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 Post subject: 6888 AM broadcaster with PCB
PostPosted: Oct Sat 04, 2014 4:20 pm 
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Location: aston, pa, usa
http://www.catahoulatech.com/productdat ... erData.pdf
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Above is new by our member Jon Stanley


http://www.catahoulatech.com/#!/Two-Tub ... ry=9646441
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PCB board available at above site


I have qty of 50, 6888 tubes available for free on a first come first serve basis for the above parts kit and PCB

Note- My first Digikey order only ran me about 15 bucks from the parts list above.

Please post pics of your builds on this thread.

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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Sat 04, 2014 5:23 pm 
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This design uses a switch-mode "boost" converter to get the B+ voltage.

Does this cause any problem with noise or whine on the transmitted signal? I have built 555-based boost converters, but never tried them with anything that had audio involved.

Also, I didn't see the spec for how much B+ voltage is generated. The IRF640 power FET is rated for 200V. Spikes on the switching transistor might get pretty high. I would use a FET with a bit more voltage. Since you are switching less than an amp., the '640 is overkill for current-rating. An IRF720 should do just fine. This will also reduce the amount of current that the 555 supplies to drive the gate of the FET.

Rich

PS: Here is a more conservative design (taken off the Internet) using a 500V FET to get 180V DC

Image

In this design, one can adjust the B+ voltage, using R5. For more adjustment range, R5 could be increased and R4 decreased.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Sun 05, 2014 1:53 pm 
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Location: aston, pa, usa
if you want to build one i'll send you everything for free

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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Sun 05, 2014 4:29 pm 
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If the switch mode supply is similar or just like nixie tube HV supplies which use a device to turn on and off voltage to a choke then it may indeed not produce RFI.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Sun 05, 2014 6:27 pm 
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Quote:
If the switch mode supply is similar or just like nixie tube HV supplies which use a device to turn on and off voltage to a choke then it may indeed not produce RFI.


Yes, it is a boost converter and is also used to power Nixies. Curious as to why you say it wouldn't produce RFI? With good design, like a snubber across the transistor, and some shielding, it could be made to work RFI-free.

Any square wave inverter can produce hi-freq. harmonics and if the circuit has some ringing, you can have all kinds of RF and/or audio noise.

Just put an AM radio near one of your switch-mode "wall warts!"

Rich


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Sun 05, 2014 8:47 pm 
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It's guaranteed that the converter will will produce RFI. However, that doesn't necessarily mean that it will interfere with the transmitter circuit. One would hope that the person who designed the PCB has tested it to ensure that the boost converter is compatible with the transmitter.

The transmitter being a high power circuit (relatively speaking) could be reasonably immune to the RFI, compared to using a boost converter to power a receiver which is amplifying extremely low level signals.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Sun 05, 2014 11:07 pm 
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yes if it is properly designed it can indeed be RFI free or at least not radiate the RFI very far.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Mon 06, 2014 5:34 pm 
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The PCB link shows a nice clean scope trace. If RFI is a problem, would it show up in that trace? I guess not since we're seeing an audio trace?


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Mon 06, 2014 5:37 pm 
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Rich, W3HWJ wrote:

Also, I didn't see the spec for how much B+ voltage is generated.

Here you go (from the description)
Quote:
The switching power supply utilizes a simple boost converter topology using a 555 timer driving a power MOSFET and an inductor to generate about 150VDC at 20mA for the tubes.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Mon 06, 2014 6:42 pm 
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Looks like it's modulated RF and looks pretty clean.

Maybe they came up with a nice clean design!

Rich


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Fri 10, 2014 3:47 am 
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Bill

That's a very nicely made PC board. Received it today.

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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Fri 10, 2014 2:34 pm 
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That is a very nice board! Very professional. I may try building one.

Pete

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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Sun 12, 2014 2:08 am 
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Received my board today. Definitely first class.

