DC Voltage supply using Zener Diodes

[TIMBO]

Hi guy's, I thought i put this up just to add to the info pile. I was wanting to have DC heaters for V1 & V3 and the schem that i followed was using a 12v power supply rectified for relays, so i thought i could adapt this for the heater supply but with a 6v tap it was giving me about 8.4v with this circuit. I found this http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm which was great. I used a 6.2v zener and a 47ohm resistor the output gives me 6.3v @ 60mA  and the 5.1v zener gives me 5.2v @ 80mA. I am using 470uf cap, but i don't know if this OK for this circuit. Any thoughts, Thanks

[PRR]

> DC heaters ... output gives me 6.3v @ 60mA

Most small tubes need 300mA at 6.3V, no?

[TIMBO]

I'm not very good at this stuff getting my A & mA mixed up.I read the tube data wrong and didn't add enough zeros. Am i in the right ballpark with this circuit or am i way off the mark.      I'm thinking that if i was to have 2x12AX7 on this circuit i would need a min of 600-700mA and if the calculation is right it would require a zenner of 5W and a resistor of 3 ohm @ 2W 
or a different circuit altogether  Thanks

[sluckey]

Zeners are great for reference voltages. But if load current is high they are not very practical for a simple shunt regulator such as your circuit. From a looong time ago, I seem to remember that the zener current needs to be considerably more than the load current to have decent regulation. I'd dump the zener idea and use simple VR chips. They're cheap and widely available and using them takes all the guess work (or calculations) out of the design. I'd use TO-220 style with a heat sink or bolt to the chassis for this application.

[TIMBO]

Thanks guys, What seem to be a simple exercise has a lot of math behind it.I am slowly getting my head around it.Thanks of the ongoing help.

[PRR]

6.3V desired at 600mA or 0.6A.

8.4V available (assume no sag, but that's unrealistic).

To be wasted: 8.4V-6.3V = 2.1V

Common 3-pin regulators are marginal with only 2V to work in. Plus the "8.4V" will surely sag. Worse: the "8.4V" is a meter average, it is probably bopping up to 9V and down below 7V.

Regulation is NOT actually needed. If you get close to 6.3V on normal wall voltage, it will be close-enough for any wall tolerable voltage. ("Tolerable" in the sense that incandescent lamps don't burn way-dim or too-bright short-life.)

IMHO, a resistor is fine and appropriate. It also adds a trace of soft-start.

Resistor should be 2.1V/0.6A or 3.5 ohms, 2.1V*0.6A= 1.26 Watts (double and round-up: use 3W part).

In fact the "8.4V" will sag. How much? Dunno. You could dummy-load it and find out. 8.4V/0.6A is 14 ohms 5 Watts. That's awkward. Radio Shed has 10 ohm 10 Watt, I'd toss one on there and see if it holds anything like 6 or 8 volts. If not disappointing, build with about 3 ohms tacked in series with the heaters, see what you get, adjust the 3 ohms down until near 6.3V at heater. The 1 ohm 1 Watt parts commonly used for big-amp bias-check would be suitable.

> I am using 470uf cap, but i don't know if this OK

Rule-o-thumb for medium voltages is 1,000uFd per one Ampere current. That says 600uFd.

This gives around 1V ripple, which may be OK for 25V or 100V supplies (or 6V relays and LEDs) but looms large for 6V "clean" DC.

A single stage of filtering will NOT make clean enough DC to route near sensitive input wiring. (And the buzz is worse than hum.) Vastly bigger capacitance will help only slowly... even 4,700uFd is not real-clean at 0.6A load.

OTOH a 2-stage filter is at least 4 times better and the added R is begging for a second cap to make a 2-stage filter.

To ballpark a value for the second cap, use R and figure what C would be needed to pass 100Hz (or 120Hz). R-C calculators are available online, but a Reactance Chart is quicker. I'll Attach one for your wall.

Find 100Hz at the bottom, and 3.5 ohms on the left. Go up to the intersection. Follow a diagonal up-left line until you find uhf numbers. Looks like something between 100uF and 1000uF. Perhaps 500uF. Now multiply by at least 10, so it will actually chop ripple, not just bunt it. 5,000uFd.

Experience says both caps could be the same. OTOH two 4,700uFd caps are big and costly, even at only 10V rating.

Go ahead and try 470uFd 3.5 ohms 470uFd. With very careful heater routing it may be clean enough. But I'd leave room for 2,200 or 4,700uFd caps in case it needs to be even cleaner.

[TIMBO]

Thanks PRR,That was great , had to read it a couple times to get a grip on it. I've learn't a lot from this exercise. I thankyou for the time you have taken to help me with this.The schem you have supplied using the voltage regulators might be a better way to go.I bought the parts for this today and it only cost me a few bucks for what i think is a better PSU. The hole exercise was to help to reduced HUM/BUZZ that is generated in the preamp so if this is the better way i'll be  Thanks

[TIMBO]

I 've put together the little PSU using the Voltage Regulators the two outputs measure 5.01v and 6.01 w/o tubes. Running two 12ax7 the voltage drops to 5.2v and from what i have read thats   Thanks

[sluckey]

I know you want to use your existing PT for this project, but in the future, if you need to use a 7806 and need the output to regulate at 6vdc, use a 12VAC PT instead of 6.3VAC.

[TIMBO]

Thanks sluckey, The project i am working on is made up of bits and pieces of other projects that just didn't make it. I'm using an 18w tranny set and it had these taps. I built my Silver jubilee w/o any dc heaters to help tame the hum and thought that trying it in this build might or may not make a difference, time will tell. I asked a question in another post but did not get a reply, can power tubes be safely turn off by disconnecting them at the cathode ground. Thanks

[sluckey]

can power tubes be safely turn off by disconnecting them at the cathode ground.

yes

[TIMBO]

Thanks Again