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 Post subject: Selenium Rectifier Substitute
PostPosted: Jan Sat 29, 2011 9:09 pm 
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I am going to build a Knight Kit AM Broadcast transmitter from scratch. The one question I had was with the selenium rectifier that is on the parts list. I know I can replace it with a simple diode but I have room on the chassis and have an extra 5U4 tube laying around that would make the project look even more nostalgic. Besides the extra transformer for the filaments is there anything else that I would have to add to the circuit? Everything I seems to point to no.

Any input?

Daniel


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 Post subject:
PostPosted: Jan Sat 29, 2011 9:15 pm 
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Daniel

You could use a 5U4 but voltage drop across the tube is higher than a selenium rectifier. This will cause DC output voltage to be lower. Might consider 5V4G as this rectifier drop is less than half that of 5U4G.

A silicon diode has the opposite effect, higher DC voltage.

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 Post subject:
PostPosted: Jan Sat 29, 2011 9:16 pm 
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The only real issue I can see is that the voltage drop for each approach is different.

The tube rectifier has the highest drop (~50 volts), then the Selenium rectifier, and the lowest being a Silicon diode.


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 Post subject:
PostPosted: Jan Sat 29, 2011 9:34 pm 
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I guess the higher voltage drop would lower the overall output of the design, which might not be a bad thing since the articles I have read on the transmitter seem to point to higher output then might be allowed now a days.

Thanks for the fast response any more input on the overall design would be great. Has anyone else built one from scratch?

Daniel


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 Post subject:
PostPosted: Jan Sat 29, 2011 9:52 pm 
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A 5U4 would hardly know the transmitter is there and wouldn't drop much voltage, I expect. Depends somewhat on the input filter cap size. This is a good time for a breadboard.


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 Post subject:
PostPosted: Jan Sat 29, 2011 10:01 pm 
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I was reading on another page which had some good info on rectifier tubes. It said that a 5AR4 was the closest to a solid state set up voltage wise. (In the test the solid state had 449.8 volts to a 5AR4s 413, the 5U4 on the same set up only achived 357) its a good reference.

http://www.guitaramplifierblueprinting. ... fiers.html

The input filter cap is 40mfd @150v


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 Post subject:
PostPosted: Jan Sat 29, 2011 10:27 pm 
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If you are going to build from scratch you will have much better results with some of the designs the forum members have designed http://antiqueradios.com/forums/viewtop ... ransmitter

I think you will be disappointed with the results from the knightkit design.

Dave


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 Post subject:
PostPosted: Jan Sat 29, 2011 10:29 pm 
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If the Knight Kit you want to replicate is similar to the one in the 1960 Allied catalog, the power requirements are pretty minimal. That kit had a 12AV6 audio preamp stage, a 50C5 oscillator, and a 50C5 "modulator" (which was there mostly so the heater voltages added up to nearly 115 volts, but could be used to drive a speaker if you wanted to connect one up).

When the selenium and the electrolytics were new, the original circuit had about 110 to 120 VDC of B+ voltage. Very little current was required, maybe only around 25 mA or a little more on voice peaks.

So just about any rectifier that can withstand the voltage will work in this circuit. A 5U4 would be massive overkill, but if you've already got one on hand, there's no reason you couldn't use it.

A bigger problem is going to be finding a transformer that has a 125-0-125 (250 volt center tapped) secondary, so your output voltage can be close to the original value. Most old radio transformers have higher voltage secondaries. It may be easier to connect both plates of the 5U4 together and use it as a half-wave rectifier.

If necessary, you can change the B+ voltage within certain limits by varying the size of the electrolytics (the bigger the caps, the higher the voltage you'll get under load), or by changing the filter arrangement. For the same power transformer and rectifier, a capacitor input filter will give a higher output voltage than a resistor or choke input filter will. But the higher voltage of the cap input filter comes with less regulation than the other two filters can provide.

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 Post subject:
PostPosted: Jan Sat 29, 2011 10:53 pm 
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A few corrections. A 12AX7 was used as a preamp in all three versions as far as I know. The 50C5 modulator was actually a modulator and modulated the plate using Heising Modulation. The early versions did not have a provision to install a speaker as they used a choke on the plate supply for the 50C5 modulator. The later models had the speaker hook up.. The early versions had open sides on the chassis while the later models were enclosed. There were actually three versions of this unit, the very first ones used a 12AX7, 35W4, and a 50B5

Dave


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 Post subject:
PostPosted: Jan Sun 30, 2011 12:41 am 
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The one I had was undoubtedly the el-cheapo version (maybe somewhat later than 1960) ... it had a 12AV6 but there was no magnetic phono input, which the deluxe models with the 12AX7's had. There were so many variations on this theme, one could make a lifelong hobby out of trying to collect all of them.

One thing about any of these transmitters is, they do not sound like AM broadcast stations when you play your music through them. The "AM sound" is the result of the volume compression in the station's processing chain. If the goal is to duplicate that sound, the transmitter should be fed with a compressor, or a transmitter that has it built-in (such as the SSTran AMT3000) should be used.

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 Post subject:
PostPosted: Jan Sun 30, 2011 1:29 am 
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Right now I am running a Lil'7 that I built that uses a single 117L7. If it sounds as good as that I will be fine. I am looking for a bit more power so I can get the music to some of the other antique radios that I have around the house. Plus it will be a cool conversation piece sitting in my office.

Just for kicks here are the radios that I have restored:
Philco's are my specialty:

38-10, 40-190, 38-12, 41-220, and a 41-630 that's on its its way to be restored. I have restored numerous other Philcos for friends as well as a trans-oceanic (won't do one of those again) and am currently working on a Fender Amp for the neighbor.
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 Post subject:
PostPosted: Jan Sun 30, 2011 4:00 pm 
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I've got a 38-10 myself, and have had it nearly 40 years! It is the second radio in my collection (I started at a very early age). I also like the "slope front" Philcos from 1938 through 1940, especially the ones that have tuned RF stages on the front end and push-pull 41 outputs. My idea of what a table radio is supposed to be all about!

I am familiar with single tube 117N7 phono oscillators, having built a couple from plans in old 1940s radio experimenter's handbooks. Replicating the Knight Kit (with the Heising modulator) will be an improvement. Less incidental FM, so not as much distortion, for one thing. But if I remember correctly, that circuit doesn't produce a whole lot of modulation, only perhaps 30% to 40%, so it may not sound as "loud." However, that cloud has a big silver lining, which is that it's impossible to over-modulate the Heising circuit and drive it into "splatter" by accident! You may be able to improve on the loudness of the sound a little by putting some audio processing ahead of the transmitter, as mentioned previously.

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 Post subject:
PostPosted: Feb Sun 06, 2011 10:22 pm 
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After reading everyones post on the Knight kit and looking around at the other designs that are on the site I decided to take a crack at the 6GY6 broadcaster. (Unless someone else has another design tip)

It looks easy enough and sounds like it will do much better then the Lil'7 that I am using right now. (The lil'7 can go to my office at work for the radio I have there)

I still might use a rect. tube for that nostalgic look and just bump the transformer up to compensate. Either way it wil be a functional conversation piece.

Daniel


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