williampage wrote:

...try to figure out how to get the load for all 4 of the tubes... At this point, I feel totally lost, and cannot go forward until I get this figured out. If you know how to do this, how about figuring the amps needed for two 12sk7gt tubes, one 12sj7gt tube, and one 12v6gt tube? Or. Could you direct me to a site which lists, in order, the steps to accomplish this? I have looked for info on building a 240 volt, diode bridge rectified, CLC power supply out of my new EDCOR xpwr119-120 power transformer. The transformer is 120 volt, 60 hz primary, and the secondary is 240 volt@ 200 ma. , and has 12.6 volt, 3 amp filament winding. I need info on the microfarad rating for my electrolytic capacitors and need to know if a choke of 30 H would be ok. I would like to know what value resistor would be best for the bleeder resistor. What I am asking is for someone to direct me to a site with an existing 240 or 250 volt power supply schematic is shown. I can just copy that schematic and use the values given there. Thanks.... I think I am dealing with a total of 69 milliamps current at the plates of the 4 tubes totaled... William

William,

Known:

1. Desired output voltage 250.

2. Transformer secondary rated 240 V ac at 200 milliamps = 48 VA.

3. Full wave solid state bridge rectification.

4. Circuit to be supplied uses 70 mA plate current.

4. Dc output voltage from full wave rectifier with no load and no bleeder = 240 x 1.4 = Eacp = 339 volts.

5. 300 V chosen as maximum permissible output voltage with bleeder but no load on supply.

4. Capacitor input filter connected to rectifier.

5. An additional LC (inductance / capacitance) filter is desired to follow the rectifier capacitor.

6. A 5 henry, 150 mA choke is on hand. Measure resistance of choke. 105 ohms will be used here.

To be determined:

1. Resistance of transformer 240 V secondary, Rs. Use ohmmeter to measure. 50 ohms will be used in this example.

2. Resistance of choke, Rchoke. Use ohmmeter.

3. Filter capacitance.

4. Bleeder resistance and power rating.

Method:

1. Look up data on plate current for each tube, add up all. Result = 70 mA (0.07 amp) needed by tubes.

2. Calculate equivalent load resistance of tubes. Rl = 250 / 0.07 = 3571 ohms

3. Determine C1 capacitance needed on rectifier positive terminal for omega x C1 x Rl = 30. C1 = 30 / (6.28 x 120 x 3571)

Result: C1 = 10 microfarads (uF or mF). The 47 uF you already have is okay.

3. Calculate /Rs / Rl. For example 50 ohms / 3571 ohms = 0.014

4. Use Fig. 15.9 graph, above, to determine Edc / Eacp with /Rs / Rl = 0.014 and omega x C1 x Rl = 30. Result: Edc / Eacp = 0.93.

5. 339 x .93 = around 315 V dc output from rectifier at the 70 mA load.

6. 315 V dc out is too high, need to drop 315 - 250 = 65 V.

7. A series resistance, R1, can be used to drop the 65 volts.

8. Resistance to drop 65 V at 70 mA = 65 V / 0.07 A = 928 ohms. 928 - Rchoke = 928 - 105 = 823 ohms. 820 ohms will be suitable for R1.

9. 820 ohms x 0.07 amps squared = 4 watts. Will need 5 to 10 watt rating for R1. The dropping resistor is placed between the rectifier positive and the choke.

10. Reactance of 5 H choke at 120 Hz = 6.28 x 120 x 5 = 3768 ohms.

11. The reactance of the output capacitor can be one tenth of 3768 ohms. 3768 / 10 = 377 ohms. C2 = 1 / (6.28 x 120 x 377) = 3.5 uF. For convenience, C2 can be 10 uF or 47 uF instead.

12. Bleeder resistance, R2, calculation. Roughly 315 V - 300 V = 15 V to be dropped across the 925 ohm (R1 + Rchoke). 15 V / 925 ohms = 16 mA. 300 V / 0.016 A = 18,750 ohm bleeder, R2. I would use 20 k ohms. The additional current due to the bleeder will drop the output voltage to around 230 V when the 70 mA load is on the supply.

13. 300 V x 0.015 = 4.5 watts for the bleeder. I'd use a 10 watt rated resistor.

14. Approximate total dc power: 4.5 watts bleeder + 4.5 watts dropping resistor + (.07 x 230) = 25 watts.

15. 48 VA / 1.23 = 39 watts. 25 watts is less than 39 watts, so sufficient power is available from the transformer.

Attachment:

RECT_FILTER_01.jpg [ 70.18 KiB | Viewed 1821 times ]
----------------

Eric LaGess

WB5HDF