Hello! I'm prepping for a build of this Nobu Shishido 2A3 DC amplifier. I should note that it's my second non-kit build... That said, I have a question about the power supply in the attached schematic and specifically why there is a 50 Ohm resistor before the first filter cap. From what I understand the first filter cap for a 5AR4 should be around 47uF. Is the 50 Ohm resistor serving as a buffer after the rectifier? Maybe I'm completely missing something obvious. Thanks in advance!
The primary DCR x the step-up turns-ratio of the primary to secondary; and the secondary DCR, and the 50 Ohms will limit the peak current into the 100uF first filter cap.
How much limiting, according to:
For a 5AR4, both DCRs need to be measured, primary DCR multiplied by the step up ratio, and the total current limiting capability of the primary DCR, secondary DCR, 50 Ohms means that the 5AR4 can either safely drive 100uF, or it can not safely drive 100uF, at power up.
There should be required minimum series resistance in the full data sheet of a 5AR4, according to the secondary 1/2 winding rms or peak voltage, and various first capacitor uF values.
Even worse, if there is ever a Hot Start condition (caused by a brief mains power dropout, and quick mains power back on . . . when the 5AR4 cathode is still hot, which exactly what happens during a Hot Start event.
Put a 40uF cap right After the 50 Ohms (where the 100uF was, until you move that 100uF from the 50 Ohms, and parallel it with the 100uF that is After the choke).
Less ripple.
Less problems.
Cooler power transformer
Cooler 5AR4
Better reliability.
Other than that, no reason to make the change.
I am not Paranoid, though I have reason to be.
I have personally seen a 5AR4 ARC when the first filter cap before the choke was 100uF.
Your arcing and sparking may differ.
You will have fun when you get it built and can listen.
How much limiting, according to:
For a 5AR4, both DCRs need to be measured, primary DCR multiplied by the step up ratio, and the total current limiting capability of the primary DCR, secondary DCR, 50 Ohms means that the 5AR4 can either safely drive 100uF, or it can not safely drive 100uF, at power up.
There should be required minimum series resistance in the full data sheet of a 5AR4, according to the secondary 1/2 winding rms or peak voltage, and various first capacitor uF values.
Even worse, if there is ever a Hot Start condition (caused by a brief mains power dropout, and quick mains power back on . . . when the 5AR4 cathode is still hot, which exactly what happens during a Hot Start event.
Put a 40uF cap right After the 50 Ohms (where the 100uF was, until you move that 100uF from the 50 Ohms, and parallel it with the 100uF that is After the choke).
Less ripple.
Less problems.
Cooler power transformer
Cooler 5AR4
Better reliability.
Other than that, no reason to make the change.
I am not Paranoid, though I have reason to be.
I have personally seen a 5AR4 ARC when the first filter cap before the choke was 100uF.
Your arcing and sparking may differ.
You will have fun when you get it built and can listen.
Not quite sure I understand this calculation. The wording is also a bit confusing and I am new to this world!
So in this situation it would be the 50 Ohm resistor followed by a 47uF cap, then the choke and two 100uF caps paralleled. Correct? Also, if the B+ needs to be adjusted up or down a little can the value of the 50 Ohm be tweaked a little? Not sure how that resistor value is calculated...
Be conservative. Use 40uF instead of 47uF.
Please look at various 5AR4 data sheets, one or more should list the minimum resistance required, versus the secondary voltage, and the first (input) capacitor.
I will "invent" an example.
120V to 720V transformer (360V from ends to center tap).
Step up ratio is 3 to 1.
Primary DCR 33.3 Ohms x 3 = 100 Ohms (99.9)
Secondary is 200 Ohms total, 100 Ohms to center tap.
100 Ohms contribution from primary, = 100 Ohms from secondary 1/2 winding = 200 Ohms. With 5AR4, only one plate conducts at 1/2 the power cycle.
With 100 Ohms + 100 Ohms + 50 Ohm resistor, the rectifier "sees" 250 Ohms driving resistance.
If the 5AR4 tube data says 47uF, 350VAC, required series resistance is 225 Ohms, you are good to go; if it says 300 Ohms, you are Not good to go.
Google Franks Tube Data, then click on 5, then find the 5AR4.
Teach a man to fish (I am too old to fish myself).
I love helping those who are new to all this, and I love discussing, reminding, etc. for all the experienced posters too.
I make lots of errors, but when I do, the experienced help my out, by correcting me.
I tried to give up arguing, I have done it in the past; but I try not to do that any more, I try not to offend . . .
But I am a jokester.
Life is too short for all of us to have heated disagreement.
One poster, that I was really going to bout with (like a Knight Jousting), I regret that I can not remember his "Handle"
The last time I saw his post, his mom went to the hospital; wish I knew how she is, it is so much later in time.
I think we were discussing the Meaning and Proper Usage of the word . . .
SYNERGY
Please look at various 5AR4 data sheets, one or more should list the minimum resistance required, versus the secondary voltage, and the first (input) capacitor.
I will "invent" an example.
120V to 720V transformer (360V from ends to center tap).
Step up ratio is 3 to 1.
Primary DCR 33.3 Ohms x 3 = 100 Ohms (99.9)
Secondary is 200 Ohms total, 100 Ohms to center tap.
