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#1
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
Okay, so I like to make up cutesy names
I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm |
#2
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
"flipper" wrote in message ... Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm Nice design work...and around inexpensive NOS tubes as well! That's a great looking output waveform. I'll bet that it'll sound great playing through a vintage receiver! Best Regards : Doug Bannard |
#3
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
"flipper" wrote in message ... Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm But the smartest arses (aussies of course, he-he!) put their AM transmitters in the vintage radio cabinets. http://www.ebay.com.au/itm/140770446...84.m1555.l2649 |
#4
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Mon, 11 Jun 2012 12:50:03 +1000, "Alex Pogossov"
wrote: "flipper" wrote in message .. . Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm But the smartest arses (aussies of course, he-he!) put their AM transmitters in the vintage radio cabinets. http://www.ebay.com.au/itm/140770446...84.m1555.l2649 Interesting you bring that up because it was my original 'cost saver' idea before the UHF converters and I even bought a supposedly non working Philips radio to use. Turned out the radio only needed one tube, which I had. It could also use a recap but it's in good enough shape I didn't have the heart to scavenge it. I might resurrect the idea for the air variable and dial indicator but the biggest problem is I want power transformer isolation and most don't have one (at least not the AA5s), nor 'extra room' to fit one. I notice that Little Nipper does but it's 240 VAC. I think my 2 bucks 2 tube Beamus scope shot looks as good as his 5 tube job |
#5
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
"flipper" wrote in message ... On Mon, 11 Jun 2012 12:50:03 +1000, "Alex Pogossov" wrote: "flipper" wrote in message . .. Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm But the smartest arses (aussies of course, he-he!) put their AM transmitters in the vintage radio cabinets. http://www.ebay.com.au/itm/140770446...84.m1555.l2649 Interesting you bring that up because it was my original 'cost saver' idea before the UHF converters and I even bought a supposedly non working Philips radio to use. Turned out the radio only needed one tube, which I had. It could also use a recap but it's in good enough shape I didn't have the heart to scavenge it. I might resurrect the idea for the air variable and dial indicator but the biggest problem is I want power transformer isolation and most don't have one (at least not the AA5s), nor 'extra room' to fit one. I notice that Little Nipper does but it's 240 VAC. I think my 2 bucks 2 tube Beamus scope shot looks as good as his 5 tube job Apparently, the more tubes -- the more "decent" and "professional" it might look... But your 2-tube solution looks like gives clean modulation due to the feedback. However, the biggest (potential) problem of your design is frequency instability. If you have a parasitic coupling between the aerial and the oscillator coil, you will get the "frequency pull". If you touch the antenna (changing the voltage on it) -- you will get a frequency shift! This pull will also be modulated, so you will get a spurious FM. Probably acceptable for an AM radio, but will sound crap on an SSB receiver. So your major goal is to shield your oscillator circuit (L1, C1b and other components). And even if you make a perfect shielding, you will not be able to completely avoid parasitic coupling through the shaft impedance of the dual gang variable capacitor. |
#6
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Sun, 10 Jun 2012 19:48:23 -0400, "Doug Bannard"
wrote: "flipper" wrote in message .. . Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm Nice design work...and around inexpensive NOS tubes as well! That's a great looking output waveform. I'll bet that it'll sound great playing through a vintage receiver! Best Regards : Doug Bannard Thank you. Yes, it does sound good though my Zenith 845 but by the time I got through the cornucopia of recording problems it ended up on a circa 80's Panasonic clock radio, which is a fairly decent one with a 6x4 speaker. The single speaker helped with mic directional problems. |
#7
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Mon, 11 Jun 2012 14:16:59 +1000, "Alex Pogossov"
wrote: "flipper" wrote in message I think my 2 bucks 2 tube Beamus scope shot looks as good as his 5 tube job Apparently, the more tubes -- the more "decent" and "professional" it might look... But your 2-tube solution looks like gives clean modulation due to the feedback. Yep, NFB is the key to getting g1 linear. I did the same thing on my reactance tube FM transmitter. And the Twin Triode Transmitter, come to think of it. For not being an 'RF guy' I sure have done a lot of RF lately. However, the biggest (potential) problem of your design is frequency instability. If you have a parasitic coupling between the aerial and the oscillator coil, you will get the "frequency pull". If you touch the antenna (changing the voltage on it) -- you will get a frequency shift! This pull will also be modulated, so you will get a spurious FM. Probably acceptable for an AM radio, but will sound crap on an SSB receiver. So your major goal is to shield your oscillator circuit (L1, C1b and other components). And even if you make a perfect shielding, you will not be able to completely avoid parasitic coupling through the shaft impedance of the dual gang variable capacitor. That sounds a lot like the old joke "Doc, it hurts when I poke a stick in my eye, What should I do about that? Don't poke a stick in your eye." Don't touch the antenna. I do intend to shield the LO and, for cost reasons, dropping the gang tuning idea. (Maybe I should blame it on a 'bean counter', eh?) There's been a revision. The 70 uH was low and the scramble wind on a UHF core not so hot, which was causing low plate swing. So I've now gone to a 250 uH ferrite rod antenna for the plate load and am getting 80 Vpp at idle. And, good as it was, modulation depth also improved. It's now so close to 100% you can't tell the difference unless you blow the scale up. Looks to me like this one is a winner but, of course, I'm a bit biased Revised schematic is posted http://flipperhome.dyndns.org/Beamus.htm |
#8
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Jun 11, 12:50*pm, "Alex Pogossov" wrote:
"flipper" wrote in message ... Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm But the smartest arses (aussies of course, he-he!) put their AM transmitters in the vintage radio cabinets.http://www.ebay.com.au/itm/140770446...ELX:IT&_tr...- Hide quoted text - - Show quoted text - Hmm, I must qualify as a dumb-ass aussie because I've not tried to make an AM transmitter in such a nice package, all kinda trick stuff. But you know, I recall that while I studied FM tube radios so I could service them properly in 1995, I mentioned production of an FM test signal by means of a reactance tube FM modulator to a bunch of radio enthusiasts at a ham gathering one evening to see if they had any schematics or knowhow. None did, and one said "Mate, waddia wanna vary frequency for? We jus wanna keep the frequency stable, eh..." Well, OK, they were stuck back in 1955, most were over 70 in 1995. Now most are dead, and I'm still lingering with audio F stuff, having farnarkled muchly with RF home brew testorator checker thingies. But I was about 15 when I made my own transmitter out of an existing old radio set. I got my sister to talk into the crystal mike while walked up the street to see how far it broadcast, using my mum's new Japanese 9 transistor portable. After I'd gone about 1/4 mile dear sister starts saying "This is the illegal broadcast by Patrick Turner from No ----- street....." and then I qualified for the Olympics 400M getting back home to shut the sister up. Females can be cheeky, and sis was no exception. I have an idea to make another modulator which might work merely by having 3 cascaded variable µ pentodes, each contributing about 30% AM. maybe it works, maybe it don't, but it'll have to wait until so many other things more worthwhile are completed. In Canberra we get ABC Radio National on 845kHz AM with AF BW allowed to be 9kHz, so my AM radio has 10kHz BW using variable selectivity in the 1st IFT by varying the distance between the coils. But one could always use the Internet to get the podcasts at http://www.abc.net.au/radionational/podcasts/program/ And you just subscribe. In many places RN is broadcast on FM, and now in digital stations. So I guess one could tune into RN on the digital and use the audio to modulate a carrier for the AM radio to pick up, and that should sound better than if one tries to pick up the local AM radio station pumping out 5kW, and only 4km away. Just don't ask me to describe how digital radio works. I looked for radio schematics and found none, seems like secret business nobody wants me to know, but seems like the digital band is at UHF or around 250MHz, and just what the wave forms look like I don't know. Patrick Turner. |
#9
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
In article ,
flipper wrote: Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm Hi Flipper, Dropping the ganged capacitor was probably a good idea, if it was a superheterodyne capacitor there was probably no way you could have made it track correctly. Did you ever think of using a crystal in the oscillator circuit, that way there would be only one tuning adjustment? What is the potentiometer in the oscillator screen circuit all about, what is the criterion for adjusting it? You state that 80 Vpp on the plate equates to about 35 mW, how did you calculate that? -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#10
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Wed, 13 Jun 2012 15:48:48 -0500, John Byrns
wrote: In article , flipper wrote: Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm Hi Flipper, Dropping the ganged capacitor was probably a good idea, if it was a superheterodyne capacitor there was probably no way you could have made it track correctly. Did you ever think of using a crystal in the oscillator circuit, that way there would be only one tuning adjustment? I wanted the frequency, at least some degree, tunable. What is the potentiometer in the oscillator screen circuit all about, That adjusts osc RF amplitude to the deflector plates. what is the criterion for adjusting it? I'm still pondering that a bit but current procedure is to run it up to near max mod and adjust for maximum peak to peak. You state that 80 Vpp on the plate equates to about 35 mW, how did you calculate that? That's guesstimated EI across the plate load. |
#11
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Monday, 11 June 2012 03:39:00 UTC+10, flipper wrote:
Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm I couldn't download Fliper's page above on his transmitter last week but can now, and the circuit looks well. Possibly some improvement could be to used cathode followers to drive each beam delctor plate. If he has 35mW and say 50Vrms anode signal at antenna output, then 0.035 = 50 x 50 / RL so RL = 71kohms. This is rather high but the tuned tank circuit probably has such a high Z at Fo. The comments made about coils are of interest, and it should make no difference what type of core is used, because 270uH is easy to achieve with many recipes. But in the best old radios the 455kHz and MW RF coils are pie wound windings using litz wire and large Al cans to keep the Q high, ie, RF resistance low. In an AM receiver I built from scratch in 1999, I made copper cans for two RF input coils and I wound solid wire on 50mm long x 10mm dia pieces of ferrite rod normally used for ferrite rod antennas. The Q became higher than using air cored windings on say 20mm PVC pipes. In my receiver, the Q was so high the BW was less than 15kHz, giving slight cutting of sidebands and thus reducing possible audio BW, so I used TWO such coils, and a 3 gang cap, with 2 gands devoted to the the two RF coild and the other to the oscillator. To overcome the reduction of BW, I used antenna input to a tap on one coil, then a resistor from top of coil to top of next coil and then I tuned the coils so they were "stagger tuned", ie, slightly apart at the low end of the band where the LC Q is higher than at above say 1.2MHz. All that worked well, but compact fluorescent lamps and other junk creates a rectified version of incoming RF and I've had to change to a long rod ferrite antenna with a shielded coil and very short wires to keep out electrostatic portion of RF wave. In my RF generator which I could use as transmitter, I get the same very nice AM wave on the CRO as Flipper's site shows. I have a 6BX6 for the RF output tube, and the coil is a hand wound thing on ferrite with 3 layers of not very tightly wound windings of solid wire, and with PVC tape used between layers. The self capacitance of the coil must be kept low if you wish to be able to tune the tank to as high as possible, say 1,750kHz. Many old radios went from 550kHz to maybe 1,550kHz, a 3 fold F increase. Say you have 20pF for coil self C, then if the cap gang = 365pF max, then total max C = 385pF. Say the coil turns or ferrite position adjusted for Fo = 530kHz, then L must be 234uH. Say the C gang min C = 25pF, and you have 20pF self C then Cmin total = 45pF which gives Fo = 1,551kHz, and not as high as anyone should accept. If C minimum could be reduced to 22.5pF, Fo = 2,194kHz, but you will never see this, and besides, one has to be able to use a trimmer cap to set the top of the F range. So the less stray C or coil self C, the better. Keep the coil cans as big as possible, and never use iron cans. Patrick Turner. |
#12
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Thu, 14 Jun 2012 22:23:29 -0700 (PDT), Patrick Turner
