|
813's as Modulators
My new (first one in 50 years) is an 8000 modulated by a pair of 813's
running as triodes. The RF deck and Pw supply are up and running and I am working on the modulator now. My plans are to drive it with the Ranger and eventually do a stand alone audio section. The Range gets poor audio reports in spite of months of trying to resolve it. I am wondering if anyone here as any experience using 813's as triodes and would also like some help on the audio driver. I bought a little solid state audio pre-amp and driver that puts out about 4W at 8 ohms. This is probably not enough to drive the 813's so I need more poop and some ideas on how to drive the modulators with the 8 ohm amp. There are some in progress pics of this project at http://schmidling.com/radio.htm Jack K9ACT -- PHOTO OF THE WEEK: http://schmidling.com/pow.htm Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com |
813's as Modulators
Jack Schmidling wrote:
My new (first one in 50 years) is an 8000 modulated by a pair of 813's running as triodes. The RF deck and Pw supply are up and running and I am working on the modulator now. My plans are to drive it with the Ranger and eventually do a stand alone audio section. The Range gets poor audio reports in spite of months of trying to resolve it. I am wondering if anyone here as any experience using 813's as triodes and would also like some help on the audio driver. I bought a little solid state audio pre-amp and driver that puts out about 4W at 8 ohms. This is probably not enough to drive the 813's so I need more poop and some ideas on how to drive the modulators with the 8 ohm amp. There are some in progress pics of this project at http://schmidling.com/radio.htm Jack K9ACT If I have my numbers right, a peak envelope power of 1500 watts equates to a carrier power of 375 watts, which requires an audio amplifier to put out 200 watts (to round up a bit). If the 813's were wired as pentodes they could deliver 600 watts in AB-2 with less than a watt on the grids. I'm not at all sure of what you'd lose using them as triodes, but I'll bet they'd deliver 200W in AB-1 with no grid power consumed at all. Antique Radio Supply (www.tubesandmore.com) has a reprint of an RCA transmitting tube handbook that has extensive data on the 813 -- it should have enough data in there to figure out the design stuff, and directions in the front on how to use the data. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html |
813's as Modulators
On Feb 21, 7:27 am, Jack Schmidling wrote:
My new (first one in 50 years) is an 8000 modulated by a pair of 813's running as triodes. The RF deck and Pw supply are up and running and I am working on the modulator now. My plans are to drive it with the Ranger and eventually do a stand alone audio section. The Range gets poor audio reports in spite of months of trying to resolve it. I am wondering if anyone here as any experience using 813's as triodes and would also like some help on the audio driver. My sole experience is using them not in triode mode, but in pentode mode. The RCA specs tell about using a pair in AB1 as a modulator. Specs he http://www.mif.pg.gda.pl/homepages/f.../079/8/813.pdf In triode mode (plate connected to screen) you'd be limited to 750 volts on the plate if you follow the spec sheet religiously, which will really hurt you in terms of AF power ability. I'm guessing that a pair of 813's used this way would not put out more than 120W of audio power. Many beam power tubes will take more volts on the screen than the spec sheet will say without arcing over but I cannot tell you about this mode on an 813. In pentode mode you have to do a fairly stiff screen supply (which you might have elsewhere in your line-up). Don't forget the -80V or so grid bias too. The "classic" way of doing all this is with a pair or maybe two pairs of 811A's, which are zero-bias as a class B modulator. You can easily get 250W of audio out of a pair or 500W out of two pairs. Driving power isn't zero but it's very moderate (less than 10 watts). The tubes for all this is easy - but you've got a modulation transformer lined up already? Choosing impedances to match that transformer is probably more relevant than any dickering about tubes. Tim. |
813's as Modulators
Tim Shoppa wrote:
On Feb 21, 7:27 am, Jack Schmidling wrote: My new (first one in 50 years) is an 8000 modulated by a pair of 813's running as triodes. The RF deck and Pw supply are up and running and I am working on the modulator now. My plans are to drive it with the Ranger and eventually do a stand alone audio section. The Range gets poor audio reports in spite of months of trying to resolve it. I am wondering if anyone here as any experience using 813's as triodes and would also like some help on the audio driver. My sole experience is using them not in triode mode, but in pentode mode. The RCA specs tell about using a pair in AB1 as a modulator. Specs he http://www.mif.pg.gda.pl/homepages/f.../079/8/813.pdf In triode mode (plate connected to screen) you'd be limited to 750 volts on the plate if you follow the spec sheet religiously, which will really hurt