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Hallicrafters S-20R question
Howdy all,
My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying to gather some info regarding it. My question to you is...If this is a receiver, then why the toggle switch for send and receive? What does this do? Thanks in advance! |
"Cowboy67" wrote: My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying to gather some info regarding it. My question to you is...If this is a receiver, then why the toggle switch for send and receive? What does this do? The S-20R is a receiver. If it were used in conjuction with a transmitter, the toggle switch would be switched to "send" when transmitting to mute the receiver. If you were using separate antennas for transmitting and receiving, the routine would be : - Switch the receiver to "send" - Activate the transmitter - At the end of transmission, switch the transmitter to "standby" - Switch the receiver to "receive" Some receivers have an input on the rear panel to mute the receiver. An extra switch on the antenna changeover relay is wired to that input to mute the receiver during transmissions. Art, N2AH |
"Cowboy67" wrote in message ...
Howdy all, My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying to gather some info regarding it. My question to you is...If this is a receiver, then why the toggle switch for send and receive? What does this do? Thanks in advance! The S-20R receiver was often used by hams along with a separate transmitter. The toggle switch was used to disable the receiver when transmitting so that it would not overload, feedback, or burn out something. I do not have a schematic handy, but I think the switch turns off the B+. When using the S-20 as a receiver only, just leave thge switch on receive and forget it. |
"Cowboy67" wrote in message ... Howdy all, My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying to gather some info regarding it. My question to you is...If this is a receiver, then why the toggle switch for send and receive? The switch is would be used if the radio was used as a ham receiver. The operator would switch the radio to "send" to silence the receiver. It's a confusing label, and the later radios used "standby" rather than "send". What does this do? By modern standards, the S-20R is a poor ham radio, so the switch is almost useless. Actually, it's worse than useless, because flipping the switch puts a strain on the 80 rectifier tube. I had a marginal rectifier tube in my Hallicrafters S-40A, and I'd see sparks inside the tube when I flipped the switch back to "receive". I guess that was OK back in the days when rectifier tubes could be bought with pocket change. Now you should just leave it on receive all the time. By the way, the S-20R is still a good radio for shortwave listening. But the original paper capacitors and probably the electrolytic capacitors are bad. The paper capacitors are little more than a roll of foil and paper, and the paper holds up about as well as any other cheap paper from around 1940 would. When these capacitors fail completely, they can also ruin tubes and hard to find transformers. Modern plastic film capacitors are cheap and last indefinately. Thanks in advance! You're welcome! Frank Dresser |
On Mon, 3 May 2004 23:59:10 -0500, "Cowboy67"
wrote: Howdy all, My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying to gather some info regarding it. My question to you is...If this is a receiver, then why the toggle switch for send and receive? What does this do? Thanks in advance! The send puts the receiver in standby, filaments remain on but that is about all. I suspect it disconnects the B+, or at least mutes the audio. The idea is that when you are transmitting on the same antenna even with a T-R switch, you really don't want the receiver active. It isn't good for the receiver, or you ears. |
Frank Dresser wrote:
"Cowboy67" wrote: Howdy all, My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying to gather some info regarding it. My question to you is...If this is a receiver, then why the toggle switch for send and receive? The switch is would be used if the radio was used as a ham receiver. The operator would switch the radio to "send" to silence the receiver. It's a confusing label, and the later radios used "standby" rather than "send". What does this do? By modern standards, the S-20R is a poor ham radio, so the switch is almost useless. Actually, it's worse than useless, because flipping the switch puts a strain on the 80 rectifier tube. I had a marginal rectifier tube in my Hallicrafters S-40A, and I'd see sparks inside the tube when I flipped the switch back to "receive". I guess that was OK back in the days when rectifier tubes could be bought with pocket change. Now you should just leave it on receive all the time. By the way, the S-20R is still a good radio for shortwave listening. But the original paper capacitors and probably the electrolytic capacitors are bad. The paper capacitors are little more than a roll of foil and paper, and the paper holds up about as well as any other cheap paper from around 1940 would. When these capacitors fail completely, they can also ruin tubes and hard to find transformers. Modern plastic film capacitors are cheap and last indefinately. Thanks in advance! You're welcome! I have a soft spot for the S-20R because it was my first 'real' shortwave receiver more than 30-years ago. I bought it for $15 when I was in college. I replaced all the paper caps' and did a complete alignment. Over the years the radio became a test bed for trying different kinds of circuits. I learned a lot about shortwave receiver design from that radio. I also built a digital frequency display for it. The parts cost about $100 in the mid' 70's! Today you can buy a shortwave portable with a display and much better performance for that amount. I wouldn't recommend using the S-20R as your primary receiver but you can still have fun with it for casual listening. -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
