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Author Topic: Inverter for B+ for Tube Receivers / QRP Transmitters  (Read 4222 times)
AE7TE
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Posts: 53




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« on: September 08, 2017, 09:38:16 PM »

Hi all.

I'm working on a design that will have amateurish uses. Basically I'm going to make a simple inverter to turn 12v into 120v using a run-of-the-mill transformer. Then I'm going to run that into a rectifier/filter to make 120v DC. THEN, I'm going to install a multi-stage filter to attenuate all vestiges of switching noise and harmonics to a trivial level.

The desire will be to provide plate power to a set of vacuum tubes in a receiver. I know it is possible to run vacuum tubes from low voltages, but to get decent performance out of them you need to run them at their intended ratings. It also will remove the need to install an IC or transistor audio output stage. This would also open up the possibility of running a QRP transmitter using a smaller tube.

Although it's possible to get 220v by using a 6-0-6v transformer, it would be more advisable to use a 12-0-12v with a primary designed for 220v. Reason is, the secondary of the 6-0-6 is not designed to see 12v peaks per side of the winding. This can lead to saturation and damage.

The next challenge would be turning the 12v from a battery or power supply into 6v to power tube heaters. This is avoided by using 12 volt tubes, or a cheap switching converter can be used, too. The tube receiver can be anything from a simple receiver with one 12AT7, or something more complex. My first design used IRF3205s but IRF510 could even be used. I also found that, for a given transformer, the realistic power achievable is around half of the VA rating, as the output voltage drops if loaded too much. I think this is because of the square wave drive to the transformer.

It's also possible to do the same with a cheap inverter purchased from Ebay or Harbor freight. All you need is a rectifier, some filter capacitors, and then some ferrites to filter out the high frequency hash.

Ed AE7TE

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WA2ISE
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Posts: 1051




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« Reply #1 on: September 13, 2017, 12:46:50 PM »

Most modern inverters are really DC-DC converters, where the output DC, around 160V, is then switched with FETs at a 60Hz rate.  Thus no 60Hz transformer involved.   The duty cycle is set to replicate what a sine wave would provide in terms of power.  This is usually called a "modified sine wave" though it's just a square wave with dead periods.  Remove the switching FETS and you'd have 160VDC. 

And you can series connect two 6.3V heater tubes (they must draw the same current) to run them off a 12VDC car battery.
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AD5X
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Posts: 1547




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« Reply #2 on: September 14, 2017, 05:34:00 AM »

I used an automotive 12VDC/120VAC inverter to do this.  Cheap and easy. This is on my website at www.ad5x.com.  Here's the direct link to the article:

www.ad5x.com/images/Articles/HVPSrcvr.pdf

Phil - AD5X
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N8CBX
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Posts: 478




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« Reply #3 on: September 14, 2017, 06:58:03 AM »

...the square wave drive to the transformer.
Yes, transformers don't like a square wave input, and it will get hot. Some capacitor filtering will help.
Jan N8CBX
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Dayton Ohio - The Birthplace of Aviation
WA8UEG
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« Reply #4 on: September 14, 2017, 09:06:35 AM »

FYI - I used AD5X supply for a BC454 and it worked out great.
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K1ZJH
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Posts: 3308




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« Reply #5 on: September 14, 2017, 02:00:57 PM »

How much power do you need? 

This is a simple B battery eliminator that will power 3 or 4 tube portable radios using battery filament type tubes:


http://www.portabletubes.co.uk/sitefiles/ausbatt.htm
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VK2HHS
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Posts: 32




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« Reply #6 on: September 14, 2017, 05:49:53 PM »

I built this one a number of years ago - simple and works well.

For 6.3v heaters, I used an LM350.

http://www.sm7ucz.se/a_www_paraset_co_uk/schematics_01.htm

de VK2HHS
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KB1GMX
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Posts: 1496




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« Reply #7 on: September 15, 2017, 10:43:00 AM »

Over the years I've built the designs mentioned:

http://www.sm7ucz.se/a_www_paraset_co_uk/schematics_01.htm
http://www.portabletubes.co.uk/sitefiles/ausbatt.ht

Also a version of the portable tubes version using transistor instead of fets.

They all work well.  The biggest issue is is hash getting (y conduction or radiation)
into the receiver so the best course is building it into its own shielded box and
bypassing leads entering and leaving.  The latter has been very successful for me.


Allison
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AE7TE
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Posts: 53




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« Reply #8 on: September 29, 2017, 11:36:01 AM »

Something else has been on my mind...

Most of these use iron cored transformers because they are cheap and readily available. They are made for frequencies of 50/60 Hz, mostly.

I wonder what would happen if I upped the frequency to 400 Hz? Logically, the 60 Hz transformer can handle more power at a higher frequency. The goal would be for it to hadle the same power, but run cooler.

Others have said the steel laminations will heat up more at a higher frequency. I know the older vibrator power supplies used to run at 100-200 Hz, so that may be something I'll have to look in to.

Ed AE7TE
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W9IQ
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Posts: 1706




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« Reply #9 on: September 29, 2017, 12:00:05 PM »

The losses in the steel core will increase as the frequency is increased. This results in more heat generation and less coupling of the windings. The heat can eventually destroy the windings on the transformer.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
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