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PostPosted: 17 Feb 2018, 13:33 
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Joined: 28 Dec 2010, 22:07
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My goal is to make a converter so 12v can be turned into 160v DC and be used to power two E88CC tubes in a phono preamp.

I have a 120-18v transformer with a 1 amp rating. I'm also going to be driving the secondary with a square wave like one of the old vibrator supplies. The primary will feed a bridge rectifier and filter.

If I were driving it with an 18 VRMS sine wave, I would get 120 VRMS and a maximum capacity of 18w. That would mean 170v DC at a lower current, close to 18w but not there (a bridge rectifier can provide 63% of the RMS transformer current as DC. Diode and winding losses make up the difference).

Since I'm dealing with a square wave, my peak and RMS AC voltage are the same, and so is my DC voltage. It follows that the relationship between a rectified sine wave and rectified square wave are different with respect to the current I can expect.

If the transformer puts out a square wave 160v AC, 18 watts works out to be 112.5 mA. I sincerely doubt the maximum output DC current would be anything close to that. Maybe I'd get 60 mA but not far more than that.

Ed

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PostPosted: 17 Feb 2018, 17:38 
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Hi Ed, What's the question? Some thoughts...Depending on the frequency of the square waves you might not get squares out. They can get pretty weird when going through a transformer and this might be desirable. Rectified they might act more like sine waves. What is the power source and what output do you really need?

Good listening
Bruce

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PostPosted: 17 Feb 2018, 21:11 
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Hi Bruce,

The question is, if I drive a transformer with a square wave, how do I determine the maximum DC current I can expect out of it? As mentioned before, the peak AC and therefore DC voltage are different from the sine wave, so the DC current I'd get from a rectified sine wave is less than what I could get as RMS current had it remained AC. That's why it's more efficient to power tube heaters with AC than to rectify and filter it to DC.

In my situation, I'll already have 12v DC from a battery (or bench supply). That means using 12v tubes and powering the heaters from the main DC rail. Then, I'd want to have a plate supply of 150-200v so I could get some performance from the tubes. That's where the inverter comes in.

My intended goal is to feed a 6922 preamp using a 12v power pack. I would use an inverter to power the plates, and run the tube heaters in series.

Ed

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PostPosted: 05 May 2018, 00:40 
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An update...

I made a circuit designed to drive the transformer in the way I described. Stage one was a 555 timer putting out a 600 Hz signal. Stage two used a D flip-flop (forget the number) to make two 300 Hz signals that were opposite in phase. Stage three used a quad NAND gate to sense when/if both power switches would be ON at the same time, and blank out any activity (dead-time generator). Then I used bipolar power switches with darlington pairs. Deadtime was created by shorting the bases of the driver transistors, with resistors smaller than the ones in the drive lines.

Using a transformer rated at 18v AC, 1 Amp, in a "vibrator power supply" configuration, I got 155v with no load. I put various loads on it and measured 125v when I loaded it down to 95 mA. It would be great for powering a simple receiver. I'd have to filter out the switching noise using RF filtering techniques.

Still, it would be a good way to power tube circuits on the go. I could run it from a vehicle supply or even with a stand alone battery bank.

Ed


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PostPosted: 05 May 2018, 08:36 
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Joined: 08 Aug 2009, 03:11
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Hi,

BowToEd wrote:
.... be used to power two E88CC tubes in a phono preamp.


As one who has tried this, you won't be able to get the noise out. That's why square waves are best transformed at 50KHz+

A more standard practise is to use a back-to-back arrangement, say 120V --> 6.3V and power the fils, then 6.3V ---> 120V rectified and filtered gives about 160-180V, depending on loading.

In my Corvus 6922 based phono preamp, I use 12VAC @ 1.5A and a six stage multiplier to 100V (unloaded, about 80V loaded) and regulate it to 65V.

Cheers!

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