starman ) writes:
Frank Dresser wrote: "Cowboy67" wrote: Howdy all, My brother-in-law just gave me a Hallicrafter S-20R receiver. I am trying to gather some info regarding it. My question to you is...If this is a receiver, then why the toggle switch for send and receive? The switch is would be used if the radio was used as a ham receiver. The operator would switch the radio to "send" to silence the receiver. It's a confusing label, and the later radios used "standby" rather than "send". What does this do? By modern standards, the S-20R is a poor ham radio, so the switch is almost useless. Actually, it's worse than useless, because flipping the switch puts a strain on the 80 rectifier tube. I had a marginal rectifier tube in my Hallicrafters S-40A, and I'd see sparks inside the tube when I flipped the switch back to "receive". I guess that was OK back in the days when rectifier tubes could be bought with pocket change. Now you should just leave it on receive all the time. By the way, the S-20R is still a good radio for shortwave listening. But the original paper capacitors and probably the electrolytic capacitors are bad. The paper capacitors are little more than a roll of foil and paper, and the paper holds up about as well as any other cheap paper from around 1940 would. When these capacitors fail completely, they can also ruin tubes and hard to find transformers. Modern plastic film capacitors are cheap and last indefinately. Thanks in advance! You're welcome! I have a soft spot for the S-20R because it was my first 'real' shortwave receiver more than 30-years ago. I bought it for $15 when I was in college. I replaced all the paper caps' and did a complete alignment. Over the years the radio became a test bed for trying different kinds of circuits. I learned a lot about shortwave receiver design from that radio. I also built a digital frequency display for it. The parts cost about $100 in the mid' 70's! Today you can buy a shortwave portable with a display and much better performance for that amount. I wouldn't recommend using the S-20R as your primary receiver but you can still have fun with it for casual listening. That's interesting. I went to the Rochester hamfest in 1973 or 74, and bought the ICs (and LED readouts) for about thirty dollars. They fit in a nice small box, a tad smaller than my fist, which was useful so I could dismiss it's value at the border. Michael |
"matt weber" wrote in message ... The send puts the receiver in standby, filaments remain on but that is about all. I suspect it disconnects the B+, or at least mutes the audio. The switch is between the center tap of the high voltage secondary and ground. Opening the switch disables the B+ circuit. Closing the switch with the tubes warmed up forces a large surge current through the rectifier as it charges the filter capacitors. It's a poor circuit design which was commonly used back then. The idea is that when you are transmitting on the same antenna even with a T-R switch, you really don't want the receiver active. It isn't good for the receiver, or you ears. Disabling the B+ dosen't protect the radio in any way. It might protect the speaker, but no more than turning the volume control all the way down. The antenna coils are the first parts to be damaged by excessive power through the antenna terminals, and they are just as vunerable with the B+ on or off. Not that the antenna coils are easy to damage or anything, but I fixed up a once nice radio which was had a few goofy "ham mods". It was also one of the few radios with burned up antenna coils. Using the "send - receive" switch also reduces the radio's frequency stability. The converter tube and oscillator coils run a little warmer when they're carrying their normal current. The tube and coils cool a bit in the send position, and rewarm up in the receive position. The frequency shifts as the temperature shifts. Frank Dresser |
On Wed, 05 May 2004 13:40:55 GMT, "Frank Dresser"
wrote: "matt weber" wrote in message .. . The send puts the receiver in standby, filaments remain on but that is about all. I suspect it disconnects the B+, or at least mutes the audio. The switch is between the center tap of the high voltage secondary and ground. Opening the switch disables the B+ circuit. Closing the switch with the tubes warmed up forces a large surge current through the rectifier as it charges the filter capacitors. Not really. The vacuum tube rectifiers have very high internal resistance, it is why you can safely use cap input filtering. The tube itself is a very effective surge limiter that gets better with age as the thorium on the cathodes is evaporated off. If you did it with a solid state rectifier, the rectifiers burn up unless you protect them. Some of the high voltage/high vacuum rectifiers like the GZ34 had trouble delivering 250ma with 800 volts on the plate. That's one of the reasons really big, vintage power supplies user Mecury Vapor rectifiers. They have much lower resistance, and you haven't seen a rectifier until you have seen a big 3 phase 800 amp mercury pool rectifier. It's a poor circuit design which was commonly used back then. I disagree. Unlike a solid state rectifier, the vaccum tube rectifier provided surge protection. That is just the way they work. The idea is that when you are transmitting on the same antenna even with a T-R switch, you really don't want the receiver active. It isn't good for the receiver, or you ears. Disabling the B+ dosen't protect the radio in any way. It might protect the speaker, but no more than turning the volume control