wrote: On Monday, 11 June 2012 03:39:00 UTC+10, flipper wrote: Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm I couldn't download Fliper's page above on his transmitter last week Sorry about that. Every once in a while the servers is offline for various reasons. but can now, and the circuit looks well. Thanks. Must say I'm rather pleased with the deep mod and excellent bandwidth using only two 1 buck tubes. Possibly some improvement could be to used cathode followers to drive each beam delctor plate. Well, as is, there isn't a signal to put into a CF for the other deflector plate. They're being driven single ended. I've considered that driving the deflectors with a PP transformer might improve RF harmonics, and maybe a CF to drive it's primary, but I think it might be possible to simply wind a PP secondary on the osc coil. However, I'm not convinced it's worth the added complexity. If he has 35mW and say 50Vrms anode signal at antenna output, then 0.035 = 50 x 50 / RL so RL = 71kohms. This is rather high but the tuned tank circuit probably has such a high Z at Fo. That's 80 volts peak to peak (Vpp) so RMS is 28 V. Guesstimating 1.25mA gives the 35 mW figure and resonant impedance around 22.4 k but I think the current guess was a tad high so impedance is probably a bit higher. The comments made about coils are of interest, and it should make no difference what type of core is used, because 270uH is easy to achieve with many recipes. I don't know why you say that. It isn't a matter of 'achieving' 250 uH, at least on an LCR meter, it's how it behaves at that frequency. For example, I talked to Bob Weaver about that and he confirms 'switching power supply' ferrite cores (which I also tried) soak up RF like, his words, "a sponge." Now, I don't know whether the UHF coil is doing the same thing or exhibiting some other 'problem' at MW frequencies but the volts weren't there. Might perform better if I had some Litz wire but I'm using a ferrite antenna for the coil now and AES has them for 3 bucks so it isn't worth spending moolah on Litz when buying the thing 'ready made' is just as cheap. But in the best old radios the 455kHz and MW RF coils are pie wound windings using litz wire and large Al cans to keep the Q high, ie, RF resistance low. In an AM receiver I built from scratch in 1999, I made copper cans for two RF input coils and I wound solid wire on 50mm long x 10mm dia pieces of ferrite rod normally used for ferrite rod antennas. The Q became higher than using air cored windings on say 20mm PVC pipes. In my receiver, the Q was so high the BW was less than 15kHz, giving slight cutting of sidebands and thus reducing possible audio BW, so I used TWO such coils, and a 3 gang cap, with 2 gands devoted to the the two RF coild and the other to the oscillator. To overcome the reduction of BW, I used antenna input to a tap on one coil, then a resistor from top of coil to top of next coil and then I tuned the coils so they were "stagger tuned", ie, slightly apart at the low end of the band where the LC Q is higher than at above say 1.2MHz. All that worked well, but compact fluorescent lamps and other junk creates a rectified version of incoming RF and I've had to change to a long rod ferrite antenna with a shielded coil and very short wires to keep out electrostatic portion of RF wave. In my RF generator which I could use as transmitter, I get the same very nice AM wave on the CRO as Flipper's site shows. I have a 6BX6 for the RF output tube, and the coil is a hand wound thing on ferrite with 3 layers of not very tightly wound windings of solid wire, and with PVC tape used between layers. The self capacitance of the coil must be kept low if you wish to be able to tune the tank to as high as possible, say 1,750kHz. Many old radios went from 550kHz to maybe 1,550kHz, a 3 fold F increase. Say you have 20pF for coil self C, then if the cap gang = 365pF max, then total max C = 385pF. Say the coil turns or ferrite position adjusted for Fo = 530kHz, then L must be 234uH. Say the C gang min C = 25pF, and you have 20pF self C then Cmin total = 45pF which gives Fo = 1,551kHz, and not as high as anyone should accept. If C minimum could be reduced to 22.5pF, Fo = 2,194kHz, but you will never see this, and besides, one has to be able to use a trimmer cap to set the top of the F range. So the less stray C or coil self C, the better. I don't really care what the 'top end' is as this isn't to 'test' radios, its a broadcaster, and being able to find a 'quiet spot' on the dial is the only reason for tuning. The 'problem', so to speak, isn't the local osc but the plate tank because a 10 ft wet noodle wire antenna comes in at 30 pF-35 pF before you even get to cap minimum, stray, and coil self capacitance. I'm not sure yet but may go to another slug coil on the plate because even though less inductance lowers voltage swing, hence generated power, that may be better than cap tuning because, in that case, most of the circulating current ends up in the tuning cap rather than the antenna. On the other hand, I'm getting sufficient range even with closed plates so it may be a case of "don't fix it if it ain't broke." Btw, I've tweaked/padded values around the existing coils and caps to avoid the irritation of being able to 'dial' a frequency you can't peak on the antenna tank so the tuning range is now narrowed down to roughly 600 KHz to 1100 KHz. So, now, if you can dial it you can peak it. Basically I put 100 pF in parallel with the osc cap and lowered the coil to 190 uH. I'm listening to it at about 680 KHz as I type. Keep the coil cans as big as possible, and never use iron cans. Patrick Turner. |
#13
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
In article ,
flipper wrote: Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm Hi Flipper, This transmitter has been bouncing around in the back of my head for the past week because something didn't seem quite right but I couldn't put my finger on it until this morning when I jumped out of bed. The problem is that there is nothing wrong with the transmitter concept, except that it doesn't take advantage of the 6ME8 in the way one might have hoped based on your previous comments on the tube. Applying the modulation to G1 doesn't take advantage of the beam deflection capabilities of the tube and instead uses it in a way that a more ordinary tube, like a dual control pentode, could serve. Why not connect the 6ME8 cathode-grid circuit as a Hartley oscillator circuit, as I think you have suggested in earlier posts, and connect the modulation signal to the beam deflection plates? I suppose the downside of this scheme is that it would require a balanced push pull RF output transformer to achieve full modulation, and building such an RF transformer could be a non trivial project in itself. I forgot where you are buying your 6ME8s for a dollar apiece, could you refresh my memory? -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#14
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Mon, 18 Jun 2012 10:26:44 -0500, John Byrns
wrote: In article , flipper wrote: Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm Hi Flipper, This transmitter has been bouncing around in the back of my head for the past week because something didn't seem quite right but I couldn't put my finger on it until this morning when I jumped out of bed. The problem is that there is nothing wrong with the transmitter concept, except that it doesn't take advantage of the 6ME8 in the way one might have hoped based on your previous comments on the tube. Applying the modulation to G1 doesn't take advantage of the beam deflection capabilities of the tube and instead uses it in a way that a more ordinary tube, like a dual control pentode, could serve. A DC pentode might work similar but I'm not sure the plate/screen current transfer is as balanced as the 6ME8 deflection plates. I wondered about that, though, but the final deciding factor was 'non technical': I wanted to use a 6ME8 Might be interesting to try because I'd think it would take less RF amplitude but, ironically, it looks like a DC pentode might pull more B+, even without 'wasting' the 6ME8 plate two power, because it's screen pulls so much current. It also looks like the 6ME8 might have better plate impedance but it's hard to tell because there are no plate curves down at the relatively low voltages it's being run on. Why not connect the 6ME8 cathode-grid circuit as a Hartley oscillator circuit, as I think you have suggested in earlier posts, and connect the modulation signal to the beam deflection plates? I suppose the downside of this scheme is that it would require a balanced push pull RF output transformer to achieve full modulation, and building such an RF transformer could be a non trivial project in itself. That was my first thought too although you may be remembering the FM stereo multiplex discussion, where we needed DSB-SC output, because I think that's the only one I posted a (conceptual) schematic for. However, unbalancing the deflectors and then 'subtracting' one plate from the other via a PP transformer was, indeed, what I proposed to Patrick for the '100% mod' solution back when he was tinkering with an AM modulator for, I think, 'testing' AM radios. I did wonder, in that discussion, what the effect of the 'extra' side bands might be. I mean, you have, in essence, DSB with 'partially suppressed carrier'. I may still try that some day but, yes, the PP RF transformer kind of put me off and I'm glad I tried this one first because of what I learned: the pot core I bought for that very idea, and would have used, wouldn't have worked well at all. The other 'advantage' would be self excite but everyone keeps telling me "don't do it." Self excite 'FMs'. It could also be done 'single ended' but then you have large audio swing required and deflector 'non-linearity at the extremes with no 'simple' means of NFB. So, this topology was picked because it seemed 'simpler', reduced the number of "never done this before" things that could go Murphy's Law wrong, and required no 'special' parts. Doubly so because I already had them. I forgot where you are buying your 6ME8s for a dollar apiece, could you refresh my memory? ABC vacuum tubes and ESRC1 vacuum tubes (which seem like maybe they're the same people). Both sites have a perennial !dollar days! section, which says qty 10 minimum but I've also been able to get a few tossed in when buying others at 'normal' price. However, since those are 'normally' $3 (occasionally $4, as with the 6ME8) you really only have to want '4' (maybe 3), and get 'extras' to make up 10, to come out ahead. vacuumtubes.net has no !dollar days! section but pretty much the same tubes simply listed for 1 buck in their normal price lists, but I've never tried ordering from them. I normally use ABC, mainly out of habit. |
#15
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On 19 June, 01:26, John Byrns wrote:
In article , *flipper wrote: Okay, so I like to make up cutesy names I wanted to do 'something' with a 6ME8 so I tried using it for an AM transmitter and the first prototype works rather well. Bandwidth is 'too much', less than 1 dB down at 18 KHz, but we'll worry about that later. This is a 'dollar days' special, 2 whole bucks worth of tubes, and L2 is a UHF converter coil scramble rewound, so that was 'free'. Power supply is from the same converter and if the remaining issues get worked out it'll probably end up in that cabinet as well (but where to put the air variable?). The schematic is rough, but should be rather self explanatory, and the ganged tuning hasn't been fleshed out yet. I'm still using separate caps while jiggling things around. Orphaned web page, not yet ready for prime time, has a recording of it playing through a table radio. http://flipperhome.dyndns.org/Beamus.htm Hi Flipper, This transmitter has been bouncing around in the back of my head for the past week because something didn't seem quite right but I couldn't put my finger on it until this morning when I jumped out of bed. The problem is that there is nothing wrong with the transmitter concept, except that it doesn't take advantage of the 6ME8 in the way one might have hoped based on your previous comments on the tube. Applying the modulation to G1 doesn't take advantage of the beam deflection capabilities of the tube and instead uses it in a way that a more ordinary tube, like a dual control pentode, could serve. Why not connect the 6ME8 cathode-grid circuit as a Hartley oscillator circuit, as I think you have suggested in earlier posts, and connect the modulation signal to the beam deflection plates? *I suppose the downside of this scheme is that it would require a balanced push pull RF output transformer to achieve full modulation, and building such an RF transformer could be a non trivial project in itself. A balanced RF tranny that is tuned requires coil with more turns on the same core to get twice the inductance, fairly easy if you start with a ferrite rod. I'd probably say the best wire for low capacitance of the coil is solid telephone hook up wire, or strands taken from a cat5 cable. You then need two tuning gangs, 20-365pF are OK. The cap frame and moving plates are at 0V and if you want to avoid B+ across the tuning caps, then cap frame is bolted to chassis and 0.1uF caps from coil to fixed plates. Maybe put 2M2 from fised plates to 0V to bias them down. This allows you to have a 3 gang cap and use one gang for the oscillator coil. The old HP606 I have has this sort of set up. It used 6B4 to cathode modulate a pair of 6CL6. Oscillator is PP type using maybe 12AT7, I forget, but its designed for many ranges from 300kHz to 65MHz, all with well calibrated dial. The whole thing has very high electro- mechanical integrity; must have cost a huge amount in 1955. Its got NFB around the modulator to make the AM more linear. But my home brew SE pentode modulator also works just as well for 2 bands up to 1.7MHz so I can test the BC band or any IF channel. I don't need the more complex HP PP circuit. I can't see any problem having NFB around ANY form of AM modulator because whatever triode is used to as a modulator can be one of a pair of triodes in an LTP with TWO input ports, and one port is for the detected AF NFB signal, and the other is for AF input, so the modulator amplifies the difference between input and FB signal and applies a correction signal at the output. Such an LTP for an SE modulator isn't really a pair, because you only need one anode's output with the other anode grounded via electro cap. But the two available output could be used for a PP modulator. I forgot where you are buying your 6ME8s for a dollar apiece, could you refresh my memory? What other beam deflector tubes are usable? BTW John, be careful jumping out of bed. Lotsa guys come to grief that way :-). Patrick Turner. |