you in terms of AF power ability. I'm guessing that a pair of 813's used this way would not put out more than 120W of audio power. Many beam power tubes will take more volts on the screen than the spec sheet will say without arcing over but I cannot tell you about this mode on an 813. In pentode mode you have to do a fairly stiff screen supply (which you might have elsewhere in your line-up). Don't forget the -80V or so grid bias too. The "classic" way of doing all this is with a pair or maybe two pairs of 811A's, which are zero-bias as a class B modulator. You can easily get 250W of audio out of a pair or 500W out of two pairs. Driving power isn't zero but it's very moderate (less than 10 watts). The tubes for all this is easy - but you've got a modulation transformer lined up already? Choosing impedances to match that transformer is probably more relevant than any dickering about tubes. Tim. A better way to use the 813 in triode mode is to connect the two grids together. This creates a HI-MU zero bias triode. There is some data for this in old handbooks for grounded grid operation. However running them in grid driven mode as zero bias triodes would be identical, except for lower driving power. |
813's as Modulators
ken scharf wrote:
A better way to use the 813 in triode mode is to connect the two grids together. This creates a HI-MU zero bias triode. That is the way I am using them. Got it all together last night and had a short QSO with a good audio report. It takes very little Ranger audio to get 100% mod. Unfortunately, the plate tuning cap started arcing so I had to shut it down and have a look today. js -- PHOTO OF THE WEEK: http://schmidling.com/pow.htm Astronomy, Beer, Cheese, Fiber,Gems, Sausage,Silver http://schmidling.com |
813's as Modulators
On Feb 22, 8:35 pm, ken scharf wrote:
TimShoppawrote: On Feb 21, 7:27 am, Jack Schmidling wrote: My new (first one in 50 years) is an 8000 modulated by a pair of 813's running as triodes. The RF deck and Pw supply are up and running and I am working on the modulator now. My plans are to drive it with the Ranger and eventually do a stand alone audio section. The Range gets poor audio reports in spite of months of trying to resolve it. I am wondering if anyone here as any experience using 813's as triodes and would also like some help on the audio driver. My sole experience is using them not in triode mode, but in pentode mode. The RCA specs tell about using a pair in AB1 as a modulator. Specs he http://www.mif.pg.gda.pl/homepages/f.../079/8/813.pdf In triode mode (plate connected to screen) you'd be limited to 750 volts on the plate if you follow the spec sheet religiously, which will really hurt you in terms of AF power ability. I'm guessing that a pair of 813's used this way would not put out more than 120W of audio power. Many beam power tubes will take more volts on the screen than the spec sheet will say without arcing over but I cannot tell you about this mode on an 813. In pentode mode you have to do a fairly stiff screen supply (which you might have elsewhere in your line-up). Don't forget the -80V or so grid bias too. The "classic" way of doing all this is with a pair or maybe two pairs of 811A's, which are zero-bias as a class B modulator. You can easily get 250W of audio out of a pair or 500W out of two pairs. Driving power isn't zero but it's very moderate (less than 10 watts). The tubes for all this is easy - but you've got a modulation transformer lined up already? Choosing impedances to match that transformer is probably more relevant than any dickering about tubes. Tim. A better way to use the 813 in triode mode is to connect the two grids together. This creates a HI-MU zero bias triode. My experiments at audio have shown that this mode causes more non- linearity than simple pentode mode. But... maybe not anymore than you'd get from a similar-sized high-mu zero bias triode, as those are fairly nonlinear too! My experiments were with much smaller tubes (807's and 6L6's and the like) than the 813's we're talking about here. Most of the extant SPICE models do pretty bad when you tie the screen and control grid together (not surprising!) Tim. |
813's as Modulators
Jack Schmidling wrote:
ken scharf wrote: A better way to use the 813 in triode mode is to connect the two grids together. This creates a HI-MU zero bias triode. That is the way I am using them. Got it all together last night and had a short QSO with a good audio report. It takes very little Ranger audio to get 100% mod. Unfortunately, the plate tuning cap started arcing so I had to shut it down and have a look today. js Please post a schematic on your web site! -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html |
813's as Modulators
On Feb 23, 10:52 am, Tim Wescott wrote:
Please post a schematic on your web site! Tim - You might be surprised how few parts there are in a classical zero- bias class-B modulator. There's a schematic in the 50's/60's ARRL handbooks, and the ENTIRE circuit is two 811A's and an output transformer. That's it! This simplicity is the cunning advantage and popularity of zero-bias triodes (or zero-bias-triode operation of the other tubes). Once you get to beam tubes/pentodes with all their bias supplies and screen supplies and power-supply-sequencing etc. you start getting into having to draw schematics. Later editions added a pi filter on the output to reduce "splatter" and a control relay to remove plate voltage on receive. Tim. |