all the way down. The antenna coils are the first parts to be damaged by excessive power through the antenna terminals, and they are just as vunerable with the B+ on or off. Not that the antenna coils are easy to damage or anything, but I fixed up a once nice radio which was had a few goofy "ham mods". It was also one of the few radios with burned up antenna coils. Using the "send - receive" switch also reduces the radio's frequency stability. The converter tube and oscillator coils run a little warmer when they're carrying their normal current. Where did you learn electrical engineering. In a Vacuum tube system, the current in the oscillator coil is maybe a milliamp or two So unless R is a big number, I^2 is on the order of .000001. I have never seen the coils in a receiver get even slightly warm from I^2 R heating. They are heated far more by radiated and convection energy from the filaments, rectifier, and Audio output tube heat dissipation. In most receivers, the filament power dwarfs everything else. If you have a reciver that is rated 40 watts, and has an audio output of 1-2 watts, the power isn't in the B+. In an All America 5 design, 90+% of the power dissipated is in the filaments. Radiated heat goes up at T^4, so a reduction in power input of 10% results in a change in temperature that is tiny (on the order of 1.7%).... That is often measured in tens of wattts. What is dissipated in the coils is microwatts to milliwatts. Ambient temperature inside the cabinet had far more to do with coil temperatures then the current in the coil. The tube and coils cool a bit in the send position, and rewarm up in the receive position. The frequency shifts as the temperature shifts. Not it if was well designed. Designer did two things. They used regulated voltage on the oscillator, and NPO caps, negative temperature coefficient, so the temperature of the coils would drive the inductance one way, the caps went the other way, cancelling the changes out. Once warmed up and in steady state, these things were often stable to a few PPM. |
"matt weber" wrote in message ... On Wed, 05 May 2004 13:40:55 GMT, "Frank Dresser" wrote: "matt weber" wrote in message .. . The send puts the receiver in standby, filaments remain on but that is about all. I suspect it disconnects the B+, or at least mutes the audio. The switch is between the center tap of the high voltage secondary and ground. Opening the switch disables the B+ circuit. Closing the switch with the tubes warmed up forces a large surge current through the rectifier as it charges the filter capacitors. Not really. The vacuum tube rectifiers have very high internal resistance, it is why you can safely use cap input filtering. Yes there really is a large surge current when hot switching vacuum tube rectifiers! In fact, they went to the trouble to develop a spec for the maximum hot-switching transient plate current. The RCA tube manual says it's 2.5 amps for the 80/5Y3. That's with the tube manual's recommended input filter cap value of 20uFd. This radio uses an input cap something like 40 uFd. I'd rather play it safe, and not hot switch the rectifier. The tube itself is a very effective surge limiter that gets better with age as the thorium on the cathodes is evaporated off. The hot switching problem gets worse as the tube ages. As the emissions go down, the voltage drop goes up. Hot switching old, high voltage drop tubes can cause internal arcing. Hot switching vacuum tube rectifiers needlessly reduces their useful life. By the way, there's no thorium in the 80/5Y3. These tubes use oxide coated filiments. I'm not aware of any thoriated tungsten filament tubes used in normal consumer electronics since the days of the '01A. If you did it with a solid state rectifier, the rectifiers burn up unless you protect them. Some of the high voltage/high vacuum rectifiers like the GZ34 had trouble delivering 250ma with 800 volts on the plate. That's what happens when tubes are operated out of their ratings. The RCA tube manual says the maximum plate voltate allowed for the GZ34/5AR4 is 475V rms, with a capacitor input filter. That's a peak voltage of 672V. The recommended rms voltage with a choke input filter is 600V, but the output voltage from the choke input filter will be lower than the rms voltage input. The recommended maximum hot-switching transient plate current for the GZ34/5AR4 is 3.7 amps. I'm sure some people exceed these ratings, but at the cost of life of the tube. That's one of the reasons really big, vintage power supplies user Mecury Vapor rectifiers. They have much lower resistance, and you haven't seen a rectifier until you have seen a big 3 phase 800 amp mercury pool rectifier. I'm sure such rectifiers also had a much higher hot-switching spec than either the 80 or the GZ34. Although it doesn't matter much. Mercury vapor rectifiers were almost always used with a choke input filter to avoid letting the thing turn into a relaxation oscillator. Mercury vapor rectifiers aren't often found in consumer equipment, either. It's a poor circuit design which was commonly used back then. I disagree. Unlike a solid state rectifier, the vaccum tube rectifier provided surge protection. That is just the way they work. Exceeding the maximum hot-switching transient plate current will reduce the life of the rectifier tube. I can't think of any good reason to hot switch the rectifier in a radio, anyway. The idea is that when you are transmitting on the same antenna even with a T-R switch, you really don't want the receiver active. It isn't good for the receiver, or you ears. Disabling the B+ dosen't protect the radio in any way. It might protect the speaker, but no more than turning the volume control all the way down. The antenna coils are the first parts to be damaged by excessive power