#16
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
In article ,
flipper wrote: On Mon, 18 Jun 2012 10:26:44 -0500, John Byrns wrote: Why not connect the 6ME8 cathode-grid circuit as a Hartley oscillator circuit, as I think you have suggested in earlier posts, and connect the modulation signal to the beam deflection plates? I suppose the downside of this scheme is that it would require a balanced push pull RF output transformer to achieve full modulation, and building such an RF transformer could be a non trivial project in itself. That was my first thought too although you may be remembering the FM stereo multiplex discussion, where we needed DSB-SC output, because I think that's the only one I posted a (conceptual) schematic for. However, unbalancing the deflectors and then 'subtracting' one plate from the other via a PP transformer was, indeed, what I proposed to Patrick for the '100% mod' solution back when he was tinkering with an AM modulator for, I think, 'testing' AM radios. I did wonder, in that discussion, what the effect of the 'extra' side bands might be. I mean, you have, in essence, DSB with 'partially suppressed carrier'. I may still try that some day but, yes, the PP RF transformer kind of put me off and I'm glad I tried this one first because of what I learned: the pot core I bought for that very idea, and would have used, wouldn't have worked well at all. The other 'advantage' would be self excite but everyone keeps telling me "don't do it." Self excite 'FMs'. Would "FMing" actually occur with a "Self excited" oscillator built with a beam deflection tube, or would the grid-cathode oscillator circuit be isolated from modulation effects that would cause "FMing", by the construction of the beam deflection tube? Since the sum of the plate currents for both plates remains constant with modulation applied to the beam deflection electrodes, I would think that the cathode-grid oscillator circuit wouldn't even see the modulation applied to the deflection electrodes. G3, the accelerating electrode probably provides further isolation between the oscillator and modulation effects. The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Both of these schemes differ from the common phono oscillator circuit in that they require two tuned circuits, one for the oscillator and a second for the plate circuit feeding the antenna, the plate current can be completely cut off, neglecting leakage and so forth, without affecting the oscillator circuit to any great extent. The single tuned circuit approach of the common phono oscillator circuit would have a greater sensitivity to "FMing", and the oscillator dies completely when the plate current is cut off. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#17
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Sun, 24 Jun 2012 15:59:48 -0500, John Byrns
wrote: In article , flipper wrote: The other 'advantage' would be self excite but everyone keeps telling me "don't do it." Self excite 'FMs'. Would "FMing" actually occur with a "Self excited" oscillator built with a beam deflection tube, or would the grid-cathode oscillator circuit be isolated from modulation effects that would cause "FMing", by the construction of the beam deflection tube? Since the sum of the plate currents for both plates remains constant with modulation applied to the beam deflection electrodes, I would think that the cathode-grid oscillator circuit wouldn't even see the modulation applied to the deflection electrodes. G3, the accelerating electrode probably provides further isolation between the oscillator and modulation effects. The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Both of these schemes differ from the common phono oscillator circuit in that they require two tuned circuits, one for the oscillator and a second for the plate circuit feeding the antenna, the plate current can be completely cut off, neglecting leakage and so forth, without affecting the oscillator circuit to any great extent. The single tuned circuit approach of the common phono oscillator circuit would have a greater sensitivity to "FMing", and the oscillator dies completely when the plate current is cut off. Well, your logic is pretty much what mine was, plus the same 'constant current' theory applying to dual control pentodes as well, but over at radiomuseum (plus others) I read an exhaustive thread where the poster said "will NEVER try THAT again." I can't find the link off hand. He did try to theorize some kind of miller interaction but was vague enough that I couldn't quite follow the logic. He also dealt with your idea 'that can't be' or else a typical radio wouldn't work right. His argument was the only reason it did work was the small RF levels and LO carrier being suppressed. Anyhow, everyone tells me don't do it but I might try anyway As for the 6ME8, I thought, like you, cathode current should be 'constant' since the 'beam' is just being 'deflected' to the two plates but was surprised to find gobs of RF on the cathode. I guess the modest deflector bias, which is changing with RF, does affect it. Maybe it wouldn't if I drove both. Btw, my instinct was to 'clean it up' but that made things worse so I finally decided to not try 'fixing' what ain't broke. I'm also going to try the DC pentode because, like you said, it seems to me that ought to work about the same. |
#18
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Sun, 24 Jun 2012 19:01:43 -0500, flipper wrote:
On Sun, 24 Jun 2012 15:59:48 -0500, John Byrns wrote: In article , flipper wrote: The other 'advantage' would be self excite but everyone keeps telling me "don't do it." Self excite 'FMs'. Would "FMing" actually occur with a "Self excited" oscillator built with a beam deflection tube, or would the grid-cathode oscillator circuit be isolated from modulation effects that would cause "FMing", by the construction of the beam deflection tube? Since the sum of the plate currents for both plates remains constant with modulation applied to the beam deflection electrodes, I would think that the cathode-grid oscillator circuit wouldn't even see the modulation applied to the deflection electrodes. G3, the accelerating electrode probably provides further isolation between the oscillator and modulation effects. The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Both of these schemes differ from the common phono oscillator circuit in that they require two tuned circuits, one for the oscillator and a second for the plate circuit feeding the antenna, the plate current can be completely cut off, neglecting leakage and so forth, without affecting the oscillator circuit to any great extent. The single tuned circuit approach of the common phono oscillator circuit would have a greater sensitivity to "FMing", and the oscillator dies completely when the plate current is cut off. Well, your logic is pretty much what mine was, plus the same 'constant current' theory applying to dual control pentodes as well, but over at radiomuseum (plus others) I read an exhaustive thread where the poster said "will NEVER try THAT again." I can't find the link off hand. He did try to theorize some kind of miller interaction but was vague enough that I couldn't quite follow the logic. He also dealt with your idea 'that can't be' or else a typical radio wouldn't work right. His argument was the only reason it did work was the small RF levels and LO carrier being suppressed. Anyhow, everyone tells me don't do it but I might try anyway As for the 6ME8, I thought, like you, cathode current should be 'constant' since the 'beam' is just being 'deflected' to the two plates but was surprised to find gobs of RF on the cathode. I guess the modest deflector bias, which is changing with RF, does affect it. Maybe it wouldn't if I drove both. Btw, my instinct was to 'clean it up' but that made things worse so I finally decided to not try 'fixing' what ain't broke. I'm also going to try the DC pentode because, like you said, it seems to me that ought to work about the same. I found the page. http://www.radiomuseum.org/forum/gra...ansmitter.html Down at the bottom, "The sharp-cutoff heptode solution" by Jacob Roschy. His second circuit is "The cathode oscillator attempt." "Since this resulted simultaneously with the desired amplitude modulation into a strong undesired frequency modulation, I abandoned this experiment and scrapped this circuit very soon, I will never try this any more !" |
#19
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
"John Byrns" wrote in message ... The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Of these tubes only 6CS6 (not 6SC6) is suitable as it has sharp cut-off on G3. With 6BE6 it is almost impossible to cut it off completely, so 100% modulation os impossible, and deep modulatio will be distorted, even with feedback. By the way, in this case NFB shall be applied from the plate of a heptode, not from its cathode. It is better to use a separate oscillator and geef it to G3 of a heptode, while feeding audio to G1. In this case any heptode can be used. NFB can be taken from the cathode since plate current is *sort of* proportional to cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND and use a large electrolytic for passing AF as well, not RF only. But still it is better to take NFB from the plate (I mean AF component, not RF). Even in a best heptode, even with a separate oscillator you will have residual FM -- due to space charge and stray capacitances. A small FM of say 100...1000Hz is acceptable for listening on an AM radio, but will not be listenable on a synchrodyne or an SSB receiver. Both of these schemes differ from the common phono oscillator circuit in that they require two tuned circuits, one for the oscillator and a second for the plate circuit feeding the antenna, the plate current can be completely cut off, neglecting leakage and so forth, without affecting the oscillator circuit to any great extent. The single tuned circuit approach of the common phono oscillator circuit would have a greater sensitivity to "FMing", and the oscillator dies completely when the plate current is cut off. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#20
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov"
wrote: "John Byrns" wrote in message ... The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Of these tubes only 6CS6 (not 6SC6) is suitable as it has sharp cut-off on G3. With 6BE6 it is almost impossible to cut it off completely, so 100% modulation os impossible, and deep modulatio will be distorted, even with feedback. By the way, in this case NFB shall be applied from the plate of a heptode, not from its cathode. Yes, audio to G3 is the most common implementation with people looking to the G3 curve for linearity, which is why dual control pentodes seem to be the preferred choice. For one, as you mentioned, the 6CS6 is one of the few sharp heptodes, the ECH84 being the only other one I can think of off hand, and there's more DC pentode choices. Second, the dual control pentode G3 'linear' region remains relatively constant over bias while the 6CS6 cutoff region shifts with screen volts and bias.. That makes the DC pentode easier to bias. There's a ton of Dual Control Pentode AM broadcaster schematics online and I've breadboarded a couple of them. In fact, my LO was originally developed for a 6GY6 version using a 1 MHz brick osc. It is better to use a separate oscillator and geef it to G3 of a heptode, while feeding audio to G1. In this case any heptode can be used. NFB can be taken from the cathode since plate current is *sort of* proportional to cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND and use a large electrolytic for passing AF as well, not RF only. But still it is better to take NFB from the plate (I mean AF component, not RF). This, using a dual control pentode, is what John was suggesting as an alternate to my 'Beamus' 6ME8 modulator. It does seem like it ought to work similarly. I managed to get a dual control pentode (6HZ6) model working with Circuitmaker, although I'm not sure how 'good' it is. At any rate, it seems to not like going over 80%, or so, mod. Can't drive it to near cutoff. Another thing, input impedance to G3 seems incredibly low, at least at RF, and, if the simulation is even remotely accurate, there's no way my LO can drive it. Both those 'problem's may be the model, so I'll probably try building it anyway, but if the simulation is valid then the 6ME8 works quite a bit better. Even in a best heptode, even with a separate oscillator you will have residual FM -- due to space charge and stray capacitances. A small FM of say 100...1000Hz is acceptable for listening on an AM radio, but will not be listenable on a synchrodyne or an SSB receiver. I'm curious how space charge makes it across to a separate LO. Modulating grid capacitance, which is coupled to the LO tank? Both of these schemes differ from the common phono oscillator circuit in that they require two tuned circuits, one for the oscillator and a second for the plate circuit feeding the antenna, the plate current can be completely cut off, neglecting leakage and so forth, without affecting the oscillator circuit to any great extent. The single tuned circuit approach of the common phono oscillator circuit would have a greater sensitivity to "FMing", and the oscillator dies completely when the plate current is cut off. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#21
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
