I decided to buy a few 5C8S rectifier tubes to experiment with because they look cool but also handle a good bit of current, double that of a 5AR4. They're still in transit but I'm studying the specs. Using a translate app to read the datasheet it's saying the capacitor input value should be 4uf? That seems very low for a tube rated for this much current. It's indirectly heated so the startup should be slow enough. Wonder how they came to that figure? https://frank.pocnet.net/sheets/084/5/5C8S.pdf
Worried about transient capacitive current?
Then use higher secondary voltage, and design a critical inductance choke input filter (or design a mixed cap/inductive input, with no more than 4uF before the critical inductance choke).
Or, just dedicate the 5C8S tubes to a home museum display.
I once used an OD3 shoulder tube and a resistor from B+, to do nothing other than look pretty.
Then use higher secondary voltage, and design a critical inductance choke input filter (or design a mixed cap/inductive input, with no more than 4uF before the critical inductance choke).
Or, just dedicate the 5C8S tubes to a home museum display.
I once used an OD3 shoulder tube and a resistor from B+, to do nothing other than look pretty.
Afaik this Amp is reliable with 47uF (although the rectifier configuration is a bit unusual)
https://www.jogis-roehrenbude.de/Leserbriefe/Klaus-PP/Push-Pull.htm
(scrollt down for the PSU schematic)
https://www.jogis-roehrenbude.de/Leserbriefe/Klaus-PP/Push-Pull.htm
(scrollt down for the PSU schematic)
maybe but only one phase anode I and II togheter each valve and only @ 190VAC
try with 450V-0-450V for only one valve and you can see how nice flash inside
a
try with 450V-0-450V for only one valve and you can see how nice flash inside
a
It is "old" tube rectifier technology , current rating is not very important
If you look at 5AR4 datasheet the voltage drop vs current is significant lower
But with electronic soft start you can use much much more capacitance .
If you look at 5AR4 datasheet the voltage drop vs current is significant lower
But with electronic soft start you can use much much more capacitance .
Each of the two 5AR4 Rectifiers are used this way:
Half Wave (50/second or 60/second) You hardly ever see 1/2 wave rectification to power output tubes; capacitance requirements double for the same amount of ripple versus full wave rectification.
The cathode only conducts 1/2 the time versus a cathode in full wave rectification (100/second or 120/second).
2 Plates in Parallel drive 47uF (that is the same as 1 Plate driving 23.5uF).
The maximum capacitance the rectifier can take is dependent on the load current, plate voltage, and DCR + series resistance for each plate.
Take a look at some other tube rectifiers. The physics is the same, just re-calculate versus things like max peak plate current, etc.
And for heating consideration do not overlook the differences of half wave versus full wave (duty cycle changes the dissipation integral)
Yes, the power supply seems strange.
Half Wave (50/second or 60/second) You hardly ever see 1/2 wave rectification to power output tubes; capacitance requirements double for the same amount of ripple versus full wave rectification.
The cathode only conducts 1/2 the time versus a cathode in full wave rectification (100/second or 120/second).
2 Plates in Parallel drive 47uF (that is the same as 1 Plate driving 23.5uF).
The maximum capacitance the rectifier can take is dependent on the load current, plate voltage, and DCR + series resistance for each plate.
Take a look at some other tube rectifiers. The physics is the same, just re-calculate versus things like max peak plate current, etc.
And for heating consideration do not overlook the differences of half wave versus full wave (duty cycle changes the dissipation integral)
Yes, the power supply seems strange.
This is not about Ohm's laws , but materials science . The max capacitance ( current surge ) when the tube is cold is determined by materials in the cathode coating or the filament if is direct heated.
Later rectifiers , as they knew more about materials , can work with more capacitance
Later rectifiers , as they knew more about materials , can work with more capacitance
Depanatoru,
You are correct, materials science.
Consider the following material problem, and the physics thereof . . .
Some rectifier's cathodes have been found to have Bumps on their coating.
A Bump is closer to the plate, and it draws Much more current than the rest of the cathode.
Bang! Arc! Ouch!
There are some tube cathodes that once they get going, can self sustain with the filament off, because the cathode current of all those electrons that race to the plate causes the cathode to heat up.
You are correct, materials science.
Consider the following material problem, and the physics thereof . . .
Some rectifier's cathodes have been found to have Bumps on their coating.
A Bump is closer to the plate, and it draws Much more current than the rest of the cathode.
Bang! Arc! Ouch!
There are some tube cathodes that once they get going, can self sustain with the filament off, because the cathode current of all those electrons that race to the plate causes the cathode to heat up.
I have the follow data on this tube, hope this help.