through the antenna terminals, and they are just as vunerable with the B+ on or off. Not that the antenna coils are easy to damage or anything, but I fixed up a once nice radio which was had a few goofy "ham mods". It was also one of the few radios with burned up antenna coils. Using the "send - receive" switch also reduces the radio's frequency stability. The converter tube and oscillator coils run a little warmer when they're carrying their normal current. Where did you learn electrical engineering. Is that supposed to be a question? I read books, including tube manuals. I work on radios. I also know how to write! In a Vacuum tube system, the current in the oscillator coil is maybe a milliamp or two So unless R is a big number, I^2 is on the order of .000001. The triode section of a 6K8 will normally draw about 3 or 4 ma. All of that current flows through the oscillator coil. The mixer section draws 2 or 3 mils through the plate and maybe 6 mils through the screen. So, in a normal Hartley hookup, at least 3ma flows through the top section of the oscillator coil, and and at least 11 ma flows through the bottom section of the coil. I know that's still not much DC current, but is much higher than your estimate. But wait! There's more!! The oscillator has a parallel resonant circuit, and as all good EEs know, a parallel resonant circuit has a very large circulating current, which is limited by the Q of the circuit. Higher Q means more current. The oscillator's feedback ratio will also have a large effect on the current of the coil. More feedback means more coil current. All that extra current warms the oscillator coil a bit. Not finger burnin' hot, just a bit. I don't know how many orders of magnitude the AC current is higher than the DC current, nor do I know how many degrees the temperature of the oscillator coil changes as it warms up. I don't care. The effect is that the frequency drifts. The radio has better frequency stability if the B+ isn't interrupted. I have never seen the coils in a receiver get even slightly warm from I^2 R heating. It doesn't take a huge temperature rise of the oscillator coil to cause a few hundred parts per million drift at 10MHz. I'll get about 100 Hz just from the air conditioner cycling on and off in the same room. Very annoyiing on SSB. But these aren't very good radios for that sort of work. No, it's not the voltage shift, either. My SX-62 drifts with temperature, and that one has a voltage regulated oscillator. They are heated far more by radiated and convection energy from the filaments, rectifier, and Audio output tube heat dissipation. In most receivers, the filament power dwarfs everything else. In most receivers, the tubes are above the chassis and the coils are below the chassis. If you're saying that there's more to the observed frequency drift as the B+ is switched on and off, you have a point. But most radios, including the S-20R, have the coils under the chassis and the tubes above the chassis. However, there's some under chassis power resistors which will also contribute to oscillator coil heating and frequency drift. If you have a reciver that is rated 40 watts, and has an audio output of 1-2 watts, the power isn't in the B+. In an All America 5 design, 90+% of the power dissipated is in the filaments. Well, let's run the numbers. The typical AA5 uses 150ma tubes in a series string rated at 120V. That's 18Watts. If the input power is 40 watts, the total percentage consumed by the filament string is 45%, not 90+%. But AA5s use a higher percentage of their power in the heaters because the audio output tube has a high power heater to optimize it for lower plate voltge use. For example, the 50L6 uses a 7.5 watt heater, while the 6F6 uses a 4.4 watt heater. Hallicrafters substituted a 6K6 for the 6F6 in their later mid level radios. The 6K6 had an even more economical heater, at 2.5 watts. Anyway, these tubes are above the chassis, the coils are below. Radiated heat goes up at T^4, so a reduction in power input of 10% results in a change in temperature that is tiny (on the order of 1.7%).... Radiated from the above chassis tubes to the below chassis oscillator coil? Ignoring the actual heating effects in the oscillator coil itself, I have to figure the under chassis B+ dropping resistors have a far larger effect on the temperature of the oscillator coil than the above chassis tubes do. Anyway, the frequency drift after switching the B+ starts right away, and that implies the source of the drift is right in the oscillator circuit. That is often measured in tens of wattts. What is dissipated in the coils is microwatts to milliwatts. Ambient temperature inside the cabinet had far more to do with coil temperatures then the current in the coil. But the under chassis temperature will rise relatively slowly after the B+ is switched. The frequency drift after switching starts immediately. The tube and coils cool a bit in the send position, and rewarm up in the receive position. The frequency shifts as the temperature shifts. Not it if was well designed. Designer did two things. They used regulated voltage on the oscillator, Oh. How many S-20R radios have been designed with regulated voltage for the oscillator? and NPO caps, negative temperature coefficient, so the temperature of the coils would drive the inductance one way, the caps went the other way, cancelling the changes out. Temperature compensation might work over a small range of frequencies. As the tuning capacitor is closed, the reletive effect of the temperature compensation will be reduced. These radios use a bimetal sort of temp compensating capacitor a couple of inches away from the oscillator coil. It's nowhere near the above chassis tuning cap, which is subject to the heat from the tubes. The temperature compensation doesn't work very well. The compensation capacitor is also microphonic. As long as we're on radio design, a good frequency stability technique would be use of low expansion coefficent coil forms such as some of the ceramics. The forms on this radio are bakelite. Bakelite isn't as good as low expansion ceramic, but it better than cardboard. This radio wasn't designed for a high level of frequency stability. It was designed to be a good value for the money. From that point it was a success. Once warmed up and in steady state, these things were often stable to a few PPM. The S-20R is stable to a few PPM? That is the most incredible statement yet!!! Frank Dresser |