"flipper" wrote in message ... On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov" wrote: Yes, audio to G3 is the most common implementation with people looking to the G3 curve for linearity, which is why dual control pentodes seem to be the preferred choice. For one, as you mentioned, the 6CS6 is one of the few sharp heptodes, the ECH84 being the only other one I can think of off hand, and there's more DC pentode choices. Second, the dual control pentode G3 'linear' region remains relatively constant over bias while the 6CS6 cutoff region shifts with screen volts and bias.. That makes the DC pentode easier to bias. Likewise in a dual control pentode the "linear G3 region" shifts with the plate voltage. Dual control pentodes have one undesirable disadvantage. Their plate impedance is not high and more importantly, it is not constant. It is high at G3 cutoff (obviously!) and high at G3=0...+5V (where it works as a conventional pentode without much current returning to G2). At the very middle of the "linear" G3 zone the plate impedance is low, as in a triode where the cathode is the space charge rather than a red hot physical cathode. If such modulator runs into a LC tank, its Q factor will be modulated. Peaks and troughs will be emphasized, carrier level -- reduced. Hence -- the second harmonic of the envelope. There's a ton of Dual Control Pentode AM broadcaster schematics online and I've breadboarded a couple of them. In fact, my LO was originally developed for a 6GY6 version using a 1 MHz brick osc. It is better to use a separate oscillator and geef it to G3 of a heptode, while feeding audio to G1. In this case any heptode can be used. NFB can be taken from the cathode since plate current is *sort of* proportional to cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND and use a large electrolytic for passing AF as well, not RF only. But still it is better to take NFB from the plate (I mean AF component, not RF). This, using a dual control pentode, is what John was suggesting as an alternate to my 'Beamus' 6ME8 modulator. It does seem like it ought to work similarly. I managed to get a dual control pentode (6HZ6) model working with Circuitmaker, although I'm not sure how 'good' it is. At any rate, it seems to not like going over 80%, or so, mod. Can't drive it to near cutoff. Another thing, input impedance to G3 seems incredibly low, at least at RF, and, if the simulation is even remotely accurate, there's no way my LO can drive it. Both those 'problem's may be the model, so I'll probably try building it anyway, but if the simulation is valid then the 6ME8 works quite a bit better. Yes, 6ME8 seems a good choice, though this tube is "exotic" -- not found in evereyone's junk box, unlike say 6BE6. Even in a best heptode, even with a separate oscillator you will have residual FM -- due to space charge and stray capacitances. A small FM of say 100...1000Hz is acceptable for listening on an AM radio, but will not be listenable on a synchrodyne or an SSB receiver. I'm curious how space charge makes it across to a separate LO. Modulating grid capacitance, which is coupled to the LO tank? Exactly. Likewise, in 6ME8 density of the beam (cathode current) affects the space charge between the deflection plates which modulated capacutance between the deflection plates. I do not know the magnitude of this effect compared to a heptode. In a heptode typically space chage "equals" 0.3pF. As far as the parasitic FM is concerned, do two simple experiments. 1. On a quality comms receiver in SSB mode listen to the carrier (set beat frequency to a few hundred hertz) of your transmitter. Without applying any audio, halve the cathode resistor of 6ME8 (touch it with another resistor in parallel), effectively simulating +100% modulation excursion. Ideally, pitch of the beat tone shall not change. If it changes... you have FM. 2. On a quality comms receiver in SSB mode listen to the carrier (set beat frequency to a few hundred hertz) of your transmitter. Without applying any audio, play with the antenna tuning capacitor of your transmitter. Ideally, pitch shall not change, only the volume as you tune to and detune from the resonance peak. If it changes... If you do these tests, could you "report" the results here. Then we can discuss what causes FM in each of these cases. Regards, Alex Both of these schemes differ from the common phono oscillator circuit in that they require two tuned circuits, one for the oscillator and a second for the plate circuit feeding the antenna, the plate current can be completely cut off, neglecting leakage and so forth, without affecting the oscillator circuit to any great extent. The single tuned circuit approach of the common phono oscillator circuit would have a greater sensitivity to "FMing", and the oscillator dies completely when the plate current is cut off. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#22
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Tue, 26 Jun 2012 22:09:21 +1000, "Alex Pogossov"
wrote: "flipper" wrote in message .. . On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov" wrote: Yes, audio to G3 is the most common implementation with people looking to the G3 curve for linearity, which is why dual control pentodes seem to be the preferred choice. For one, as you mentioned, the 6CS6 is one of the few sharp heptodes, the ECH84 being the only other one I can think of off hand, and there's more DC pentode choices. Second, the dual control pentode G3 'linear' region remains relatively constant over bias while the 6CS6 cutoff region shifts with screen volts and bias.. That makes the DC pentode easier to bias. Likewise in a dual control pentode the "linear G3 region" shifts with the plate voltage. I wasn't clear. I meant on the horizontal axis. Look at the Tung-Sol (because it has the combined curves) datasheet for the 6AS6 and the G.E. datasheet for the 6CS6. On the 6AS6 all the plate curves tend to converge in the same cutoff region but it's dramatically different for the 6CS6. Now, the 'linear' region of the 6AS6 does shift some horizontally but it's nothing like the 6CS6. Dual control pentodes have one undesirable disadvantage. Their plate impedance is not high and more importantly, it is not constant. It is high at G3 cutoff (obviously!) and high at G3=0...+5V (where it works as a conventional pentode without much current returning to G2). At the very middle of the "linear" G3 zone the plate impedance is low, as in a triode where the cathode is the space charge rather than a red hot physical cathode. If such modulator runs into a LC tank, its Q factor will be modulated. Peaks and troughs will be emphasized, carrier level -- reduced. Hence -- the second harmonic of the envelope. Yeah, I noticed the plate impedance curves and the change is rather dramatic. Are you suggesting a heptode would be a better 'alternate choice', for the 6ME8 audio to G1 topology, than the dual control pentode? There's a ton of Dual Control Pentode AM broadcaster schematics online and I've breadboarded a couple of them. In fact, my LO was originally developed for a 6GY6 version using a 1 MHz brick osc. It is better to use a separate oscillator and geef it to G3 of a heptode, while feeding audio to G1. In this case any heptode can be used. NFB can be taken from the cathode since plate current is *sort of* proportional to cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND and use a large electrolytic for passing AF as well, not RF only. But still it is better to take NFB from the plate (I mean AF component, not RF). This, using a dual control pentode, is what John was suggesting as an alternate to my 'Beamus' 6ME8 modulator. It does seem like it ought to work similarly. I managed to get a dual control pentode (6HZ6) model working with Circuitmaker, although I'm not sure how 'good' it is. At any rate, it seems to not like going over 80%, or so, mod. Can't drive it to near cutoff. Another thing, input impedance to G3 seems incredibly low, at least at RF, and, if the simulation is even remotely accurate, there's no way my LO can drive it. Both those 'problem's may be the model, so I'll probably try building it anyway, but if the simulation is valid then the 6ME8 works quite a bit better. Yes, 6ME8 seems a good choice, though this tube is "exotic" -- not found in evereyone's junk box, unlike say 6BE6. Well, they're only a buck online, which is cheaper than the 6BE6. Even in a best heptode, even with a separate oscillator you will have residual FM -- due to space charge and stray capacitances. A small FM of say 100...1000Hz is acceptable for listening on an AM radio, but will not be listenable on a synchrodyne or an SSB receiver. I'm curious how space charge makes it across to a separate LO. Modulating grid capacitance, which is coupled to the LO tank? Exactly. Likewise, in 6ME8 density of the beam (cathode current) affects the space charge between the deflection plates which modulated capacutance between the deflection plates. I do not know the magnitude of this effect compared to a heptode. In a heptode typically space chage "equals" 0.3pF. Wow, my guess wasn't bad for 'not an RF guy' As far as the parasitic FM is concerned, do two simple experiments. 1. On a quality comms receiver in SSB mode listen to the carrier (set beat frequency to a few hundred hertz) of your transmitter. Without applying any audio, halve the cathode resistor of 6ME8 (touch it with another resistor in parallel), effectively simulating +100% modulation excursion. Ideally, pitch of the beat tone shall not change. If it changes... you have FM. 2. On a quality comms receiver in SSB mode listen to the carrier (set beat frequency to a few hundred hertz) of your transmitter. Without applying any audio, play with the antenna tuning capacitor of your transmitter. Ideally, pitch shall not change, only the volume as you tune to and detune from the resonance peak. If it changes... If you do these tests, could you "report" the results here. Then we can discuss what causes FM in each of these cases. I don't have a SSB receiver. Regards, Alex Both of these schemes differ from the common phono oscillator circuit in that they require two tuned circuits, one for the oscillator and a second for the plate circuit feeding the antenna, the plate current can be completely cut off, neglecting leakage and so forth, without affecting the oscillator circuit to any great extent. The single tuned circuit approach of the common phono oscillator circuit would have a greater sensitivity to "FMing", and the oscillator dies completely when the plate current is cut off. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#23
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
In article ,
flipper wrote: On Sun, 24 Jun 2012 19:01:43 -0500, flipper wrote: On Sun, 24 Jun 2012 15:59:48 -0500, John Byrns wrote: In article , flipper wrote: The other 'advantage' would be self excite but everyone keeps telling me "don't do it." Self excite 'FMs'. Would "FMing" actually occur with a "Self excited" oscillator built with a beam deflection tube, or would the grid-cathode oscillator circuit be isolated from modulation effects that would cause "FMing", by the construction of the beam deflection tube? Since the sum of the plate currents for both plates remains constant with modulation applied to the beam deflection electrodes, I would think that the cathode-grid oscillator circuit wouldn't even see the modulation applied to the deflection electrodes. G3, the accelerating electrode probably provides further isolation between the oscillator and modulation effects. The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Both of these schemes differ from the common phono oscillator circuit in that they require two tuned circuits, one for the oscillator and a second for the plate circuit feeding the antenna, the plate current can be completely cut off, neglecting leakage and so forth, without affecting the oscillator circuit to any great extent. The single tuned circuit approach of the common phono oscillator circuit would have a greater sensitivity to "FMing", and the oscillator dies completely when the plate current is cut off. Well, your logic is pretty much what mine was, plus the same 'constant current' theory applying to dual control pentodes as well, but over at radiomuseum (plus others) I read an exhaustive thread where the poster said "will NEVER try THAT again." I can't find the link off hand. He did try to theorize some kind of miller interaction but was vague enough that I couldn't quite follow the logic. He also dealt with your idea 'that can't be' or else a typical radio wouldn't work right. His argument was the only reason it did work was the small RF levels and LO carrier being suppressed. Anyhow, everyone tells me don't do it but I might try anyway As for the 6ME8, I thought, like you, cathode current should be 'constant' since the 'beam' is just being 'deflected' to the two plates but was surprised to find gobs of RF on the cathode. I guess the modest deflector bias, which is changing with RF, does affect it. Maybe it wouldn't if I drove both. Btw, my instinct was to 'clean it up' but that made things worse so I finally decided to not try 'fixing' what ain't broke. I'm also going to try the DC pentode because, like you said, it seems to me that ought to work about the same. Actually that wasn't what I was trying to suggest, that was either a "typo" on my part or I was having a "senior moment". What I was actually talking about was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how the term "dual control pentode" managed to escape my keyboard. An interesting point about your 6ME8 transmitter is that it appears to be a vacuum state analog to the solid state designs using a three transistor "cell", which seem to be the designs of choice with many old radio fans. In this circuit two of the transistors are connected as a long tailed differential pair with RF drive applied to the base of one transistor and with the modulated RF output taken from one of the collectors. The third transistor operates as a current source in the emitter "tail" circuit of the first two transistors, with audio applied to its base to modulate the RF output of the differential pair. There are also a bunch of dual plate tubes that were used in electric organs, that I have been meaning to lookup for a couple of years to see if there are any that might be useful in these circuits, so far I haven't gotten around to it. I found the page. http://www.radiomuseum.org/forum/gra...ansmitter.html Down at the bottom, "The sharp-cutoff heptode solution" by Jacob Roschy. His second circuit