Official Universal Passive Tube PSU Thread
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hikowinter
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I am thinking that I am going to upgrade the PSU for Telefunken AK-47 (not MKII).
What resistors values on R1 and R2 should I use for them?
They has about +150vdc at psu out for B+ when I measured.
What resistors values on R1 and R2 should I use for them?
They has about +150vdc at psu out for B+ when I measured.
hikowinter said:
Sounds too high, did you measured with the mic connected?
hikowinter
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TLRT said:
yes, I was connecting the mic.
At the PSU out B+ was 148vdc and 60vdc at before the capsule.
Is it wrong?
It has 5840 tube.
hikowinter
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hikowinter said:
I have assembled this psu using specified parts and works great.
I could adjust for some required voltages.
ripemedia
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I just finished building this project and everything checks out with the 180k resistor across PIN 1 and PIN 7 except B+ is at 168V instead of 120V, and that's with B+ adjust all the way negative. The components I'm using were very recently purchased from Studio 939. The donor mic is a Carvin CTM100. I've rechecked all components and connections and haven't found any visible issues. Hopefully this is something familiar to someone who can assist. Thanks!
How do we calculate R1 and R2 values for various tubes and B+ values? I am using EF802 for AMI U47 (alt), but have some other, similar microphones with related tubes (EF80, EF184, 6HM6, etc.) and would like to understand the theory. What should I be looking for on the tube data sheets and the particular B+ desired in order to compute values for R1 and R2?
Matador
Well-known member
It's a bit complicated, but you can get a rough estimate like this:
Find the DC bias current needed to power your microphone and the required voltage. For the C12, it's 120V @ approximately 1mA. There is also a bleeder resistor of 470K, so at 120V, it bleeds 0.25mA. The polarization switch is 470K, so another 0.25mA. That's 1.5mA total.
The raw DC supply from the 200VAC transformer is 200*1.414-0.7 ~ 280VDC. So we need to drop 280-120 = 160VDC while flowing 1.5mA, which means we need a total of 106K of resistance. R4 is a 100K pot, so set it to the middle (50K), which means R1 and R2 need to total 56K, or roughly 28K for each one.
In reality, you need to measure the total PSU draw and tune it, because each tube draws slightly different current, and each 1/10mA means dozens of volts difference at these impedances.
Thanks, Matador, that seems an understatement.
I suppose if I can get the desired B+ and H+ without maxing out R4 then that should be in the ballpark, as long as the output sounds good. The designs don't typically have the desired current, so I'll try measuring that to ensure it does not get either too high or too low.
Purplenoise
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Hello. I have started a new thread but maybe this is the right place to ask.
Will this psu work for a u47 with a nuvistor if I wanted to use separate heater line for the nuvistor? ( avoiding the voltage drop resistor in the u47)
Nuvistor requires 8-12v
Matador
Well-known member
The stock PSU has a 9.5VAC winding, which is roughly 12.5V before the regulator. You can easily trim it down to around 10V which leaves plenty of dropout across the regulator even at full heater current.
So yes, I can't see why it wouldn't work.
So yes, I can't see why it wouldn't work.
Purplenoise
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Great, thank you so much
I've decided to build my c12 with a 12at7 since the 6072s are getting so pricey and I used the 33k resistors for R1 and R2. With a 180k resistor between pin 1 and 7 I got my bias to 1v and heaters to 6.3v but my B+ won't go below 134v. I've backed the trim pot all the way off and that's as low as it will go. Is this within limits or do I need to get this sorted. If so what could it be?
Also, will I need to change anything on the mic for the 12at7 or was it just the power supply that needs tweaks?
cheers,
Dave
Also, will I need to change anything on the mic for the 12at7 or was it just the power supply that needs tweaks?
cheers,
Dave
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Matador
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R1 and R2 need to be larger. Try 50K in one or both.
Thanks Matador, I'll order some. Since I'll need to place an order anyway, do you know if any parts need to be swapped out on the mic itself for a 12at7? I haven't seen any mention of it but I want to make sure and I'd rather get all parts at once.
Matador
Well-known member
It's common to lower R17 down from 100K to 68K or even 47K, in order to center the signal at the plate at half-120V (60V) for maximum output headroom. However if you like the sound with 100K you can leave it in and experiment.
Thanks again. I have (hopefully) one more question. On a whim I decided to put the stock 68k resistors back in R1 and R2. While my understanding was that these are good for the 6072 but the 33k are supposed to be better for the 12at7, I'm getting all the correct numbers with the 68k resistors.
Now it occurs to me: I'm using a 180k resistor in the 7pin connection as a dummy load, but should I be using a different value dummy load if I'll be using a 12at7? There's so many variables it's a little hard to keep straight. I know that when Chunger did the v1.4 prototype build he used a 12at7 and kept the 68k resistors for R1 and R2 so maybe it's fine, but then why suggest the 33k? I feel like I must be missing something.
Trickyricky
Member
Did you ever figure out this problem? I’m having this same issue with mine, everything was great with the 68k resistors in, but I decided to go with a 12at7 tube. So I popped in the recommended 33k resistors and now can’t get the B+ below 135.
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