On Thu, 06 May 2004 17:19:30 GMT, "Frank Dresser"
wrote: "matt weber" wrote in message .. . On Wed, 05 May 2004 13:40:55 GMT, "Frank Dresser" wrote: "matt weber" wrote in message .. . The send puts the receiver in standby, filaments remain on but that is about all. I suspect it disconnects the B+, or at least mutes the audio. The switch is between the center tap of the high voltage secondary and ground. Opening the switch disables the B+ circuit. Closing the switch with the tubes warmed up forces a large surge current through the rectifier as it charges the filter capacitors. Not really. The vacuum tube rectifiers have very high internal resistance, it is why you can safely use cap input filtering. Yes there really is a large surge current when hot switching vacuum tube rectifiers! In fact, they went to the trouble to develop a spec for the maximum hot-switching transient plate current. The RCA tube manual says it's 2.5 amps for the 80/5Y3. That's with the tube manual's recommended input filter cap value of 20uFd. This radio uses an input cap something like 40 uFd. I'd rather play it safe, and not hot switch the rectifier. The tube itself is a very effective surge limiter that gets better with age as the thorium on the cathodes is evaporated off. The hot switching problem gets worse as the tube ages. As the emissions go down, the voltage drop goes up. Hot switching old, high voltage drop tubes can cause internal arcing. Hot switching vacuum tube rectifiers needlessly reduces their useful life. Not appreciably. Rectifier failures are almost invariable the result of filaments burning out. Remember that both the anode and cathode have considerable mass, so they tolerate short term gross overloads very very well. In fact that is the basis of the so called vacuum tube sound. Most tubes that are rated for a few watts, can easily put out tens of watts for a few seconds without damage, and a Tube like a 4PR400 which is rated 400 watts can actually take a several hundred thousand Kilowatts for a few milliseconds at a time. If you tried that with a solid state device, it would be toast . Short of getting them hot enough for the seals to fail, or elements or connections to the elements to vaporize, there really isn't any such thing as short term damage from overload unless the tube is already at the end of service life anyway. (you eventually boil off all of the thorium on the cathode). By the way, there's no thorium in the 80/5Y3. These tubes use oxide coated filiments. I'm not aware of any thoriated tungsten filament tubes used in normal consumer electronics since the days of the '01A. If you did it with a solid state rectifier, the rectifiers burn up unless you protect them. Some of the high voltage/high vacuum rectifiers like the GZ34 had trouble delivering 250ma with 800 volts on the plate. That's what happens when tubes are operated out of their ratings. The RCA tube manual says the maximum plate voltate allowed for the GZ34/5AR4 is 475V rms, with a capacitor input filter. That's a peak voltage of 672V. The recommended rms voltage with a choke input filter is 600V, but the output voltage from the choke input filter will be lower than the rms voltage input. I suggest you review the design of the EICO 720 transmitter, which used a GZ34, and plate voltage on the 6146 was a nice round 600 volts. Never heard of the GZ34 failing, The 5AR5 is no an absolute replacement for the GZ34. The GZ34 was a lot tougher. I would add that it was permissible to operate tubes well above the commercial ratings, in fact many had two rating CCS (Continuous Commercial Service), and a much higher, manufacturer sanctions, ICAS (Intermittent Commercial and Amateur Service). The ICAS ratings were often 25-30% higher. The recommended maximum hot-switching transient plate current for the GZ34/5AR4 is 3.7 amps. I'm sure some people exceed these ratings, but at the cost of life of the tube. That's one of the reasons really big, vintage power supplies user Mecury Vapor rectifiers. They have much lower resistance, and you haven't seen a rectifier until you have seen a big 3 phase 800 amp mercury pool rectifier. I'm sure such rectifiers also had a much higher hot-switching spec than either the 80 or the GZ34. Although it doesn't matter much. Mercury vapor rectifiers were almost always used with a choke input filter to avoid letting the thing turn into a relaxation oscillator. Mercury vapor rectifiers aren't often found in consumer equipment, either. It's a poor circuit design which was commonly used back then. I disagree. Unlike a solid state rectifier, the vaccum tube rectifier provided surge protection. That is just the way they work. Exceeding the maximum hot-switching transient plate current will reduce the life of the rectifier tube. Why do you believe that? What is the physics involved? Unlike solid state devices, they have such large thermal mass that you really needed GROSS long term overload to damage them. Eimac built a whole line of tubes that were designed for massive (orders of magnitude) short term overload. The triode section of a 6K8 will normally draw about 3 or 4 ma. All of that current