is "The cathode oscillator attempt." "Since this resulted simultaneously with the desired amplitude modulation into a strong undesired frequency modulation, I abandoned this experiment and scrapped this circuit very soon, I will never try this any more !" That's an interesting page, but back to that in a moment. Re unintentional FMing of AM carriers, that seems to have been a traditional part of early AM broadcasting, although the radios back then may have had a much wider bandwidth than today's radios, so the FMing may not have mattered much. Consider the FMing that was likely present in the following circuit taken from an old notebook that belonged to a gentleman doing research on building a radio station back in the day, which I bought on eBay. http://fmamradios.com/Old_Notebook/K..._Schematic.jpg And here is a photograph of what is probably a later but similar transmitter. Unfortunately this photo doesn't include the power supply and modulation reactor. http://www.hammondmuseumofradio.org/...y-kdka-exp.jpg While this is obviously not a photograph of the exact transmitter described by the schematic, I suspect that it is similar, possibly with improvements to eliminate FMing, but probably not to improve modulation capability. The Radiomuseum web page really took me back as I owned a Motorola FM converter using the 12-volt tubes back in the 1962-66 time frame. The Motorola was probably a little more upscale than the Granco discussed on the Radiomuseum page, IIRC it had an extra IF stage and a nicer case than the Granco, however it used a "phono oscillator" circuit similar to the Granco's to drive the existing AM radio. At the time I was impressed with how good the "transmitter" circuit in the Motorola FM converter sounded, and the audio level was not especially lower than on an AM station. I had been wondering how much of the FMing with the self-excited heptodes is due to coupling within the tube, and how much was due to layout and shielding issues. Joe seems to have gone into that to some extent, although a deeper exploration of the subject would be more satisfying to me. Jacob's use of the ECH84 is interesting in that it presumably eliminates any direct "electron cloud" coupling issues, but the oscillator and modulated amplifier elements are still in close proximity so that there could still be inter-electrode coupling issues. None of Jacob's circuits include a buffer stage between the oscillator and modulated amplifier stage to completely eliminate the last vestiges of "electron cloud" coupling affecting the oscillator frequency. Another issue that Jacob addressed that I have been wondering about is what he calls "G3 Blurring", which he reduces by using a cathode follower to drive G3. It occurs to me that a refinement of this approach would be to use a direct-coupled cathode follower to allow driving G3 positive, although that would probably require an additional power supply, increasing complexity. I was not previously aware of the 6BY6 heptode mentioned on this page, it appears to have a considerably more linear G3 voltage vs. plate current curve than the 6CS6 most people on this side of the pond seem to talk about. Something else that I learned from this page, that should have been obvious to me before, is that the two tuned circuit heptode circuit that I suggested provides a degree of isolation between varying antenna capacity and the oscillator frequency. One thing that really strikes me about pages like this Radiomuseum web page is that the authors rarely state what all their design goals are. If I were building a small transmitter using four tube elements as Jacob did, I would probably go with a plate modulation approach, eliminating several problems presented by the grid modulation approach Jacob uses. The attractiveness of the heptodes to me are the possibilities for building a single tube transmitter -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#24
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
In article ,
flipper wrote: On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov" wrote: "John Byrns" wrote in message ... The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Of these tubes only 6CS6 (not 6SC6) is suitable as it has sharp cut-off on G3. With 6BE6 it is almost impossible to cut it off completely, so 100% modulation os impossible, and deep modulatio will be distorted, even with feedback. By the way, in this case NFB shall be applied from the plate of a heptode, not from its cathode. Yes, audio to G3 is the most common implementation with people looking to the G3 curve for linearity, which is why dual control pentodes seem to be the preferred choice. For one, as you mentioned, the 6CS6 is one of the few sharp heptodes, the ECH84 being the only other one I can think of off hand, and there's more DC pentode choices. Second, the dual control pentode G3 'linear' region remains relatively constant over bias while the 6CS6 cutoff region shifts with screen volts and bias.. That makes the DC pentode easier to bias. There's a ton of Dual Control Pentode AM broadcaster schematics online and I've breadboarded a couple of them. In fact, my LO was originally developed for a 6GY6 version using a 1 MHz brick osc. It is better to use a separate oscillator and geef it to G3 of a heptode, while feeding audio to G1. In this case any heptode can be used. NFB can be taken from the cathode since plate current is *sort of* proportional to cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND and use a large electrolytic for passing AF as well, not RF only. But still it is better to take NFB from the plate (I mean AF component, not RF). This, using a dual control pentode, is what John was suggesting as an alternate to my 'Beamus' 6ME8 modulator. It does seem like it ought to work similarly. Actually that wasn't what I was trying to suggest, that was either a "typo" on my part or I was having a "senior moment". What I was actually talking about was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how the term "dual control pentode" managed to escape my keyboard. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#25
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Tue, 26 Jun 2012 15:36:48 -0500, John Byrns
wrote: In article , flipper wrote: snip. I'm also going to try the DC pentode because, like you said, it seems to me that ought to work about the same. Actually that wasn't what I was trying to suggest, that was either a "typo" on my part or I was having a "senior moment". What I was actually talking about was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how the term "dual control pentode" managed to escape my keyboard. You're quite right. You said the "6SA7/6BE6/6SC6 family of heptodes" and it was I who 'converted' it to dual control heptodes because 'total cathode current appears to be largely independent of the voltage on G3' on those too. An interesting point about your 6ME8 transmitter is that it appears to be a vacuum state analog to the solid state designs using a three transistor "cell", which seem to be the designs of choice with many old radio fans. In this circuit two of the transistors are connected as a long tailed differential pair with RF drive applied to the base of one transistor and with the modulated RF output taken from one of the collectors. The third transistor operates as a current source in the emitter "tail" circuit of the first two transistors, with audio applied to its base to modulate the RF output of the differential pair. Yes, I know the topology and was 'that close' to building one but am not sure which was the cart and which the horse here. I originally started thinking of the 6ME8 back when Patrick was tinkering with Gilbert Cells because I noticed a 'similarity' between it and the 3 transistor 'cell', of which the Gilbert has two on top a current source. Anyway, when working on this transmitter I tried 'emulating' something akin to the beam deflection tube because I don't have a model for one and constructed a 'long tail pair' using a pentode for the tail, which is also akin to the 3 transistor 'cell'. That did point out one difference, though. The deflection plates do not depend on cathodes (emitters) driving the 'other side' to get the 'see saw' effect. There are also a bunch of dual plate tubes that were used in electric organs, that I have been meaning to lookup for a couple of years to see if there are any that might be useful in these circuits, so far I haven't gotten around to it. The one I'm aware of is the 12FQ8 but have never been able to figure out how it was intended to function. I found the page. http://www.radiomuseum.org/forum/gra...ansmitter.html Down at the bottom, "The sharp-cutoff heptode solution" by Jacob Roschy. His second circuit is "The cathode oscillator attempt." "Since this resulted simultaneously with the desired amplitude modulation into a strong undesired frequency modulation, I abandoned this experiment and scrapped this circuit very soon, I will never try this any more !" That's an interesting page, but back to that in a moment. Re unintentional FMing of AM carriers, that seems to have been a traditional part of early AM broadcasting, although the radios back then may have had a much wider bandwidth than today's radios, so the FMing may not have mattered much. Consider the FMing that was likely present in the following circuit taken from an old notebook that belonged to a gentleman doing research on building a radio station back in the day, which I bought on eBay. http://fmamradios.com/Old_Notebook/K..._Schematic.jpg And here is a photograph of what is probably a later but similar transmitter. Unfortunately this photo doesn't include the power supply and modulation reactor. http://www.hammondmuseumofradio.org/...y-kdka-exp.jpg While this is obviously not a photograph of the exact transmitter described by the schematic, I suspect that it is similar, possibly with improvements to eliminate FMing, but probably not to improve modulation capability. The Radiomuseum web page really took me back as I owned a Motorola FM converter using the 12-volt tubes back in the 1962-66 time frame. The Motorola was probably a little more upscale than the Granco discussed on the Radiomuseum page, IIRC it had an extra IF stage and a nicer case than the Granco, however it used a "phono oscillator" circuit similar to the Granco's to drive the existing AM radio. At the time I was impressed with how good the "transmitter" circuit in the Motorola FM converter sounded, and the audio level was not especially lower than on an AM station. I had been wondering how much of the FMing with the self-excited heptodes is due to coupling within the tube, and how much was due to layout and shielding issues. Joe seems to have gone into that to some extent, although a deeper exploration of the subject would be more satisfying to me. Jacob's use of the ECH84 is interesting in that it presumably eliminates any direct "electron cloud" coupling issues, but the oscillator and modulated amplifier elements are still in close proximity so that there could still be inter-electrode coupling issues. None of Jacob's circuits include a buffer stage between the oscillator and modulated amplifier stage to completely eliminate the last vestiges of "electron cloud" coupling affecting the oscillator frequency. Yeah, and he also doesn't provide any measurements of the 'FMing' in either. Just that it's apparently 'bad' in one and the other circuit works well, whatever that means. Another issue that Jacob addressed that I have been wondering about is what he calls "G3 Blurring", which he reduces by using a cathode follower to drive G3. It occurs to me that a refinement of this approach would be to use a direct-coupled cathode follower to allow driving G3 positive, although that would probably require an additional power supply, increasing complexity. I was not previously aware of the 6BY6 heptode mentioned on this page, it appears to have a considerably more linear G3 voltage vs. plate current curve than the 6CS6 most people on this side of the pond seem to talk about. I've got both and they're both '1 buck' tubes. I think people kind of like the 6CS6 because of the unusually high G3 gm, so it can potentially be driven without a preamp. Something else that I learned from this page, that should have been obvious to me before, is that the two tuned circuit heptode circuit that I suggested provides a degree of isolation between varying antenna capacity and the oscillator frequency. One thing that really strikes me about pages like this Radiomuseum web page is that the authors rarely state what all their design goals are. If I were building a small transmitter using four tube elements as Jacob did, I would probably go with a plate modulation approach, eliminating several problems presented by the grid modulation approach Jacob uses. The attractiveness of the heptodes to me are the possibilities for building a single tube transmitter Well, the 6CS6 mat be the choice, then, because of that high G3 gm, if the FMing could be solved. Joe Sousa, who has another interesting article on what he calls his 'hi-fi' transmitter, seems to think it could be neutralized out. Btw, his 'hi-fi' transmitter is the same idea I was proposing to Patrick with the 6ME8 but he's using a dual control pentode to do it. http://www.radiomuseum.org/forum/hi_...ansmitter.html One tube job, like your goal. Btw, he speaks of neutralization in that article too (3- Frequency Stability) but decides his circuit doesn't really need it. |
#26
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Tue, 26 Jun 2012 15:38:20 -0500, John Byrns