100 Ohms contribution from primary, = 100 Ohms from secondary 1/2 winding = 200 Ohms. With 5AR4, only one plate conducts at 1/2 the power cycle.
With 100 Ohms + 100 Ohms + 50 Ohm resistor, the rectifier "sees" 250 Ohms driving resistance.
If the 5AR4 tube data says 47uF, 350VAC, required series resistance is 225 Ohms, you are good to go; if it says 300 Ohms, you are Not good to go.
Google Franks Tube Data, then click on 5, then find the 5AR4.
Teach a man to fish (I am too old to fish myself).
I love helping those who are new to all this, and I love discussing, reminding, etc. for all the experienced posters too.
I make lots of errors, but when I do, the experienced help my out, by correcting me.
I tried to give up arguing, I have done it in the past; but I try not to do that any more, I try not to offend . . .
But I am a jokester.
Life is too short for all of us to have heated disagreement.
One poster, that I was really going to bout with (like a Knight Jousting), I regret that I can not remember his "Handle"
The last time I saw his post, his mom went to the hospital; wish I knew how she is, it is so much later in time.
I think we were discussing the Meaning and Proper Usage of the word . . .
SYNERGY
This is super helpful! A couple quick things related to this example:
How is this calculated? I understand the step up ratio of 3 to 1. Is the 33.3 Ohm DCR a PT spec? Or derived from the step up ratio? If derived from the step ratio, why multiplied by 3?
If the required series resistance from the data sheet is 225 Ohms but it is "seeing" 250 Ohms (100 ohm + 100 ohm + 50 ohm resistor) wouldn't this not be good? Also, if the tube data says it can handle 350VAC, is it also not good that the 5AR4 is seeing 360VAC from the PT?
I'm getting there!
How is this calculated? I understand the step up ratio of 3 to 1. Is the 33.3 Ohm DCR a PT spec? Or derived from the step up ratio? If derived from the step ratio, why multiplied by 3?
If the required series resistance from the data sheet is 225 Ohms but it is "seeing" 250 Ohms (100 ohm + 100 ohm + 50 ohm resistor) wouldn't this not be good? Also, if the tube data says it can handle 350VAC, is it also not good that the 5AR4 is seeing 360VAC from the PT?
I'm getting there!
250 Ohms total is good for a spec that requires at least 225 Ohms.
250 Ohms total is Not good, if the spec requires a minimum of 300 Ohms.
DCRs are Measured by me. A DMM fills the need.
Specified DCR, you may find in a data sheet, you may not, but . . .
Trust but Verify.
Hint: When checking DCR of a power transformer, and also when checking the DCR of an output transformer . . .
Be sure to short the Other windings. That reduces the inductance of the winding you are trying to measure DCR of.
Inductance often causes a DMM to auto-range . . . jumping ranges up and down, and never getting the correct answer.
Jumping means you are Mostly measuring the inductive effect, and not the DCR.
I only made up the numbers, they may not be the real numbers on the 5AR4 data sheet. I think some power transformer primaries are 33 Ohms
In my example we had 250 Ohms total, accounting for DCRs, and 50 Ohm resistor.
For a tube requirement of at least 225 Ohms we are OK
For a tube requirement of at least 300 Ohms, 250 Ohms is not good.
If 250 Ohms works for 200VAC, and a 47 uF capacitor,
Then for 400VAC, we will get 2 X the inrush current on a 47uF cap, so we will need 500 Ohms total, to get the transient current back to a safe value.
Look at the 5AR4, and perhaps some other tube types 5Y3, or others, you may see those factors coming into play.
Usually, a capacitor input filter supply, requires a minimum series resistance, sometimes just the 2 DCRs are enough, sometimes you need a resistor too.
Usually, a choke input filter supply, does Not require a minimum series resistance.
Good tube rectifier data sheets show data for both cap input, and for choke input, B+ filter circuits.
250 Ohms total is Not good, if the spec requires a minimum of 300 Ohms.
DCRs are Measured by me. A DMM fills the need.
Specified DCR, you may find in a data sheet, you may not, but . . .
Trust but Verify.
Hint: When checking DCR of a power transformer, and also when checking the DCR of an output transformer . . .
Be sure to short the Other windings. That reduces the inductance of the winding you are trying to measure DCR of.
Inductance often causes a DMM to auto-range . . . jumping ranges up and down, and never getting the correct answer.
Jumping means you are Mostly measuring the inductive effect, and not the DCR.
I only made up the numbers, they may not be the real numbers on the 5AR4 data sheet. I think some power transformer primaries are 33 Ohms
In my example we had 250 Ohms total, accounting for DCRs, and 50 Ohm resistor.
For a tube requirement of at least 225 Ohms we are OK
For a tube requirement of at least 300 Ohms, 250 Ohms is not good.
If 250 Ohms works for 200VAC, and a 47 uF capacitor,
Then for 400VAC, we will get 2 X the inrush current on a 47uF cap, so we will need 500 Ohms total, to get the transient current back to a safe value.
Look at the 5AR4, and perhaps some other tube types 5Y3, or others, you may see those factors coming into play.
Usually, a capacitor input filter supply, requires a minimum series resistance, sometimes just the 2 DCRs are enough, sometimes you need a resistor too.
Usually, a choke input filter supply, does Not require a minimum series resistance.
Good tube rectifier data sheets show data for both cap input, and for choke input, B+ filter circuits.