flows through the oscillator coil. The mixer section draws 2 or 3 mils through the plate and maybe 6 mils through the screen. So, in a normal Hartley hookup, at least 3ma flows through the top section of the oscillator coil, and and at least 11 ma flows through the bottom section of the coil. I know that's still not much DC current, but is much higher than your estimate. But wait! There's more!! The oscillator has a parallel resonant circuit, and as all good EEs know, a parallel resonant circuit has a very large circulating current, which is limited by the Q of the circuit. Higher Q means more current. Perhaps, but the more resistance there is in the coil, the lower the Q will be. Q is the ratio of reactance to resistance. So if you have enough resistance to make the I^2 losses reach milliwatts, the Q will be lousy (and usually is). even if you work from 10ma, 10 ohms will give you a whole milliwatt to dissipate. The tubes in fact cool by both convenction and radiation, both are in large part take up by the chassis, which is metal. Metal is a pretty good conductor of heat, so to suggest that heat generated on the top of the chassis doesn't also end up underneath is nonsense, but I will concede that resistor to set up screen and bias voltages as well as the bleeder across the B+ cap are likely to generate substantial amounts of heat as well, far more than the I^2R losses in any coil except perhaps in a filter choke. The oscillator's feedback ratio will also have a large effect on the current of the coil. More feedback means more coil current. All that extra current warms the oscillator coil a bit. Not finger burnin' hot, just a bit. I don't know how many orders of magnitude the AC current is higher than the DC current, nor do I know how many degrees the temperature of the oscillator coil changes as it warms up. I don't care. The effect is that the frequency drifts. The radio has better frequency stability if the B+ isn't interrupted. I have never seen the coils in a receiver get even slightly warm from I^2 R heating. It doesn't take a huge temperature rise of the oscillator coil to cause a few hundred parts per million drift at 10MHz. I'll get about 100 Hz just from the air conditioner cycling on and off in the same room. How much of that is temperature, and how much is a result of voltage reduction as a result of the AC unit cycling on? Very annoyiing on SSB. But these aren't very good radios for that sort of work. No, it's not the voltage shift, either. My SX-62 drifts with temperature, and that one has a voltage regulated oscillator. They are heated far more by radiated and convection energy from the filaments, rectifier, and Audio output tube heat dissipation. In most receivers, the filament power dwarfs everything else. In most receivers, the tubes are above the chassis and the coils are below the chassis. If you're saying that there's more to the observed frequency drift as the B+ is switched on and off, you have a point. But most radios, including the S-20R, have the coils under the chassis and the tubes above the chassis. However, there's some under chassis power resistors which will also contribute to oscillator coil heating and frequency drift. If you have a reciver that is rated 40 watts, and has an audio output of 1-2 watts, the power isn't in the B+. In an All America 5 design, 90+% of the power dissipated is in the filaments. Well, let's run the numbers. The typical AA5 uses 150ma tubes in a series string rated at 120V. That's 18Watts. If the input power is 40 watts, the total percentage consumed by the filament string is 45%, not 90+%. But AA5s use a higher percentage of their power in the heaters because the audio output tube has a high power heater to optimize it for lower plate voltge use. If what you say is true, then were pray tell does the other 22 watts go. It certainly cannot be output. A 50C5 cannot delivery anything like that, and frankly, if you tried to dissipate 22 watts in the other 4 tubes, they'd burn out in a matter of hours. YOu are also assuming an AA5 uses 40 watts, most are more like 25. For example, the 50L6 uses a 7.5 watt heater, while the 6F6 uses a 4.4 watt heater. Hallicrafters substituted a 6K6 for the 6F6 in their later mid level radios. The 6K6 had an even more economical heater, at 2.5 watts. Anyway, these tubes are above the chassis, the coils are below. Radiated heat goes up at T^4, so a reduction in power input of 10% results in a change in temperature that is tiny (on the order of 1.7%).... Radiated from the above chassis tubes to the below chassis oscillator coil? Ignoring the actual heating effects in the oscillator coil itself, I have to figure the under chassis B+ dropping resistors have a far larger effect on the temperature of the oscillator coil than the above chassis tubes do. Anyway, the frequency drift after switching the B+ starts right away, and that implies the source of the drift is right in the oscillator circuit. That is often measured in tens of wattts. What is dissipated in the coils is microwatts to milliwatts. Ambient temperature inside the cabinet had far more to do with coil temperatures then the current in the coil. But the under chassis temperature will rise relatively slowly after the B+ is switched. The frequency drift after switching starts immediately. The tube and coils cool a bit in the send position, and rewarm up in the receive position. The frequency shifts as the temperature shifts. Not it if was well designed. Designer did two things. They used regulated voltage on the oscillator, Oh. How many S-20R radios have been designed with regulated voltage for the oscillator? and NPO caps, negative temperature coefficient, so the temperature of the coils would drive the inductance one way, the caps went the other way, cancelling the changes