wrote: In article , flipper wrote: On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov" wrote: "John Byrns" wrote in message ... The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Of these tubes only 6CS6 (not 6SC6) is suitable as it has sharp cut-off on G3. With 6BE6 it is almost impossible to cut it off completely, so 100% modulation os impossible, and deep modulatio will be distorted, even with feedback. By the way, in this case NFB shall be applied from the plate of a heptode, not from its cathode. Yes, audio to G3 is the most common implementation with people looking to the G3 curve for linearity, which is why dual control pentodes seem to be the preferred choice. For one, as you mentioned, the 6CS6 is one of the few sharp heptodes, the ECH84 being the only other one I can think of off hand, and there's more DC pentode choices. Second, the dual control pentode G3 'linear' region remains relatively constant over bias while the 6CS6 cutoff region shifts with screen volts and bias.. That makes the DC pentode easier to bias. There's a ton of Dual Control Pentode AM broadcaster schematics online and I've breadboarded a couple of them. In fact, my LO was originally developed for a 6GY6 version using a 1 MHz brick osc. It is better to use a separate oscillator and geef it to G3 of a heptode, while feeding audio to G1. In this case any heptode can be used. NFB can be taken from the cathode since plate current is *sort of* proportional to cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND and use a large electrolytic for passing AF as well, not RF only. But still it is better to take NFB from the plate (I mean AF component, not RF). This, using a dual control pentode, is what John was suggesting as an alternate to my 'Beamus' 6ME8 modulator. It does seem like it ought to work similarly. Actually that wasn't what I was trying to suggest, that was either a "typo" on my part or I was having a "senior moment". What I was actually talking about was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how the term "dual control pentode" managed to escape my keyboard. Sorry. I'm the one who 'converted' it to dual control pentode. |
#27
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
In article ,
flipper wrote: On Tue, 26 Jun 2012 15:38:20 -0500, John Byrns wrote: In article , flipper wrote: On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov" wrote: "John Byrns" wrote in message ... The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Of these tubes only 6CS6 (not 6SC6) is suitable as it has sharp cut-off on G3. With 6BE6 it is almost impossible to cut it off completely, so 100% modulation os impossible, and deep modulatio will be distorted, even with feedback. By the way, in this case NFB shall be applied from the plate of a heptode, not from its cathode. Yes, audio to G3 is the most common implementation with people looking to the G3 curve for linearity, which is why dual control pentodes seem to be the preferred choice. For one, as you mentioned, the 6CS6 is one of the few sharp heptodes, the ECH84 being the only other one I can think of off hand, and there's more DC pentode choices. Second, the dual control pentode G3 'linear' region remains relatively constant over bias while the 6CS6 cutoff region shifts with screen volts and bias.. That makes the DC pentode easier to bias. There's a ton of Dual Control Pentode AM broadcaster schematics online and I've breadboarded a couple of them. In fact, my LO was originally developed for a 6GY6 version using a 1 MHz brick osc. It is better to use a separate oscillator and geef it to G3 of a heptode, while feeding audio to G1. In this case any heptode can be used. NFB can be taken from the cathode since plate current is *sort of* proportional to cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND and use a large electrolytic for passing AF as well, not RF only. But still it is better to take NFB from the plate (I mean AF component, not RF). This, using a dual control pentode, is what John was suggesting as an alternate to my 'Beamus' 6ME8 modulator. It does seem like it ought to work similarly. Actually that wasn't what I was trying to suggest, that was either a "typo" on my part or I was having a "senior moment". What I was actually talking about was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how the term "dual control pentode" managed to escape my keyboard. Sorry. I'm the one who 'converted' it to dual control pentode. Are you trying to play with my mind? I was the first to use the term "dual control pentode in this thread when I made the statement "Applying the modulation to G1 doesn't take advantage of the beam deflection capabilities of the tube and instead uses it in a way that a more ordinary tube, like a dual control pentode, could serve." when I meant to say heptode. I'm the one that converted it! -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#28
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Tue, 26 Jun 2012 22:29:35 -0500, John Byrns
wrote: In article , flipper wrote: On Tue, 26 Jun 2012 15:38:20 -0500, John Byrns wrote: In article , flipper wrote: On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov" wrote: "John Byrns" wrote in message ... The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Of these tubes only 6CS6 (not 6SC6) is suitable as it has sharp cut-off on G3. With 6BE6 it is almost impossible to cut it off completely, so 100% modulation os impossible, and deep modulatio will be distorted, even with feedback. By the way, in this case NFB shall be applied from the plate of a heptode, not from its cathode. Yes, audio to G3 is the most common implementation with people looking to the G3 curve for linearity, which is why dual control pentodes seem to be the preferred choice. For one, as you mentioned, the 6CS6 is one of the few sharp heptodes, the ECH84 being the only other one I can think of off hand, and there's more DC pentode choices. Second, the dual control pentode G3 'linear' region remains relatively constant over bias while the 6CS6 cutoff region shifts with screen volts and bias.. That makes the DC pentode easier to bias. There's a ton of Dual Control Pentode AM broadcaster schematics online and I've breadboarded a couple of them. In fact, my LO was originally developed for a 6GY6 version using a 1 MHz brick osc. It is better to use a separate oscillator and geef it to G3 of a heptode, while feeding audio to G1. In this case any heptode can be used. NFB can be taken from the cathode since plate current is *sort of* proportional to cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND and use a large electrolytic for passing AF as well, not RF only. But still it is better to take NFB from the plate (I mean AF component, not RF). This, using a dual control pentode, is what John was suggesting as an alternate to my 'Beamus' 6ME8 modulator. It does seem like it ought to work similarly. Actually that wasn't what I was trying to suggest, that was either a "typo" on my part or I was having a "senior moment". What I was actually talking about was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how the term "dual control pentode" managed to escape my keyboard. Sorry. I'm the one who 'converted' it to dual control pentode. Are you trying to play with my mind? I was the first to use the term "dual control pentode in this thread when I made the statement "Applying the modulation to G1 doesn't take advantage of the beam deflection capabilities of the tube and instead uses it in a way that a more ordinary tube, like a dual control pentode, could serve." when I meant to say heptode. I'm the one that converted it! LOL. Okay, sorry again For this application I tend to think of them as almost the same thing but am beginning to realize that the 'extra' G4 screen is probably what gives a heptode the higher plate impedance. Is that why, despite the 'accidental' mentioning of dual control pentode, you seem to be adamant about heptodes, or is there another reason? |
#29
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
"flipper" wrote in message news On Tue, 26 Jun 2012 22:09:21 +1000, "Alex Pogossov" wrote: As far as the parasitic FM is concerned, do two simple experiments. 1. On a quality comms receiver in SSB mode listen to the carrier (set beat frequency to a few hundred hertz) of your transmitter. Without applying any audio, halve the cathode resistor of 6ME8 (touch it with another resistor in parallel), effectively simulating +100% modulation excursion. Ideally, pitch of the beat tone shall not change. If it changes... you have FM. 2. On a quality comms receiver in SSB mode listen to the carrier (set beat frequency to a few hundred hertz) of your transmitter. Without applying any audio, play with the antenna tuning capacitor of your transmitter. Ideally, pitch shall not change, only the volume as you tune to and detune from the resonance peak. If it changes... If you do these tests, could you "report" the results here. Then we can discuss what causes FM in each of these cases. I don't have a SSB receiver. Oops, surely you are not an RF guy. Then do another trick instead. Tune any AM radio into a strong local AM station within tuning range of your transmitter. Then tune your transmitter to a close frequency so that you hear beat frequency on the radio. It is better to have the signal from the transmitter stronger than this station so that you do not hear the audio but only the beat whining (with some sideband chatter of course). Then proceed with the above described tests. (A carrier of an AM station is even more stable than any communications receiver.) Regards, Alex |
#30
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Wed, 27 Jun 2012 16:21:53 +1000, "Alex Pogossov"
wrote: "flipper" wrote in message news On Tue, 26 Jun 2012 22:09:21 +1000, "Alex Pogossov" wrote: As far as the parasitic FM is concerned, do two simple experiments. 1. On a quality comms receiver in SSB mode listen to the carrier (set beat frequency to a few hundred hertz) of your transmitter. Without applying any audio, halve the cathode resistor of 6ME8 (touch it with another resistor in parallel), effectively simulating +100% modulation excursion. Ideally, pitch of the beat tone shall not change. If it changes... you have FM. 2. On a quality comms receiver in SSB mode listen to the carrier (set beat frequency to a few hundred hertz) of your transmitter. Without applying any audio, play with the antenna tuning capacitor of your transmitter. Ideally, pitch shall not change, only the volume as you tune to and detune from the resonance peak. If it changes... If you do these tests, could you "report" the results here. Then we can discuss what causes FM in each of these cases. I don't have a SSB receiver. Oops, surely you are not an RF guy. Then do another trick instead. Tune any AM radio into a strong local AM station within tuning range of your transmitter. Then tune your transmitter to a close frequency so that you hear beat frequency on the radio. It is better to have the signal from the transmitter stronger than this station so that you do not hear the audio but only the beat whining (with some sideband chatter of course). Then proceed with the above described tests. (A carrier of an AM station is even more stable than any communications receiver.) Regards, Alex Wouldn't my frequency counter be simpler? hehe With carrier at 640 KHz frequency deviation from idle to twice idle is roughly 70 Hz. Actual measurement was .64015 MHz (resolution limit) to ..64022 MHz |
#31
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Jun 27, 12:15*pm, flipper wrote:
On Tue, 26 Jun 2012 15:36:48 -0500, John Byrns wrote: In article , flipper wrote: snip. I'm also going to try the DC pentode because, like you said, it seems to me that ought to work about the same. Actually that wasn't what I was trying to suggest, that was either a "typo" on my part or I was having a "senior moment". *What I was actually talking about was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how the term "dual control pentode" managed to escape my keyboard. You're quite right. You said the "6SA7/6BE6/6SC6 family of heptodes" and it was I who 'converted' it to dual control heptodes because 'total cathode current appears to be largely independent of the voltage on G3' on those too. An interesting point about your 6ME8 transmitter is that it appears to be a vacuum state analog to the solid state designs using a three transistor "cell", which seem to be the designs of choice with many old radio fans. *In this circuit two of the transistors are connected as a long tailed differential pair with RF drive applied to the base of one transistor and with the modulated RF output taken from one of the collectors. The third transistor operates as a current source in the emitter "tail" circuit of the first two transistors, with audio applied to its base to modulate the RF output of the differential pair. Yes, I know the topology and was 'that close' to building one but am not sure which was the cart and which the horse here. I originally started thinking of the 6ME8 back when Patrick was tinkering with Gilbert Cells because I noticed a 'similarity' between it and the 3 transistor 'cell', of which the Gilbert has two on top a current source. Yeah, I was going to say the talk about 3 transistors was actually about a Gilbert Cell. I couldn't spend another 3 months perfecting what I did with 3 transistors because I always have other things to do. Using a gilbert cell chip probably would have worked better then me trying to fiddle with discretes. But could a Gilbert cell be made using 3 triodes? Another tube with strong g3 action is the 7 pin 6DT6 commonly used for quadrature FM detection. Another one I was intersted in was "gated beam pentode" 6BN6, which some said never worked well as 6DT6. Maybe these have apps at BC band F. Patrick Turner. |
#32
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Wed, 27 Jun 2012 17:30:35 -0700 (PDT), Patrick Turner
