100VAC. Power tubes draw different bias voltage and I have encountered situations where -30vdc is still too cold and I need it lower than -30vdc. THIS is the resitor you'd want to adjust if that's the case. Raise the 22K resistor to 33K and you'll be able to get the bias range more like -15vdc through -35vdc.

Hope this helps.
Walter--your info is very helpful--thank you! I feel like I understand it a bit better... But I have some new questions.

Recently (and the reason this has come up at all), I decided to kinda switch my JTM45 to be more like a JTM50. I basically took the KT66's out and took the tube rectifier out, put some EL34's in and put a solid state rectifier "plug" in place of the rectifier tube. (The amp has a 50w PT and 50w OT, and the kit originally came with EL34's. So I'm confident that this is ok to do. And it seems to sound and respond more like a JTM50 or a 50w Bass spec. Pretty killer.) When I had been running the KT66's, instead of the 47k resistor pictured (to ground), I had to use an 82k resistor to get my bias window where I needed it. (I understand now, from what you've explained, that this is not the resistor I should be swapping out.) But before I understood what you explained, I was swapping out that 47k resistor. Anyway, when I put the EL34's back in, with that 82k resistor in place, I couldn't get the bias hot enough for my EL34's. Way too cold. I don't think it was getting above 10ma bias even with the bias pot maxed out. So I put the 47k back in there, and presto--I was back in business.

For clarification, I'm measuring my bias by using the Bias Probe tool sold by Eurotubes. It has an octal base that goes in the tube socket and the power tube plugs in the top of that octal base. It reads my plate voltage from pin 3 and displays it on one screen. Then it reads my mA and displays it on another screen. So I take my 25 watts (max dissipation for an EL34), divide it by my plate voltage (which is around 470v with the solid state recto plug), multiply that by .7 (70%) to get an idle bias, and then turn my bias pot to hit that mA value (about 37mA). When I switched from the 82k resistor back to the 47k resistor, my bias window that was originally too cold for the EL34's (maxing out around 10mA) came up enough that I could bias the EL34's at about 37mA.

Now my question--are you saying that, instead of changing that 47k resistor at all, I should leave it alone, and instead adjust the 22k resistor value... and then my "bias window" (controlled by my 10k pot) will move accordingly? Increasing the value of the 22k resistor will have a negative impact on the voltage, which will have a negative impact on my current--right? So if I'm not getting enough current to get the right mA bias reading, I need to lower that 22k resistor(to say an 18k, for example), and then my bias window (reading mA on my tubes) will go up... Am I getting this correct?

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Re: Help understanding bias resistors, please!

Post by danman » Tue Feb 11, 2020 6:16 pm

There is very little current going through the bias circuit. This is the reason you do not see any voltage drop across the 220k bias splitters. If their was any appreciable current flowing through the bias circuit, you would see a voltage drop across those. The circuit's only job is to apply a small negative voltage to the power tube's grid which in turn controls the flow of current from cathode to plate. As wdelaney72 mentioned, use the first resistor (bias feed resistor) to adjust the voltage coming into the circuit. The second resistor (bias range resistor) is there to fine tune the final voltage.

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Re: Help understanding bias resistors, please!

Post by wdelaney72 » Wed Feb 12, 2020 11:14 am

You got it, except your going the wrong direction on the 22K voltage dropping resistor. Keep the 47K resistor that's in series with the trimpot and RAISE the 22K to 33K or 39K. You shouldn't have to go any higher than 47K here. 39K is the highest I've ever had to go. If you don't have a 33K or 39K, you can do (2) 68K in parellel to get a 34K resistor.

Nothing wrong with your conversion to EL34 and diode rectifier....the OT can handle it. What you CANNOT do is the reverse....taking an EL34 based OT and pop in KT66.....that's a no-no.

In terms of bias setting.....35mA for EL34 is more than sufficient. Most current production power tubes like JJ or the New Sensor tubes won't last all that long sitting at 35mA or higher. I typically keep it at 32mA
Walter

"There's no great thing in being a soloist. I think the hardest thing is to play together with a lot of people, and do it right." - Angus Young, 1984

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Re: Help understanding bias resistors, please!

Post by ESR » Wed Feb 12, 2020 3:42 pm

danman wrote:
Tue Feb 11, 2020 6:16 pm
There is very little current going through the bias circuit. This is the reason you do not see any voltage drop across the 220k bias splitters. If their was any appreciable current flowing through the bias circuit, you would see a voltage drop across those. The circuit's only job is to apply a small negative voltage to the power tube's grid which in turn controls the flow of current from cathode to plate. As wdelaney72 mentioned, use the first resistor (bias feed resistor) to adjust the voltage coming into the circuit. The second resistor (bias range resistor) is there to fine tune the final voltage.
Thanks danman!

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Re: Help understanding bias resistors, please!

Post by ESR » Wed Feb 12, 2020 3:43 pm

wdelaney72 wrote:
Wed Feb 12, 2020 11:14 am
You got it, except your going the wrong direction on the 22K voltage dropping resistor. Keep the 47K resistor that's in series with the trimpot and RAISE the 22K to 33K or 39K. You shouldn't have to go any higher than 47K here. 39K is the highest I've ever had to go. If you don't have a 33K or 39K, you can do (2) 68K in parellel to get a 34K resistor.

Nothing wrong with your conversion to EL34 and diode rectifier....the OT can handle it. What you CANNOT do is the reverse....taking an EL34 based OT and pop in KT66.....that's a no-no.

In terms of bias setting.....35mA for EL34 is more than sufficient. Most current production power tubes like JJ or the New Sensor tubes won't last all that long sitting at 35mA or higher. I typically keep it at 32mA
Thanks wdelaney72!

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Re: Help understanding bias resistors, please!

Post by ESR » Wed Feb 12, 2020 4:24 pm

One last question--when I swap out the 22k resistor, am I correct in thinking that a 1/2 watt resistor will suffice?

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Re: Help understanding bias resistors, please!

Post by wdelaney72 » Thu Feb 13, 2020 11:48 am

yeah, 1/2 watt is fine on bias current.
Walter

"There's no great thing in being a soloist. I think the hardest thing is to play together with a lot of people, and do it right." - Angus Young, 1984

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