out. Temperature compensation might work over a small range of frequencies. As the tuning capacitor is closed, the reletive effect of the temperature compensation will be reduced. These radios use a bimetal sort of temp compensating capacitor a couple of inches away from the oscillator coil. It's nowhere near the above chassis tuning cap, which is subject to the heat from the tubes. The temperature compensation doesn't work very well. The compensation capacitor is also microphonic. As long as we're on radio design, a good frequency stability technique would be use of low expansion coefficent coil forms such as some of the ceramics. The forms on this radio are bakelite. Bakelite isn't as good as low expansion ceramic, but it better than cardboard. This radio wasn't designed for a high level of frequency stability. It was designed to be a good value for the money. From that point it was a |
"matt weber" wrote in message ... On Thu, 06 May 2004 17:19:30 GMT, "Frank Dresser" wrote: Hot switching vacuum tube rectifiers needlessly reduces their useful life. Not appreciably. Rectifier failures are almost invariable the result of filaments burning out. That is simply not true. Rectifiers generally fail from poor emissions. The oxide coating on the cathodes gets used up. Rectifier filaments sometimes burn out, but that risks a catastrophic failure in which the broken filament touches the plate and shorts. The rectifier short will ruin the power transformer quickly. There's still alot of old radios around, although most of the original rectifier tubes have been replaced. The rectifiers in AA5s are an exception. They have a crimped area in one of the internal wires which acts as a fuse. Problems in the radio, such as a heater-cathode short in one of the tubes will blow the rectifier's internal fuse, rather than risk burning the radio. Remember that both the anode and cathode have considerable mass, so they tolerate short term gross overloads very very well. By "mass", I'll guess you mean thermal mass. OK, short term overloads don't necessarily overheat the tube's internal parts. So what? It's the cathode's oxide coating that gets used up. Drawing excessive current through the rectifier is like chirping the tires on a car. Does chirping the tires cause extra wear? Yes. Will it ruin the tires right away? No. Will it shorten the life of the tires? Of course. In fact that is the basis of the so called vacuum tube sound. Whaaa.... No, please, please -- don't explain. Most tubes that are rated for a few watts, can easily put out tens of watts for a few seconds without damage, and a Tube like a 4PR400 which is rated 400 watts can actually take a several hundred thousand Yes, that's nice. What piece of consumer gear uses a 4PR400? The damage from exceeding the hot switching transient current spec on rectifiers isn't immediate, it's cumulative. Is this a difficult concept? Kilowatts for a few milliseconds at a time. If you tried that with a solid state device, it would be toast . Short of getting them hot enough for the seals to fail, or elements or connections to the elements to vaporize, there really isn't any such thing as short term damage from overload unless the tube is already at the end of service life anyway. (you eventually boil off all of the thorium on the cathode). Ahh, I see. Here's a tube tech update. The thoriated cathode became obselete in consumer radios around 1930. The oxide coated cathode is much more efficent. I suppose I should have mentioned the oxide coated cathode earlier. By the way, there's no thorium in the 80/5Y3. These tubes use oxide coated filiments. I'm not aware of any thoriated tungsten filament tubes used in normal consumer electronics since the days of the '01A. Oh. Looks like I did mention the oxide coated filament. Although I did misspell filament. If you did it with a solid state rectifier, the rectifiers burn up unless you protect them. Some of the high voltage/high vacuum rectifiers like the GZ34 had trouble delivering 250ma with 800 volts on the plate. That's what happens when tubes are operated out of their ratings. The RCA tube manual says the maximum plate voltate allowed for the GZ34/5AR4 is 475V rms, with a capacitor input filter. That's a peak voltage of 672V. The recommended rms voltage with a choke input filter is 600V, but the output voltage from the choke input filter will be lower than the rms voltage input. I suggest you review the design of the EICO 720 transmitter, which used a GZ34, and plate voltage on the 6146 was a nice round 600 volts. Never heard of the GZ34 failing, The 5AR5 is no an absolute replacement for the GZ34. The GZ34 was a lot tougher. Maybe it's just me, but I do think there's a difference between 600V and 800V. Here's a spec sheet on the GZ34: http://www.mif.pg.gda.pl/homepages/f...010/g/GZ34.pdf Philips only allows 450V rms input at 250 ma on the GZ 34, while RCA allows 475V input at 250 ma on the 5AR4. Philips does allow higher voltages, but at lower currents. I don't see much discrepency between the specs of the GZ34 and the 5AR4. In fact, the choke input rating charts are identical, if you take into account that the RCA book uses ratings for one plate: http://hereford.ampr.org/cgi-bin/tube?tube=5ar4 I would add that it was permissible to operate tubes well above the commercial ratings, in fact many had two rating CCS (Continuous Commercial Service), and a much higher, manufacturer sanctions, ICAS (Intermittent Commercial and Amateur Service). The ICAS ratings were often 25-30% higher. Sure, run 'em harder, but at the expense of shorter life. It's in the manuals! Exceeding the maximum hot-switching transient plate current will reduce the life of the rectifier tube. Why do you believe that? Because the the tube manufacturers developed a spec for maximum hot