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Author Topic: Voltage doubler for filament supply?  (Read 9186 times)
AA4PB
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Posts: 12847




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« Reply #15 on: December 03, 2012, 04:59:31 PM »

Just make sure that the switch mode power supply doesn't generate RFI that will get into your receiver. The receive signal probably passes through antenna relay contacts inside the amp so there isn't a great deal of isolation.

Also note that you are applying RF drive to the filaments. The RF choke and bypass capacitors are supposed to isolate the RF drive from the filament transfomer. A center tap is used on the transformer to keep things balanced. You **may** have problems with RF drive getting into the switch mode power supply and causing regulation problems.
« Last Edit: December 03, 2012, 05:38:18 PM by AA4PB » Logged
KE3WD
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Posts: 5689




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« Reply #16 on: December 03, 2012, 07:17:54 PM »

AA4PB voices the same concerns as I. 

But worded a lot better than I probably could.


73
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G3RZP
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« Reply #17 on: December 04, 2012, 02:59:13 AM »

Another warning about switchers. When you first switch on and the filament is cold, it draws more current than when hot. That means that you need  a switcher with a much higher current capability than you would first imagine, or one that goes into current limit but still starts. A lot don't.

Add to that the possible RFI problems, and an extra transformer is a better route.
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N8CBX
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« Reply #18 on: December 04, 2012, 06:27:12 AM »

Very interesting, thanks for pointing out your concerns of using the switcher. Now I understand why there is a center tap on the filament transformer.
Jan
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Dayton Ohio - The Birthplace of Aviation
K1DA
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« Reply #19 on: December 05, 2012, 09:53:36 AM »

I hope that switcher is rf quiet and not subject to rfi itself.  Lotta junctions in a switcher. You are feeding a high power RF amp, not a stereo.
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KC4MOP
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« Reply #20 on: December 06, 2012, 07:08:32 AM »

Does the filament need to be AC? What difference does it make if AC or DC. I would think no difference.
Jan
AC current is easier to make happen and manage than DC...You might need 30 amps of DC current to light those tubes.
And noted earlier, the switcher supply will not like the cold start filament that it will see to get those tubes to light up. Transformers handle all this overloads and cold filaments pretty nicely
Fred
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N2EY
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« Reply #21 on: December 06, 2012, 08:01:31 AM »

The best solution is a completely separate 12.6 volt transformer. Here's why:

1) The GI-7B tubes are indirectly-heated and require warmup time. They're not "instant on" the way 3-500Z, 572B and similar directly-heated tubes are. A separate transformer can be switched on first, and the B+ switched on later. Increases tube life, adds flexibility and can save energy.

2) Less turn-on current surge

3) Less load on the SB-200 transformer.

Do not use a switching supply, voltage doubler, etc. Those are complex solutions to a simple problem.

73 de Jim, N2EY
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K2QPN
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« Reply #22 on: December 06, 2012, 06:10:51 PM »

Just wondering why you are bothering to change the tube? 572Bs are available and not expensive. They are rugged - I built my SB200 in 1968 and still has the original tubes. The power supply will limit you power output. Seems like a lot of work for little gain.

Just curious.

73, Bob K2QPN
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N8CBX
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Posts: 147




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« Reply #23 on: December 07, 2012, 07:14:06 AM »

Bob,
Why? I aready restored a SB-200 (check out my QRZ.com page) and like it. I bought a CBer's version of a SB-200 recently (it was junk & burnt up, and paid too much) and I want to use & do something different than with the 572Bs.
Jan N8CBX
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Dayton Ohio - The Birthplace of Aviation
NO9E
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Posts: 403




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« Reply #24 on: December 07, 2012, 10:48:59 AM »

Jan,
It would be interesting to see how the switcher works. It might create a distortion or it may not since it will be inside a metal cage. The peak input current will be reduced, which may be good and extend the life of the tube. Like inrush protector for free.
Ignacy
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KE3WD
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Posts: 5689




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« Reply #25 on: December 07, 2012, 02:53:18 PM »

Most switchers will try to supply as much Current as the load demands.  This is one reason I get to repair so many switchers - and also the stuff they power. 

Some are limited, lots are not. 


As far as using a switcher to power filaments in a qro linear amp, it can only be defined at this point in time to the realm of the experimenter. 

That experimenter should already be rather well versed in switcher supply design and implementation IMO. 


73
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K4RVN
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Posts: 772




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« Reply #26 on: December 07, 2012, 04:28:18 PM »

Jan,
It would probably be easier to add a transformer for the tube you mentioned. For 20 bucks If you have the room to mount it take a look at this link if interested.
http://www.allelectronics.com/make-a-store/item/TX-124/12VCT-4A-POWER-TRANSFORMER/1.html
Too bad the ct is grounded on the 12.6 volt transformer I think in your SB 200. I suppose it is used for relays and other stuff or you might could use it for the filaments and put a smaller one in for 6.3 volts for relays , pilot lamps, etc. I guess you want to substitue the
tube you mentioned for the 572.
Frank
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KE3WD
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Posts: 5689




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« Reply #27 on: December 07, 2012, 06:50:42 PM »

Jan,
It would probably be easier to add a transformer for the tube you mentioned. For 20 bucks If you have the room to mount it take a look at this link if interested.
http://www.allelectronics.com/make-a-store/item/TX-124/12VCT-4A-POWER-TRANSFORMER/1.html
Too bad the ct is grounded on the 12.6 volt transformer I think in your SB 200. I suppose it is used for relays and other stuff or you might could use it for the filaments and put a smaller one in for 6.3 volts for relays , pilot lamps, etc. I guess you want to substitue the
tube you mentioned for the 572.
Frank

There's the twenty dollar problem solve.

AllElectronics surpluses many good deals typically purchased as excess inventory and the like, I've shopped there for years, great place for the electronics experimenter, but remember, things don't stay on their shelves forever, when gone, they are gone and usually that's the end of it.  That said, I've never gotten a dud from them in all these years and at one point they were the goto for production runs on a couple or three certain items where the price and quality was beating every other source. 

Smart designer would pick up two of these trannies and put the second one on the shelf.  That's a good way to just about guarantee that the first one never fails, right? 

73
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NM3G
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Posts: 47




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« Reply #28 on: December 08, 2012, 07:49:51 AM »

Not quite true. Some of the MRI amplifiers I've worked on (including those built in the 1980s through the mid 2000s had switched power supplies for the filaments. These supplies were current limited and voltage regulated, and microprocessor-monitored. Most of these amplifiers ran 15-30 kW (pulsed) and were linear to around 0.5 dB over a 40 dB dynamic range.

Mind you, you aren't going to just yank a PC power supply out of an old case and drop it into an amplifier without some work ... but there ARE plenty of current controlled, voltage regulated power supplies already built, available surplus and new that will provide your filament power.

73,

Rick
NM3G


...

As far as using a switcher to power filaments in a qro linear amp, it can only be defined at this point in time to the realm of the experimenter. 

That experimenter should already be rather well versed in switcher supply design and implementation IMO. 
 ...
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N8CBX
Member

Posts: 147




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« Reply #29 on: December 08, 2012, 08:49:15 AM »

...For 20 bucks If you have the room to mount it take a look at this link if interested...
Frank,
I found a 12.6VCT transformer from RF Parts that I'm going to buy. They have a wide selection, by the way.
http://rfparts.com/transformer.html#low
Jan
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Dayton Ohio - The Birthplace of Aviation
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