The Official Dual Rail Thread

[JC1]

Stabilised power supply

There are simpler ways of doing it of course

I thought you said "There are simpler ways of doing it of course" that circuit, though very well designed & my hat's off to you for doing it, seems a bit more involved than a couple of screen zeners in series from the B+ screen node. I don't even think I have enough realestate left inside my amp to install all of those components. I don't mind a little P.S. as long as my screen voltages are operating in a safe enough level not to destroy EL34's or KT77's.

Plan B zeners?

[Steve Conner]

Well, have a look at Q3 and surrounding circuitry in this schematic. http://scopeboy.com/scopeblog/wp-con...HybridRev2.pdf About the simplest screen voltage dropper possible, a MOSFET and a pot. Cheaper than zeners and about as robust.

For a bigger amp I would add the current limiter from the Ninja Toaster PS (Q1 and R11-R14) R12= 4.7 or 10 ohms would be more appropriate for a screen supply.

[JC1]

Well, have a look at Q3 and surrounding circuitry in this schematic. http://scopeboy.com/scopeblog/wp-con...HybridRev2.pdf About the simplest screen voltage dropper possible, a MOSFET and a pot. Cheaper than zeners and about as robust.

For a bigger amp I would add the current limiter from the Ninja Toaster PS (Q1 and R11-R14) R12= 4.7 or 10 ohms would be more appropriate for a screen supply.

I'll take another look at that.

[JC1]

Steve,

have a couple of questions, the data on the Mosfet say 500v, I have 500v or a little more already on the screens. With a say 50v zener in circuit, would you think that Mosfet would be able handle it or do you know of a higher rated voltage Mosfet that I could subsitude?

What's the wattage rating on that 18v zener you have in the circuit presented and the actual voltage drop from plate to screen? The 1Meg Pot, I'm assuming it's to adjust the screen voltage to within from approx. plate voltage to the maximum rating of the zener?

[Steve Conner]

In the Ninja Corvette project I used a 500V MOSFET because this was a little EL84 amp with B+ that didn't rise above 350V even unloaded. (and I had a drawer full of IRF840s looking for a use )

For a 50W amp you should use an 800 or 900V part. The BFC62 I used in the Ninja Toaster circuit is long obsolete, a modern equivalent would be something like this: STP6NK90Z - STMICROELECTRONICS - MOSFET, N, TO-220 | Farnell United Kingdom

The zener in the Ninja Corvette circuit plays no part in regulation, it is just insurance to protect the MOSFET gate against any overvoltage that might destroy it. You can spend time proving to yourself that this couldn't possibly happen, but Murphy's law of electronics says that it will happen regardless, so I always just use the zener anyway. It only costs pennies. I used the smallest size of zener, 0.6W or whatever it is.

The adjustment range of the Ninja Corvette circuit is from zero to the full B+ voltage. Note that the pot sees the full B+ voltage across its track and has to be suitably rated. If you know what voltage you want, and don't intend to vary it, you can replace the pot with two fixed resistors.

For extra geek points you can derive the B+ reference to the pot from further downstream in the power supply's RC filtering chain. This does a great job of removing hum from the screen supply with no additional parts. I implemented this as a mod and it made the amp significantly quieter.

[exclamationmark]

I'm incredibly fond of mosfets that have the gate-source protection mosfet built into them. I have really dry skin so I tend to accidentally destroy mosfets just by looking at them! In many cases I've blown them up before I can even solder them into my circuits. As per Steve's advice, if you can provide a mosfet source follower with a well filtered gate voltage, it can reduce the hum pretty dramatically if the drain is powered directly off the main filter cap.

[Alf]

I would live to see the schem ! Something went wrong ??

[troposnet]

Hi all, since this tread addresses the PT and HT winding, i hv put my case here for help.
My PT HT winding looks like the attached, 0,60,250 and 400Vac. they are all in the same
winding with no center tap. i have used the 0 and 400Vac taps for the B+ of around 500Vdc
(via a bridge and caps setup) for 2X6550 in the output stage. i'm trying to use the 0 and
60Vac taps to do a negative bias cct with a separate bridge but the fuse on the HT side
keeps blowing. i know just a single diode would be able to do the job but it seems that
if its directly connected, its a short somewhere that the fuse blows.

could someone please help me the right direction in getting this to work. thanks.
(also i hear of some pump charge cct.)

[loudthud]

With a full-wave bridge on the 0V and 400V leads, assuming the negtive side of the bridge is grounded, the 60V tap cannot be used to generate negative bias. First of all, it never swings below ground, and secondly, on alternate half cycles, it swings positve by the peak of 340V.

[Pedro Vecino]

With a full wave bridge, to obtain a bias voltage you can use the Vox AC50 circuit:
http://www.voxamps.com/downloads/circuits/ac50_1.jpg

[troposnet]

yes the -ve side of the HT bridge goes into the first cap star which then is chasis grounded. i've redrawn with the ac50 bias cct and what terminals i have. would the wiring shown there work or i just don't have a choice but to use the 400Vac terminal for a -ve supply? that would be way too much Vac i presume.

[Pedro Vecino]

This circuit is for 380VAC. If you have 400VAC, no problem. Use it in the same way.

[imaradiostar]

Did you have this transformer custom wound?

The vox circuit relies on having separate windings, not one continuous winding.

Is it possible to separate the 60 volt winding from the others?

Jamie

[troposnet]

yes its custom wound and does not have center tap. the 60V tap is part of the HT winding. i'm seeing if the 0 volt point can be lifted to the 60V point in order to emulate a center tap type winding. attached below is the cct but could someone clarify if this would work. thanks.

[imaradiostar]

You're not trying to emulate a center tapped winding, you want different windings such as the hiwatt 200 mentioned earlier in this thread. Center tapped windings are a holdover from the days of tube rectifiers- it made for a simpler circuit with only two diodes. Modern solid state bridge rectifiers only need one continuous winding (of half the voltage) with no center tap, and are more efficient.

Technically your design isn't a "dual rail" design but I'm sure we're happy to help. Perhaps you'd have gotten better results by starting a new thread with an appropriate title.

Jamie