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AA5 AM Tube Radios on 240V AC

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AA5 AM Tube Radios on 240V AC AA5 AM tube radios on 240V AC http://pw2.netcom.com/~wa2ise/radios/aa5240v.html Designing an AA5 AM tube radio for use on 240V AC 5ive methods, just below is the "heater current in series with the plate current" method. Second method is the "higher B+ on the 50L6 audio output tube, AC cap in the heater string" method. A third is to use a capacitor in series with the entire radio circuit. And a 4th radio where the usual tubes are replaced by those whose heater currents are 90 (117L7) and 70ma (three 26x6). Yet another using 100ma heater tubes . My objective here is more of an electrical engineering "what-if" than just being a way to convert an existing radio. Like a "How would the engineers at the XYZ Radio Corp design an "AA5" for sale and use in 240V countries". Given a large enough production volume, the cost of the 26V tubes used in radio four would come down to nearly that of the common AA5 tubes. The diagram on the right shows the modifications done to an otherwise standard AA5 AM radio circuit. How a manufacturer might have designed an AA5 for 240V AC countries. If it is desired to convert a normal "All American 5ive" 120V AC AM tube radio design to run off of 240V AC, the following is a relatively low energy consumption way to do it. Especially for the collector living in Europe or other parts of the 240V AC powerline world. Olympic back in '53 made an adaptor cord, which consisted of a 430 ohm power resistor around 40W in series with the entire set, for their AA5 sets. Curtain Burner? Cheap, but burns a lot of power (about 30W in the resistor plus the 30W for the AA5). My following mod saves about 20W. It does require the use of some "sand", though. Not totally out of place, as solid state silicon rectifier diodes did exist in the last decade of AA5 production. Or use a selenium rectifier for earlier sets, to be "authentic" to the technology available when your radio was manufactured. Also need a 500 to 600 ohm power resistor, of about 20 watt rating (about twice the power rating needed, but I like to run them cool). What we are going to do is place the heater string in series with the B+ current of the radio, and the diode and resistor (wired in series) is to provide heater current for the opposite direction of the flow of the radio B+ current. Thus, after warm-up, the heater string will see a reasonably balanced AC current. The diode and resistor also provides a critical start-up function to get the tube heaters warmed up enough so the radio B+ current starts pulling more current thru the heater string. Can't do that until the cathodes are reasonably hot first. It takes several minutes before everything gets warmed up and balanced out. Idea is to have the heater string act as the series dropping resistance to operate the radio's B+ circuits from, and also to use the radio's B+ circuits current as a series dropper for the heater string. Rather than burning more power by passing all the heater current AND B+ current thru a single big resistor. More detail: first step is to make sure the radio works before doing any mods. Then, at the rectifier tube, the rectifier plate needs to be separated from the top of the heater string. The top of the heater string gets connected to one side of the 240V AC line (let's call it "A"), the rectifier tube's plate is connected to the other side of the 240V AC line (call this "R"). The heater string 1 of 14 2/02/2013 7:16 AM AA5 AM tube radios on 240V AC http://pw2.netcom.com/~wa2ise/radios/aa5240v.html connection to the radio's B minus line stays in place as it was. This places the heater string in series with the radio's B+ supply circuit. Now take the silicon rectifier diode (1N4007 for example) and connect its cathode to the point where the rectifier tube's plate is connected, point "R". The anode of this diode connects to the power resistor, and then the other end of the power resistor connects to the radio's B minus line. Be sure to connect to the spot where the heater string itself connects to the B minus line. This avoids inducing hum into the radio circuits due to ground loop currents. You may note that the AC current thru the heater string has a DC bias current (in other words, more current in one direction than the other). This is not a problem, heaters are just resistive elements. But it can make taking AC voltage measurements difficult to get meaningful information from. However, you can meaningfully measure the B+ from B minus voltage, I read 114V DC, which is reasonable. As for the heaters, use your experience and look at the color of the heaters and cathodes to see if they "look" the right color and brightness. Or use a 'scope: Radio now consumes about 40 watts, vs. 30 watts for a standard AA5 set. To design the above mod, I used a bit of calculus type math. I enjoyed calculus class in college so much, I took it twice! :-) But the following is fairly easy calculus. To determine power consumption over one cycle of the AC powerline, integrate, from 0 to 2p, (150(sine 2(X))) dx. Sine squared because power is current squared times resistance of the heaters. This gives a reference number we need to match with the B+ and diode/resistor currents. Number was about 470. We now figure the power dissipated in the heater string by passing the radio's B+ current thru it. Figure 60ma. This is effectively an RMS value, the actual waveform is anything but even. So we do an integral, from 0 to 2p, (70x) dx. This yielded a number of 200 or so, and the below diode/resistor needs to make it up to 470. A reality check would be: heater power = 120V * 150ma = 18 watts. Radio B+ power would be 150V (don't forget losses in the rectifier tube) * 60mA = 9 watts. Half the heater power. We are going to put in "series" the 2 of 14 2/02/2013 7:16 AM AA5 AM tube radios on 240V AC http://pw2.netcom.com/~wa2ise/radios/aa5240v.html heater power, and the B+ power plus an extra 9 watts or so of power in the power resistor. Now we figure the power for a half cycle of the powerline, as the diode/resistor gives this. The diode makes the other half cycle zero power. So we do the integral, from 0 to p, (Y(sine 2(x))) dx. And just keep picking a value for Y that makes things add up with the above 240 to get a total of 470. Once you nail down a value for Y, do: 120/Y= value of power resistor for use with the diode. I got about 600 ohms. Reality check: figure (1/2)(diode cuts out half the cycle)*(120v 2/600)=12 watts, ahhhh... a little high. Turns out I need to run a bit more than half the usual heater current thru the tubes on turn-on to get them hot enough to start functioning and conducting current thru their cathodes. After that starts, more heater current starts warming up the tubes more, eventually reaching a steady state condition. Your calculus prof in college would probably shudder in horror at the crudeness of the above use of calculus, but it yielded an answer that made a working circuit when built the first time! And the above is roughly what real engineers in the real world actually do when designing stuff. No rigour, no proofs. 2nd method if radio uses octal tubes This method rectifies the 220V AC line directly, to create higher B+ voltage for the audio output stage (50L6). The 50L6 is designed to operate at this plate voltage; however the 50C5 is not. The load on the output tube needs to be doubled for use with this higher B+. This can easily done by replacing the 4 ohm speaker with an 8 ohm speaker. The output transformer reflects this change to the output tube. There will be less bass response, but that is usually not an issue in an AA5. You should get more audio output power. Also the cathode resistor should be increased as per the diagram. 3 of 14 2/02/2013 7:16 AM AA5 AM tube radios on 240V AC http://pw2.netcom.com/~wa2ise/radios/aa5240v.html An alternative to changing the speaker is to change the output tube to a 50C6. That's right, fifty-cee-SIX. But be sure it can physically fit inside the radio, as it is bigger. Same basing as the 50L6. You'll need to change the cathode resistor to 220 ohms. The 50C6's load impedance is rated for 2600 ohms at 200V plate voltage, close enough to the 2000 ohm load for the 50L6 at 120V plate installed in the radio. The 50C6 is nearly the same as the 6Y6. BTW the 50Y6 is a rectifier. A new B+ capacitor rated for 250VDC will be needed. And a 6K seven or so watts will need to replace the old 1K resistor to run the output tube screen grid and the rest of the set.
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