5C8S DATASHEET
Filament voltage 5 V
Filament current 5 A
Variable effective voltage of the secondary winding of the transformer
RMS voltage on the secondary of the transformr 2 x500V
Filter capacitance 4 uF
Rectified current 400 mA
Guaranteed lifespan 500 h
Limit Operating Data Maximal Values:
The filament voltage Filament voltage 4.5 ... 5.5 V
The anode current amplitude Peak anode current 1.2 A
Anode current at the moment of switching on Anode current during powering on 5 A
The amplitude of the reverse voltage of the anode Peak negative anode voltage 1700 V
Power dissipated by the anode Anode power dissipation 30 W
Rectified current 420 mA
5C8S DATASHEET
Filament voltage 5 V
Filament current 5 A
Variable effective voltage of the secondary winding of the transformer
RMS voltage on the secondary of the transformr 2 x500V
Filter capacitance 4 uF
Rectified current 400 mA
Guaranteed lifespan 500 h
Limit Operating Data Maximal Values:
The filament voltage Filament voltage 4.5 ... 5.5 V
The anode current amplitude Peak anode current 1.2 A
Anode current at the moment of switching on Anode current during powering on 5 A
The amplitude of the reverse voltage of the anode Peak negative anode voltage 1700 V
Power dissipated by the anode Anode power dissipation 30 W
Rectified current 420 mA
Just an off-the-cuff estimate:
Full wave rectification; 120Hz
4uF, Xc @ 120Hz = 330 Ohms
500V / 330 Ohms = approximately 1.5A inrush current at power up.
(Some rectifiers require a long filament/cathode warm-up time before B+ is applied, does the 5C8S require that?
Full wave rectification; 120Hz
4uF, Xc @ 120Hz = 330 Ohms
500V / 330 Ohms = approximately 1.5A inrush current at power up.
(Some rectifiers require a long filament/cathode warm-up time before B+ is applied, does the 5C8S require that?
It's not that simple. For example, 420mA is only for short (5min) use. Long term max is 380ma. There are other conditions listed in the datasheet: https://www.istok2.com/data/303/
Turning on the heater and anode voltage at the same time is allowed.
Only one parameter is allowed to reach the limit at one time.
Unfortunatly is not like that , the capacitor is like a short when charging in the first few AC cycles , only external resistance like transformer primary , secundary and aditional resistors limit the current . And the tube itself
Lower max capacitance means a shorter time for this condition , important second constrain besides max inrush current if the cathode is "weak" .
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Depanatoru,
You are absolutely correct.
Dead short in the beginning (not the big-inning of baseball).
But look at it this way . . .
Given a warmed up filament and cathode . . . (like in a hot-start, when the power goes out, the massive 5A filament and cathode stays Hot, and the power comes back on.
If you have 500V peak, an uncharged 4uF cap, a resistive load across the 4uF cap, and zero DCR in the primary and zero DCR in the secondary,
Until the cap is charged, there might be up to 'at least 1.5A' of current while the cap is charging.
The first transient depends on three things:
The plate impedance as the only series resistance that is in series with 330 Ohms of capacitive reactance.
Yes, the capacitive reactance is much lower to the first transient, the extreme bandwidth during the first alternation when the switch closes.
The sine of the phase angle at the moment the switch closes, times 500V.
If you close the switch at 80, 100, 170, or 190 degrees, you have 500V x 0.9848 = 492V.
Pray for some leakage reactance from the primary to the secondary, it helps reduce the transient current a little bit.
I bet some readers were not expecting anywhere as large as 1.5A of current at any time.
You are absolutely correct.
Dead short in the beginning (not the big-inning of baseball).
But look at it this way . . .
Given a warmed up filament and cathode . . . (like in a hot-start, when the power goes out, the massive 5A filament and cathode stays Hot, and the power comes back on.
If you have 500V peak, an uncharged 4uF cap, a resistive load across the 4uF cap, and zero DCR in the primary and zero DCR in the secondary,
Until the cap is charged, there might be up to 'at least 1.5A' of current while the cap is charging.
The first transient depends on three things:
The plate impedance as the only series resistance that is in series with 330 Ohms of capacitive reactance.
Yes, the capacitive reactance is much lower to the first transient, the extreme bandwidth during the first alternation when the switch closes.
The sine of the phase angle at the moment the switch closes, times 500V.
If you close the switch at 80, 100, 170, or 190 degrees, you have 500V x 0.9848 = 492V.
Pray for some leakage reactance from the primary to the secondary, it helps reduce the transient current a little bit.
I bet some readers were not expecting anywhere as large as 1.5A of current at any time.
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