wrote: On Jun 27, 12:15*pm, flipper wrote: On Tue, 26 Jun 2012 15:36:48 -0500, John Byrns wrote: In article , flipper wrote: snip. I'm also going to try the DC pentode because, like you said, it seems to me that ought to work about the same. Actually that wasn't what I was trying to suggest, that was either a "typo" on my part or I was having a "senior moment". *What I was actually talking about was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how the term "dual control pentode" managed to escape my keyboard. You're quite right. You said the "6SA7/6BE6/6SC6 family of heptodes" and it was I who 'converted' it to dual control heptodes because 'total cathode current appears to be largely independent of the voltage on G3' on those too. An interesting point about your 6ME8 transmitter is that it appears to be a vacuum state analog to the solid state designs using a three transistor "cell", which seem to be the designs of choice with many old radio fans. *In this circuit two of the transistors are connected as a long tailed differential pair with RF drive applied to the base of one transistor and with the modulated RF output taken from one of the collectors. The third transistor operates as a current source in the emitter "tail" circuit of the first two transistors, with audio applied to its base to modulate the RF output of the differential pair. Yes, I know the topology and was 'that close' to building one but am not sure which was the cart and which the horse here. I originally started thinking of the 6ME8 back when Patrick was tinkering with Gilbert Cells because I noticed a 'similarity' between it and the 3 transistor 'cell', of which the Gilbert has two on top a current source. Yeah, I was going to say the talk about 3 transistors was actually about a Gilbert Cell. I couldn't spend another 3 months perfecting what I did with 3 transistors because I always have other things to do. I thought you were trying to build a Gilbert cell, back then. That takes more than 3 transistors. Using a gilbert cell chip probably would have worked better then me trying to fiddle with discretes. But could a Gilbert cell be made using 3 triodes? Well, no, because a Gilbert Cell isn't '3 transistors'. For the '3 transistor' cell I think pentodes might be more appropriate because the bipolar collector current vs collector-emitter voltage looks just like a pentode. Although, I'm not sure what the effect of screen current mixed in might be. Or a single 6ME8 because that's essentially how it works with the deflector plates acting as the two upper transistors and G1 the bottom. That's why I used a 6ME8: it's 'conceptually pure' with equivalent 'ins and outs'. As used, however, one plate is simply tied to B+ so, if screen current and plate current 'see saw' in a heptode, which the curves indicate is the case, then it might work similarly. I.E. what goes into the 'unused' second plate externally connected to B+ is 'internally attached' screen current in the heptode. At least it seems to be, in theory, but I didn't know what 'subtleties' might crop up and, especially when doing something 'new', like to cut down the unknowns as much as possible. Another tube with strong g3 action is the 7 pin 6DT6 commonly used for quadrature FM detection. Another one I was intersted in was "gated beam pentode" 6BN6, which some said never worked well as 6DT6. Maybe these have apps at BC band F. Patrick Turner. Gee, I don't know about the 6BN6. That thing is a gated beam discriminator made for FM demod and has a 'limiter' built in, which is sort of the antithesis of AM. Of course, one doesn't have to use it 'as intended' but it's beyond me. |
#33
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
In article ,
flipper wrote: On Tue, 26 Jun 2012 22:29:35 -0500, John Byrns wrote: In article , flipper wrote: On Tue, 26 Jun 2012 15:38:20 -0500, John Byrns wrote: In article , flipper wrote: On Mon, 25 Jun 2012 19:36:07 +1000, "Alex Pogossov" wrote: "John Byrns" wrote in message ... The 6SA7/6BE6/6SC6 family of heptodes would also seem to provide isolation between a cathode-grid "self excited" oscillator circuit and modulation applied to G3 as the total cathode current appears to be largely independent of the voltage on G3, minimizing "FMing". If this weren't the case, the local oscillator frequency, in an AM receiver using one of these tubes, would vary with changes in the AGC voltage due to fading. Of these tubes only 6CS6 (not 6SC6) is suitable as it has sharp cut-off on G3. With 6BE6 it is almost impossible to cut it off completely, so 100% modulation os impossible, and deep modulatio will be distorted, even with feedback. By the way, in this case NFB shall be applied from the plate of a heptode, not from its cathode. Yes, audio to G3 is the most common implementation with people looking to the G3 curve for linearity, which is why dual control pentodes seem to be the preferred choice. For one, as you mentioned, the 6CS6 is one of the few sharp heptodes, the ECH84 being the only other one I can think of off hand, and there's more DC pentode choices. Second, the dual control pentode G3 'linear' region remains relatively constant over bias while the 6CS6 cutoff region shifts with screen volts and bias.. That makes the DC pentode easier to bias. There's a ton of Dual Control Pentode AM broadcaster schematics online and I've breadboarded a couple of them. In fact, my LO was originally developed for a 6GY6 version using a 1 MHz brick osc. It is better to use a separate oscillator and geef it to G3 of a heptode, while feeding audio to G1. In this case any heptode can be used. NFB can be taken from the cathode since plate current is *sort of* proportional to cathode current. Do not forget to decouple G2+G4 to cathode (!), not to GND and use a large electrolytic for passing AF as well, not RF only. But still it is better to take NFB from the plate (I mean AF component, not RF). This, using a dual control pentode, is what John was suggesting as an alternate to my 'Beamus' 6ME8 modulator. It does seem like it ought to work similarly. Actually that wasn't what I was trying to suggest, that was either a "typo" on my part or I was having a "senior moment". What I was actually talking about was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how the term "dual control pentode" managed to escape my keyboard. Sorry. I'm the one who 'converted' it to dual control pentode. Are you trying to play with my mind? I was the first to use the term "dual control pentode in this thread when I made the statement "Applying the modulation to G1 doesn't take advantage of the beam deflection capabilities of the tube and instead uses it in a way that a more ordinary tube, like a dual control pentode, could serve." when I meant to say heptode. I'm the one that converted it! LOL. Okay, sorry again For this application I tend to think of them as almost the same thing but am beginning to realize that the 'extra' G4 screen is probably what gives a heptode the higher plate impedance. Is that why, despite the 'accidental' mentioning of dual control pentode, you seem to be adamant about heptodes, or is there another reason? The reason I am "adamant about heptodes", if that is what you want to call it, is not because of the plate resistance, it is simply because I have no familiarity at all with DC pentodes and their characteristics. I don't even know any DC Pentode type numbers and am not sophisticated enough to know that the plate resistance of a DC pentode is lower than a heptode. Assuming the plate resistance of a DC Pentode is lower than a heptode, is that a bad thing? I am not "adamant about heptodes", in fact I don't particularly care for grid modulation, preferring plate modulation instead, however it is hard to conceive of how to build a plate modulated transmitter using only one single section tube, leaving the heptode, or possibly the DC Pentode, as the only easy choice for a single tube transmitter. I do have a design for a single tube transmitter using your 6ME8 that meets my design brief, however it would require me to build my own well balanced push-pull RF antenna coupling transformer. -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#34
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Wed, 27 Jun 2012 22:06:46 -0500, John Byrns
wrote: In article , flipper wrote: For this application I tend to think of them as almost the same thing but am beginning to realize that the 'extra' G4 screen is probably what gives a heptode the higher plate impedance. Is that why, despite the 'accidental' mentioning of dual control pentode, you seem to be adamant about heptodes, or is there another reason? The reason I am "adamant about heptodes", if that is what you want to call it, is not because of the plate resistance, it is simply because I have no familiarity at all with DC pentodes and their characteristics. Oh, okay. Well, just saying you're not familiar with them was enough I don't even know any DC Pentode type numbers and am not sophisticated enough to know that the plate resistance of a DC pentode is lower than a heptode. Assuming the plate resistance of a DC Pentode is lower than a heptode, is that a bad thing? Well, it affects plate tank Q. 6AS6, 6DT6, 6GY6, and 6888 are some of the more 'popular' ones for broadcasters. I am not "adamant about heptodes", in fact I don't particularly care for grid modulation, preferring plate modulation instead, however it is hard to conceive of how to build a plate modulated transmitter using only one single section tube, leaving the heptode, or possibly the DC Pentode, as the only easy choice for a single tube transmitter. I do have a design for a single tube transmitter using your 6ME8 that meets my design brief, however it would require me to build my own well balanced push-pull RF antenna coupling transformer. Yeah, a PP transformer is in some of my designs too but I'm skittish about trying to wind one. |
#35
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
"flipper" wrote in message ... On Wed, 27 Jun 2012 16:21:53 +1000, "Alex Pogossov" wrote: Oops, surely you are not an RF guy. Then do another trick instead. Tune any AM radio into a strong local AM station within tuning range of your transmitter. Then tune your transmitter to a close frequency so that you hear beat frequency on the radio. It is better to have the signal from the transmitter stronger than this station so that you do not hear the audio but only the beat whining (with some sideband chatter of course). Then proceed with the above described tests. (A carrier of an AM station is even more stable than any communications receiver.) Regards, Alex Wouldn't my frequency counter be simpler? hehe With carrier at 640 KHz frequency deviation from idle to twice idle is roughly 70 Hz. Actual measurement was .64015 MHz (resolution limit) to .64022 MHz. It is not bad. I expected worse. Probably 6ME8 has less space charge coupling than a regular heptode. Still 70Hz is not acceptable for listening on a synchronous or SSB detector. In these cases parasitic FM index shall not exceed about 0.25, so with the lowest audio frequency of 50Hz, FM deviation shall not exceed 15Hz. But who would deliberately listening to your transmitter on an SSB radio? No one. So, well done anyway! |
#36
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
In article ,
flipper wrote: On Wed, 27 Jun 2012 22:06:46 -0500, John Byrns wrote: In article , flipper wrote: For this application I tend to think of them as almost the same thing but am beginning to realize that the 'extra' G4 screen is probably what gives a heptode the higher plate impedance. Is that why, despite the 'accidental' mentioning of dual control pentode, you seem to be adamant about heptodes, or is there another reason? The reason I am "adamant about heptodes", if that is what you want to call it, is not because of the plate resistance, it is simply because I have no familiarity at all with DC pentodes and their characteristics. Oh, okay. Well, just saying you're not familiar with them was enough I don't even know any DC Pentode type numbers and am not sophisticated enough to know that the plate resistance of a DC pentode is lower than a heptode. Assuming the plate resistance of a DC Pentode is lower than a heptode, is that a bad thing? Well, it affects plate tank Q. But that is a good thing, as we need to keep the plate tank Q within reason to avoid sideband clipping and provide High Fidelity audio. 6AS6, 6DT6, 6GY6, and 6888 are some of the more 'popular' ones for broadcasters. I am not "adamant about heptodes", in fact I don't particularly care for grid modulation, preferring plate modulation instead, however it is hard to conceive of how to build a plate modulated transmitter using only one single section tube, leaving the heptode, or possibly the DC Pentode, as the only easy choice for a single tube transmitter. I do have a design for a single tube transmitter using your 6ME8 that meets my design brief, however it would require me to build my own well balanced push-pull RF antenna coupling transformer. Yeah, a PP transformer is in some of my designs too but I'm skittish about trying to wind one. After a little out of the box thinking, it occurs to me that it may be possible to make lemonade out of these lemons. It appears that by intentionally unbalancing the transformer you can generate AM without the need for biasing the deflectors to produce a carrier. Tight coupling between the two sections of the winding would probably still be required to insure that no weird phase shifts are produced which would prevent achieving 100% negative modulation. Hopefully using a toroidal transformer core would provide adequate coupling between the two plates? -- Regards, John Byrns Surf my web pages at, http://fmamradios.com/ |
#37
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Thu, 28 Jun 2012 17:47:29 -0500, John Byrns