Darrell


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Wed 15, 2014 4:31 am 
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What is the efficiency of that switch mode supply? I'm wondering how it would do supplying B+ for a Zenith tube Transoceanic.

Pete

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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Wed 15, 2014 5:49 am 
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I was aiming for 67 V at about 40 mA when I built a 555 boost converter. I think it was about 60 or 70% efficiency.

You can easily get 70 or 80% with a dedicated switch-mode controller chip and optimized MOSFET. Especially if you can start from at least 10V. The 555 is an old chip and never intended as an SMPS controller.
http://desmith.net/NMdS/Electronics/Nix ... ematic.png

I was trying to run mine from 4 D-cells. The peak currents are pretty high.

Lots of info on boost converters for Nixie supplies.

The problem with a boost converter as a B+ supply is keeping the RFI down.

Rich


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Wed 15, 2014 7:29 am 
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The 555 wastes quite a bit of power all by itself. If you use the 7555 CMOS version of the timer, then the efficiency is better. I got about 82% efficiency with a CMOS version when supplying 25mA at 90 volts to the load which is about what a Zenith TO would require. With good shielding and good filtering I was able to power a radio and not have any problem with RFI.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Wed 15, 2014 8:45 am 
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BobWeaver wrote:
The 555 wastes quite a bit of power all by itself. If you use the 7555 CMOS version of the timer, then the efficiency is better. I got about 82% efficiency with a CMOS version when supplying 25mA at 90 volts to the load which is about what a Zenith TO would require. With good shielding and good filtering I was able to power a radio and not have any problem with RFI.
Part of the problem is MOSFET gate capacitance. I've been toying with one and efficiency is improved by adding a PP transistor driver, at least in SPICE.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Wed 15, 2014 4:05 pm 
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Quote:
Part of the problem is MOSFET gate capacitance


--Don't use a larger MOSFET than needed. The higher the current rating the larger the gate capacitance. IRF740 = 1400 pF; IRF710 = 170 pF. The 710 may have too much conduction loss, but the 740 size is a real hog for gate drive.

--Whether you drive it from an IC or a "totem-pole" bipolar arrangement, you still have to furnish gate charge and that means current

--Keep the switching frequency modest. Some folks like to build 100 KHz +, but that means more drive current needed. Anything above the audible range (say 20 KHz) is probably OK.

Rich


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Sat 18, 2014 10:24 am 
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Rich, W3HWJ wrote:
--Don't use a larger MOSFET than needed. The higher the current rating the larger the gate capacitance. IRF740 = 1400 pF; IRF710 = 170 pF. The 710 may have too much conduction loss, but the 740 size is a real hog for gate drive.

--Whether you drive it from an IC or a "totem-pole" bipolar arrangement, you still have to furnish gate charge and that means current

--Keep the switching frequency modest. Some folks like to build 100 KHz +, but that means more drive current needed. Anything above the audible range (say 20 KHz) is probably OK.

Rich
I'm working on a switcher for a 2 W guitar amp at 210 V, 50mA, and get a smidgen better than 81%, in SPICE, and you got it with the Rdson. The IRF710 more than eats up the 'savings' from the gate capacitance.


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 Post subject: Re: 6888 AM broadcaster with PCB
PostPosted: Oct Sun 19, 2014 10:49 am 
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That's a good point about not using a higher current rated MOSFET than necessary. It looks as if there's going to be a tradeoff though, because the MOSFETs with lower current ratings typically have a higher on resistance. I bought a bunch of IRF730's a couple of years ago, mainly because they were cheaper than anything else I could find at the time. They fall halfway between the IRF740 and IRF710 in almost all parameters. I've had quite decent results with them.

Efficiencies in the low 80's seem to be achievable with reasonably careful design, using good quality components. But beyond that, I think it quickly becomes the law of diminishing returns. To get much above 80% efficiency, you'd likely be spending a lot more money on very low loss components.

BTW, one component that hasn't come up in the discussion so far is the inductor. A good quality low loss inductor is also very important for efficiency.


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