switch transient plate current. Because I know tube emissions go down as the tube ages, and it makes sense that stressing the tube will make emissions go down faster. Because I've seen a marginal rectifier arc internally as it was hot switched. What is the physics involved? No doubt the physics of accelerated tube aging from hot switching into a capacitor are the same as the slower aging from reduced emissions from the oxide filament. By the way, the oxides normally used are of barium and strontium. They haven't been using thorium in production consumer tubes since the days of the Model A. Unlike solid state devices, they have such large thermal mass that you really needed GROSS long term overload to damage them. Eimac built a whole line of tubes that were designed for massive (orders of magnitude) short term overload. Wow. Did Hallicrafters use those tubes as rectifiers in S-20Rs? Perhaps, but the more resistance there is in the coil, the lower the Q will be. Q is the ratio of reactance to resistance. So if you have enough resistance to make the I^2 losses reach milliwatts, the Q will be lousy (and usually is). even if you work from 10ma, 10 ohms will give you a whole milliwatt to dissipate. It isn't a DC current/resistance problem. Due to feedback, the oscillator's tuned circuit draws power from the oscillator tube. I'll try to illustrate it with an Armstrong feedback oscillator, although the priciple is the same for all oscillators. Imagine a normal Armstrong oscillator with a tuned circuit at the grid fed through a blocking cap. The grid leak is connected to ground. The feedback coil from the plate is wound on the same form, but there's no DC connection to the tuned circuit. But, when the oscillator starts up, there's no DC on the tuned circuit but considerable AC. 5 - 10 Volts peak AC is common. In this oscillator, all the power in the tuned circuit is induced from the plate circuit. Let's say there's 5V peak AC at the top of the tuned circuit. 5V peak AC across 10 ohms is 1.25 watts, right? No! This isn't a regular resistance problem, it's a tuned circuit. The impedance of the oscillator's parallel tuned circuit is much higher than 10 ohms. But we do get a significant AC voltage and a significant AC current in the tuned circuit, without a bit of DC. The upper limit for the power dissapation in the tuned circuit would be somewhat less than the DC input to the oscillator tube. If it's 3.5 mils at 100V, the upper limit would be .35W. I'm sure the actual dissapated power is much less than that, but it's still enough AC power to warm the coil slightly and make the oscillator's frequency drift upward until the coil temperature stabilizes. The Radio Amateur's handbook recommends minimal feedback in Variable Frequency Oscillators to minimize RF heating of the oscillator coils. They also recommend against using bakelite forms and ferrite slugs, because they change so much with temperature. The S-20R uses both bakelite coil forms and ferrite slugs, but it wasn't designed with super stability in mind. It was made to be a popular radio, and it was. The tubes in fact cool by both convenction and radiation, both are in large part take up by the chassis, which is metal. Metal is a pretty good conductor of heat, so to suggest that heat generated on the top of the chassis doesn't also end up underneath is nonsense, but I will concede that resistor to set up screen and bias voltages as well as the bleeder across the B+ cap are likely to generate substantial amounts of heat as well, far more than the I^2R losses in any coil except perhaps in a filter choke. Of course, a small part of the tube heat gets under the chassis. But this heat would take time to reach the under chassis oscillator coil. The frequency drift caused by switching the B+ happens right away. I'm sure the under chassis power resistors will eventually cause some oscillator coil temperature rise and frequency drift, as well. The frequency drift from switching the B+ is a fact. But if you want to believe under chassis resistors or tubes cause the drift, that' OK with me. Well, let's run the numbers. The typical AA5 uses 150ma tubes in a series string rated at 120V. That's 18Watts. If the input power is 40 watts, the total percentage consumed by the filament string is 45%, not 90+%. But AA5s use a higher percentage of their power in the heaters because the audio output tube has a high power heater to optimize it for lower plate voltge use. If what you say is true, The tube manuals say it's true. If you want to nit-pick, the series string of an AA5 adds up to 121Volts, not 120. All the tubes have 150ma heaters. 18 watts of heater dissapation is close enough. then were pray tell does the other 22 watts go. The additional dissapation is more like 12 watts, the 22 watts comes from your 40 watt AA5 number. Anyway, the 12 watts heat the tubes. Do I have to mention that tubes are also heated by plate dissapation and grid dissapation? Let's look at the 50L6. At 120 V, it's plate draws about 50 ma and it's screen draws about 4ma. That's a dissapation of about 6.5 watts, just for the audio output tube. Since the tube's plate dissapation is rated at about 10 watts, it's well within it's tube manual ratings, and should last a long time. The same excercise can be repeated for each tube, but I hope you get the point. It certainly cannot be output. A 50C5 cannot delivery anything like that, and frankly, if you tried to dissipate 22 watts in the other 4 tubes, they'd burn out in a matter of hours. All the tubes in AA5s are operated within their normal dissapation ratings. The tube manuals say so! YOu are also assuming an AA5 uses 40 watts, most are more like 25. It's more like 30 watts, but 40 watts was YOUR number. Frank Dresser |
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