wrote: In article , flipper wrote: On Wed, 27 Jun 2012 22:06:46 -0500, John Byrns wrote: In article , flipper wrote: For this application I tend to think of them as almost the same thing but am beginning to realize that the 'extra' G4 screen is probably what gives a heptode the higher plate impedance. Is that why, despite the 'accidental' mentioning of dual control pentode, you seem to be adamant about heptodes, or is there another reason? The reason I am "adamant about heptodes", if that is what you want to call it, is not because of the plate resistance, it is simply because I have no familiarity at all with DC pentodes and their characteristics. Oh, okay. Well, just saying you're not familiar with them was enough I don't even know any DC Pentode type numbers and am not sophisticated enough to know that the plate resistance of a DC pentode is lower than a heptode. Assuming the plate resistance of a DC Pentode is lower than a heptode, is that a bad thing? Well, it affects plate tank Q. But that is a good thing, as we need to keep the plate tank Q within reason to avoid sideband clipping and provide High Fidelity audio. I don't know if that's necessarily a 'good thing' or not but you seem to presuming 110k, or whatever the particular DC pentode comes out as, results in 'the optimum value'. One can always lower Q but you can't do better than the device limits so, if all else were equal (which is seldom the case), my gut instinct would be to 'err' on the side of 'too much' because that's readily correctable. 6AS6, 6DT6, 6GY6, and 6888 are some of the more 'popular' ones for broadcasters. I am not "adamant about heptodes", in fact I don't particularly care for grid modulation, preferring plate modulation instead, however it is hard to conceive of how to build a plate modulated transmitter using only one single section tube, leaving the heptode, or possibly the DC Pentode, as the only easy choice for a single tube transmitter. I do have a design for a single tube transmitter using your 6ME8 that meets my design brief, however it would require me to build my own well balanced push-pull RF antenna coupling transformer. Yeah, a PP transformer is in some of my designs too but I'm skittish about trying to wind one. After a little out of the box thinking, it occurs to me that it may be possible to make lemonade out of these lemons. It appears that by intentionally unbalancing the transformer you can generate AM without the need for biasing the deflectors to produce a carrier. Tight coupling between the two sections of the winding would probably still be required to insure that no weird phase shifts are produced which would prevent achieving 100% negative modulation. Hopefully using a toroidal transformer core would provide adequate coupling between the two plates? I've debated myself into recursive circles trying to decide which 'imbalance' is the least problematic but, at the moment, I'm on the 'balanced transformer', offset deflectors, side of the circle precisely because of the potential phase shift issue you mentioned. I also kind of like the idea that deflector offset is adjustable. Btw, that previously posted 'hi-hi' link is the same thing using a dual control pentode. He does use an imbalanced transformer but I'm not sure how that compares because, in a DC pentode, the plate and screen are not 'equal' like the dual plates on a 6ME8. I also kind of wonder about screen modulation but it seems to work, maybe because amplitudes are so small. |
#38
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Jun 28, 1:03*pm, flipper wrote:
On Wed, 27 Jun 2012 17:30:35 -0700 (PDT), Patrick Turner wrote: On Jun 27, 12:15 pm, flipper wrote: On Tue, 26 Jun 2012 15:36:48 -0500, John Byrns wrote: In article , flipper wrote: snip. I'm also going to try the DC pentode because, like you said, it seems to me that ought to work about the same. Actually that wasn't what I was trying to suggest, that was either a "typo" on my part or I was having a "senior moment". What I was actually talking about was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how the term "dual control pentode" managed to escape my keyboard. You're quite right. You said the "6SA7/6BE6/6SC6 family of heptodes" and it was I who 'converted' it to dual control heptodes because 'total cathode current appears to be largely independent of the voltage on G3' on those too. An interesting point about your 6ME8 transmitter is that it appears to be a vacuum state analog to the solid state designs using a three transistor "cell", which seem to be the designs of choice with many old radio fans. In this circuit two of the transistors are connected as a long tailed differential pair with RF drive applied to the base of one transistor and with the modulated RF output taken from one of the collectors. The third transistor operates as a current source in the emitter "tail" circuit of the first two transistors, with audio applied to its base to modulate the RF output of the differential pair. Yes, I know the topology and was 'that close' to building one but am not sure which was the cart and which the horse here. I originally started thinking of the 6ME8 back when Patrick was tinkering with Gilbert Cells because I noticed a 'similarity' between it and the 3 transistor 'cell', of which the Gilbert has two on top a current source. Yeah, I was going to say the talk about 3 transistors was actually about a Gilbert Cell. I couldn't spend another 3 months perfecting what I did with 3 transistors because I always have other things to do. I thought you were trying to build a Gilbert cell, back then. That takes more than 3 transistors. Using a gilbert cell chip probably would have worked better then me trying to fiddle with discretes. But could a Gilbert cell be made using 3 triodes? Well, no, because a Gilbert Cell isn't '3 transistors'. For the '3 transistor' cell I think pentodes might be more appropriate because the bipolar collector current vs collector-emitter voltage looks just like a pentode. Although, I'm not sure what the effect of screen current mixed in might be. Or a single 6ME8 because that's essentially how it works with the deflector plates acting as the two upper transistors and G1 the bottom. That's why I used a 6ME8: it's 'conceptually pure' with equivalent 'ins and outs'. As used, however, one plate is simply tied to B+ so, if screen current and plate current 'see saw' in a heptode, which the curves indicate is the case, then it might work similarly. I.E. what goes into the 'unused' second plate externally connected to B+ is 'internally attached' screen current in the heptode. At least it seems to be, in theory, but I didn't know what 'subtleties' might crop up and, especially when doing something 'new', like to cut down the unknowns as much as possible. Another tube with strong g3 action is the 7 pin 6DT6 commonly used for quadrature FM detection. Another one I was intersted in was "gated beam pentode" 6BN6, which some said never worked well as 6DT6. Maybe these have apps at BC band F. Patrick Turner. Gee, I don't know about the 6BN6. That thing is a gated beam discriminator made for FM demod and has a 'limiter' built in, which is sort of the antithesis of AM. Of course, one doesn't have to use it 'as intended' but it's beyond me.- Its awhile since I mucked around with a Gilner, or Turbert Cell or whatever I tried to use as an am modulator with some simularity to Gilbert, which does have more than 3 transistors, 6 as you say. But looking at my notes and the collection of Gilbert cell schematics I have, what I used was a 1/2 Gilbert cell with an LTP driving balanced RF OPT, one side of LTP to 0V, other side to RF carrier F, and tail is transistor collector with AF applied to base. Very simple, but not as good as using tubes as I had already. Maybe Gilbert himself used 6 triodes to make his original Cell, so 3 x 12AT7 might work fine, although with two LTPs using one 12AT7, Ik is twice Ia in one tube, so the cathode drive might best be a 6DJ8 or a pentode. I've never had time to findoutabout and do my own complete research and development. But one reason behind good sounding stereo FM is due to Gilbert cells. Paravicini tried to con the world with a design for an "all tubed MPX decoder" over 10 years ago, but it was just a chip with Gibert within, and buffer tubes on output. The guy didn't do a real tubed MPX decoder with tubed Gilbert cell. Manufacturers avoided doing it like the plague because the cost of 3 twin triodes was far higher than the usual crappy diode matrix. I recall Fisher used 6BN6. I never bothered to build anything using the tube. But 6DT6 was used in very many TV sets to get the audio from the FM audio signal and chief benefit was the large amount of audio output compared to using Foster Seely discriminator or ratio detector. The 6DT6 gave quite low enough THD, and its use could eliminate the use of a preamp tube so hence it "paid for itself" The g3 has fairly high gm, and in FM demod, the circuit connected to g3 is an oscillator at Fm frequency, very easy to get running reliably. Anyway, maybe the tube could work well as an Am modulator. I know you are hooked on using just one tube, and someone said stuff about FM being created and maybe that's true, because the audio changes the effective C across a coil that determines the F so the tuning F changes. I have a separate triode oscillator, then CF buffer, and audio cannot get near anything oscillating at RF. I have a pentode RF amp tube, and fairly large Rk bypassed for RF, but not for AF, so when AF is applied to g1 with the RF from resistance divider from CF RF source, the tube current is changed fairly linearly because there is local current FB at AF, so wave form wasn't so terribly bad as say 50% mod. But adding more tubes to get NFB applied to audio stage much reduced THD in wave form, and got me closer to 100% mod. I found many old radios make a real mess when they try to detect audio when modulation exceeds 50%, and of course back in good old days AM was not fully modulated. But when you look at radio station waves, there is high % mod, and of course compression maybe. And many superhet radio sets can't produce a clean 100% mod IF wave when there is a 100% mod RF wave input. I'm not sure how much NFB you have but certainly it does work to make AM better you have non linear tubes or feeble tubes to deal with. I've not tried to use plate modulation which ppl say is best, one reason being that the modulator adds power to the output signal, and % mod is more linear to AF input to modulator. There must be many ways to make AM. I googled "tube gilbert cell" and other like things but no tubed Gilbert Cell came onto my screen when I clicked 'images' ....so all trial and error needed. Patrick T. |
#39
Posted to rec.audio.tubes
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Jun 28, 1:59*pm, flipper wrote:
On Wed, 27 Jun 2012 22:06:46 -0500, John Byrns wrote: In article , flipper wrote: For this application I tend to think of them as almost the same thing but am beginning to realize that the 'extra' G4 screen is probably what gives a heptode the higher plate impedance. Is that why, despite the 'accidental' mentioning of dual control pentode, you seem to be adamant about heptodes, or is there another reason? The reason I am "adamant about heptodes", if that is what you want to call it, is not because of the plate resistance, it is simply because I have no familiarity at all with DC pentodes and their characteristics. Oh, okay. Well, just saying you're not familiar with them was enough I don't even know any DC Pentode type numbers and am not sophisticated enough to know that the plate resistance of a DC pentode is lower than a heptode. Assuming the plate resistance of a DC Pentode is lower than a heptode, is that a bad thing? Well, it affects plate tank Q. 6AS6, 6DT6, 6GY6, and 6888 are some of the more 'popular' ones for broadcasters. I am not "adamant about heptodes", in fact I don't particularly care for grid modulation, preferring plate modulation instead, however it is hard to conceive of how to build a plate modulated transmitter using only one single section tube, leaving the heptode, or possibly the DC Pentode, as the only easy choice for a single tube transmitter. *I do have a design for a single tube transmitter using your 6ME8 that meets my design brief, however it would require me to build my own well balanced push-pull RF antenna coupling transformer. Yeah, a PP transformer is in some of my designs too but I'm skittish about trying to wind one. PP RF tranny is easy. You can have just one winding with CT to B+, and double tuning gang with frame and moving plates at 0V with 0.01uF caps from fixed plated to coil ends. the thing is to allow the tubes to drive a nice high ohm RLa-a load, and have a secondary wound over the middle part of the winding, ie, the earthy part, and have this winding about 1/5 of the total P turns, so you get a 25:1 impedance match so antena C has hardly any effect on coil tuning. if the RLa-a in parallel with tube Ra was say 25k, then Zout from sec is 1k0, quite low enough to get over effects of C across OP terminals etc. probably an air cored coil is OK using 40mm PVC pipe former with cat 5 strands of solid wire for coil. But maybe best if you drill many holes as possible in PVC to reduce diaelectric losses. Layout and keeping coil away from other metal and keeping leads tidy assist in overall Q which need not be huge. Another way to get low Z output is to have a tap on one side of PP coil, at say 20% of turns so you can use 0.01 cap to take out SE signal. No sec winding, and you still get benefits of the Z transformation and stray output C from antenna won't worry the coil. Patrick Turner. |
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"Beam Me Up, Scotty" (Beamus) AM Transmitter -- first prototype
On Thu, 28 Jun 2012 20:31:40 +1000, "Alex Pogossov"
wrote: "flipper" wrote in message .. . On Wed, 27 Jun 2012 16:21:53 +1000, "Alex Pogossov" wrote: Oops, surely you are not an RF guy. Then do another trick instead. Tune any AM radio into a strong local AM station within tuning range of your transmitter. Then tune your transmitter to a close frequency so that you hear beat frequency on the radio. It is better to have the signal from the transmitter stronger than this station so that you do not hear the audio but only the beat whining (with some sideband chatter of course). Then proceed with the above described tests. (A carrier of an AM station is even more stable than any communications receiver.) Regards, Alex Wouldn't my frequency counter be simpler? hehe With carrier at 640 KHz frequency deviation from idle to twice idle is roughly 70 Hz. Actual measurement was .64015 MHz (resolution limit) to .64022 MHz. It is not bad. I expected worse. Were you thinking maybe a CF would be needed? Probably 6ME8 has less space charge coupling than a regular heptode. I suspect so and it's one of the things I think are 'different' because of the beam deflection. Still 70Hz is not acceptable for listening on a synchronous or SSB detector. In these cases parasitic FM index shall not exceed about 0.25, so with the lowest audio frequency of 50Hz, FM deviation shall not exceed 15Hz. But who would deliberately listening to your transmitter on an SSB radio? No one. Right. What I'd rather know is how much deviation is 'acceptable' for the BCB. So, well done anyway! Thanks. I'm getting the impression it might not be worth even trying a heptode or DC pentode. |
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