Question about tube rectifier voltage drop and current

[kagliostro]

This question is specific to a circuit but I would like to know how to solve it as to apply also to other circuits with other rectifier tubes

I've a schematic where there is a CT transformer with 225v-0-225v AC

the tube rectifier is a 6x4 tube and the DC voltage measured after the rectifier is 270v

assuming that a SS rectify will give a DC of ~315v I can estimate the voltage drop in 45v

Is possible to estimate which is the current consumption of the circuit that gives as result a drop of 45v using a 6x4 tube ?

there is in the datasheet of the tube an element that can be used as to establish the current value ? (the cathode / plate resistance ???)

to me is still difficult to understand the whole datasheet of a tube, I understand only a part of it

the schematic to which I refer is attached

Thanks

K

[John]

K, if I'm understanding right, the 6X4 supplies 70 mA which makes sense since the datasheet for it says it's got a resistance of 525 ohms. 45 volts/525 ohms = .085 which maybe is "close enough"? Then, IF I've done the math right on the B+ rail, you're drawing about 42 mA through the tubes. I'll look up the datasheets for those tubes and see how much current they're rated at.

Always remember, I could be - and probably am - wrong! But getting slapped down helps me learn too.

[HotBluePlates]

the tube rectifier is a 6x4 tube and the DC voltage measured after the rectifier is 270v

assuming that a SS rectify will give a DC of ~315v I can estimate the voltage drop in 45v

Is possible to estimate which is the current consumption of the circuit that gives as result a drop of 45v using a 6x4 tube ?

The current that causes the voltage drop is the entire B+ current being drawn, because the rectifier must pass all current drawn by the B+ supply.

This question is specific to a circuit but I would like to know how to solve it as to apply also to other circuits with other rectifier tubes

So you really want to know how to estimate the output voltage of a tube rectifier in other amps, right?

How exact do you need to be? If just "close enough" then Tubenit has posted a chart with observed multipliers when going from a.c. input voltage to d.c. output voltage for a number of common rectifiers in a cap-input power supply.

If you want to be exact, download a copy of Radiotron Designer's Handbook, 4th Edition from the Library of Information (download link is just after the "Windows Printing Tip" box). Go to Chapter 30, Section 2 pages 1170-1179.

An example takes up most of page 1179, and has 14 steps and 8-9 graphs to use in lieu of complex calculations.

The problem is many factors impact the actual output voltage, including information about the power transformer winding resistances, winding ratio, input a.c. voltage, peak and average diode resistances, any additional series resistance, load current, load impedance, filter cap value, etc.

You also have to know a lot about how your proposed amp circuit will operate. If class A, you can simplify and assume load current stays essentially constant, but for class AB amps you may have large changes in amp current during operation. If you want to predict how far the voltage will sag under full power conditions, you'd have to take those changes into account and calculate the performance separately for idle and full-power.

[kagliostro]

Thanks John & HotBluePlates

@ John

I'll try to do a bit of math according with what you told

@  HotBluePlates

I'll try to read the Radiotron 4 chapter (I hope I'll understand)

however, I'd be happy to get closer to the value without the perfection of the data

I was wondering about this thing for two reasons

personal educational reasons with maybe a possible practical application

Looking to the schematic I posted previously I was thinking about the use of back to back transformer (or an isolation transformer) so I'll have disposable 0-220v instead of 225v-0-225v with the use of a SS bridge as rectifier

So the necessity to use a SAG resistor as to drop the voltage for the first node from ~308v to the 270v stated on the schematic

In this case I require a drop of 38v but a priori (beforehand) I can't know the current consumption of the circuit

so I'm looking for a way to try to estimate it knowing the fact that the 6x4 tube in that circuit causes a drop of 45v

I know, it is a reasoning somewhat 'twisted, but I try to do somehow with what I know

K

[jjasilli]

You can go to the tube charts and get the current draw of the power tube(s) at your plate voltage: plate + screen. The current draw of the preamp tubes probably won't matter.  Then use Ohm's Law to compute the value of a dropping resistor. 

Or, build the circuit; measure the current draw; then add the resistor.

Or, use the Duncan Power Supply Calculator.  I haven't been able to figure out how to use it.  Especially the transformer values and the "Load".

[HotBluePlates]

The problem is the voltage drop doesn't follow ohms law perfectly.

Resistance and current aren't the only considerations. You also have a non-linear process happening (rectification), and influence of the capacitive reactance of the first filter cap and how that divides voltage with resistance in the circuit. Add in the fact that the a.c. output of the rectifier are sine half-waves (and probably not pure sine shape anyway), and the picture for what d.c. voltage you get out is complicated.

You will run into exactly the same trouble trying to use ohm's law to calculate the d.c. bias voltage output with a known a.c. voltage input. Nothing seems to add up without all those other considerations.

That's why RDH4 never provides a formula for calculating this stuff. They provide a formula which makes reference to data from various charts to simplify the mass of complex equations.

[kagliostro]

Thanks to All

so it seems that the old way is the best way

No math, build and try and try again till you have what you want  

Thanks again

K

[smackoj]

I love school when the 'math class' gets canceled for the day.....cheers

 

[PRR]

http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6X4.pdf

[kagliostro]

Many Thanks PRR

K

[DummyLoad]

I haven't been able to figure out how to use it.  Especially the transformer values and the "Load".

hammond is kind enough to provide the DCR and NL (off-load) voltages for their transformers - even the guitar amp replacement line, however, with care, you can measure your own and make fairly close approximations of what the PS output voltages will be under differing load conditions with PSUD2. for more complex power supply designs use the sim at circuitlab.com or fiddle with SPICE variants, but be warned that SPICE tools are an engineers tool and are fairly complex applications.

with PSUD2 it's easy:

launch PSUD2
1.select source - full-wave SS or full-wave VT
2.click on the transformer symbol.
3."edit transformer properties" window opens
4. click on the "... ohms" button
5. menu opens
6. enter the RMS value of the mains voltage for your region
7. enter primary winding DC resistance  (note: measure it yourself with your DMM!)
8. enter 1/2 secondary off load RMS voltage. (note: measure it yourself -terminate the PT leads on a barrier strip and measure CAREFULLY!)
10. enter the 1/2 secondary DC resistance.
11. tab to or click in the results field to update the estimated Z, then click OK.
12. click simulate button
13. click check boxes for V across each cap and the load.

i recorded a video short demonstration.

http://youtu.be/r-Qd1LLtRSs

[kagliostro]

Hi DummyLoad

Your explanation encourages me to give a try to that software

Thanks

K