813's as Modulators
Tim Shoppa wrote:
On Feb 23, 10:52 am, Tim Wescott wrote: Please post a schematic on your web site! Tim - You might be surprised how few parts there are in a classical zero- bias class-B modulator. There's a schematic in the 50's/60's ARRL handbooks, and the ENTIRE circuit is two 811A's and an output transformer. That's it! This simplicity is the cunning advantage and popularity of zero-bias triodes (or zero-bias-triode operation of the other tubes). Once you get to beam tubes/pentodes with all their bias supplies and screen supplies and power-supply-sequencing etc. you start getting into having to draw schematics. Later editions added a pi filter on the output to reduce "splatter" and a control relay to remove plate voltage on receive. Tim. I guess I'm more interested in knowing what the supply and grid voltages are, transformer ratios, a bill of materials, that sort of thing. Yes, you are correct that the schematic of a one cubic foot, 100 pound monster amplifier could be drawn on a business card, but still... -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html |
813's as Modulators
On Feb 23, 12:30 pm, Tim Wescott wrote:
I guess I'm more interested in knowing what the supply and grid voltages are, transformer ratios, a bill of materials, that sort of thing. I too am interested in the transformers that he's using. Tubes are easy to come by in comparison to appropriate modulation transformers. And it can't be the super-simplest implementation because I see some pictures with little power transformers with diodes and electrolytics forming some kind of bias supply. Then you have to work hard to not burn up your favorite modulation and plate transformers :-). Plate chokes, I can rewind them when they catch on fire now! Tim. |
813's as Modulators
On Feb 21, 8:42?pm, Tim Wescott wrote:
If I have my numbers right, a peak envelope power of 1500 watts equates to a carrier power of 375 watts, which requires an audio amplifier to put out 200 watts (to round up a bit). Not exactly! You're mixing input and output powers. Current FCC rules limit us to 1500 watts peak output. On AM, that typically works out to 375 watts carrier output. To make the math simple, let's assume our Class C plate modulated final stage has 75% efficiency. Then we will need to run 500 W DC plate input to get 375 watts carrier output, and 1500 watts peak output. The audio power needed to plate-modulate that 500 watt DC input stage is 250 watts - exactly half the DC input. More modulator power is OK - it's a good idea to figure on 300 watts or so, to allow for various losses, and so the modulator tubes aren't running flat out to give you 100% modulation. You can easily see how the design could get complicated in a hurry. Suppose your Class C modulated stage is only 50% efficient, instead of 75%. Then you would need 750 watts DC input to the stage to get 375 watts carrier output. And you'd need at least 375 watts of audio! -- When reading old Handbooks and other amateur radio publications, it's important to remember that in the past hams rated transmitters by DC input, not output. If you build a "100 watt" transmitter from those days and expect to get 100 watts of RF from it, you will either be sadly disappointed, or will run it way over design ratings in order to get those 100 watts. In the bad old days before 1984, Part 97 simply limited US hams to 1000 watts DC plate input to the final. Which meant you simply needed 500 or so watts of audio to plate modulate a legal-limit AM rig, regardless of efficiency.' The old rules are one big reason plate modulation was the AM method of choice for the Ancient Ones. The carrier efficiency of most other AM techniques (grid modulation, screen modulation, suppressor modulation, cathode modulation, conventional AM linear, etc.) is only 30-35%. So for a kilowatt DC input, you could only get about 300 watts of carrier output, and you'd need a final that could dissipate 700 W! Under today's rules, the DC plate input doesn't matter - only the peak RF output matters. btw, there's all sorts of tube data info online. No need to buy tube manuals anymore unless you are really into paper! 73 es GL de Jim, N2EY |
813's as Modulators
Tim Shoppa wrote:
On Feb 22, 8:35 pm, ken scharf wrote: TimShoppawrote: On Feb 21, 7:27 am, Jack Schmidling wrote: My new (first one in 50 years) is an 8000 modulated by a pair of 813's running as triodes. The RF deck and Pw supply are up and running and I am working on the modulator now. My plans are to drive it with the Ranger and eventually do a stand alone audio section. The Range gets poor audio reports in spite of months of trying to resolve it. I am wondering if anyone here as any experience using 813's as triodes and would also like some help on the audio driver. My sole experience is using them not in triode mode, but in pentode mode. The RCA specs tell about using a pair in AB1 as a modulator. Specs he http://www.mif.pg.gda.pl/homepages/f.../079/8/813.pdf In triode mode (plate connected to screen) you'd be limited to 750 volts on the plate if you follow the spec sheet religiously, which will really hurt you in terms of AF power ability. I'm guessing that a pair of 813's used this way would not put out more than 120W of audio power. Many beam power tubes will take more volts on the screen than the spec sheet will say without arcing over but I cannot tell you about this mode on an 813. In pentode mode you have to do a fairly stiff screen supply (which you might have elsewhere in your line-up). Don't forget the -80V or so grid bias too. The "classic" way of doing all this is with a pair or maybe two pairs of 811A's, which are zero-bias as a class B modulator. You can easily get 250W of audio out of a pair or 500W out of two pairs. Driving power isn't zero but it's very moderate (less than 10 watts). The tubes for all this is easy - but you've got a modulation transformer lined up already? Choosing impedances to match that transformer is probably more relevant than any dickering about tubes. Tim. A better way to use the 813 in triode mode is to connect the two grids together. This creates a HI-MU zero bias triode. My experiments at audio have shown that this mode causes more non- linearity than simple pentode mode. But... maybe not anymore than you'd get from a similar-sized high-mu zero bias triode, as those are fairly nonlinear too! My experiments were with much smaller tubes (807's and 6L6's and the like) than the 813's we're talking about here. Most of the extant SPICE models do pretty bad when you tie the screen and control grid together (not surprising!) Tim. The problem with 807/6L6's in zero bias is that the suppressor cage is grounded to the cathode and messes things up. With the 813 you can connect the suppressor to the plate and the two grids together (or connect the suppressor to the grids). One way or the other will work better than grounding the suppressor. There have been triode circuits using the 807/6L6/1625 or even sweep tubes (such as the 6AV5) in which G1 was tied to G2 with a resistor so G2 was driven harder than G1. One circuit used a dual cathode follower driver with a center tapped choke to provide a low resistance ground for the grids (actually used one winding of an interstage transformer). A member of a radio club I was in years ago built a high power two meter AM rig using a 5894 final modulated by a pair of 6AV5GA's in such a circuit. It always got glowing reports on audio quality. |
813's as Modulators
Tim Shoppa wrote:
On Feb 21, 7:27 am, Jack Schmidling wrote: My new (first one in 50 years) is an 8000 modulated by a pair of 813's running as triodes. The RF deck and Pw supply are up and running and I am working on the modulator now. My plans are to drive it with the Ranger and eventually do a stand alone audio section. The Range gets poor audio reports in spite of months of trying to resolve it. I am wondering if anyone here as any experience using 813's as triodes and would also like some help on the audio driver. My sole experience is using them not in triode mode, but in pentode mode. The RCA specs tell about using a pair in AB1 as a modulator. Specs he http://www.mif.pg.gda.pl/homepages/f.../079/8/813.pdf In triode mode (plate connected to screen) you'd be limited to 750 volts on the plate if you follow the spec sheet religiously, which will really hurt you in terms of AF power ability. I'm guessing that a pair of 813's used this way would not put out more than 120W of audio power. Many beam power tubes will take more volts on the screen than the spec sheet will say without arcing over but I cannot tell you about this mode on an 813. In pentode mode you have to do a fairly stiff screen supply (which you might have elsewhere in your line-up). Don't forget the -80V or so grid bias too. The "classic" way of doing all this is with a pair or maybe two pairs of 811A's, which are zero-bias as a class B modulator. You can easily get 250W of audio out of a pair or 500W out of two pairs. Driving power isn't zero but it's very moderate (less than 10 watts). The tubes for all this is easy - but you've got a modulation transformer lined up already? Choosing impedances to match that transformer is probably more relevant than any dickering about tubes. Tim. BTW I have 5 or 6 used 813's in the junk box gathering dust. I'd like to keep one for static display and the rest are available for nominal cost and shipping. If anybody can use them let me know. I'll also take 2 or 3 good 811A's in trade. |
813's as Modulators
On Feb 24, 11:23�pm, Tim Wescott wrote:
wrote: On Feb 21, 8:42?pm, Tim Wescott wrote: If I have my numbers right, a peak envelope power of 1500 watts equates to a carrier power of 375 watts, which requires an audio amplifier to put out 200 watts (to round up a bit). Not exactly! You're mixing input and output powers. Current FCC rules limit us to 1500 watts peak output. On AM, that typically works out to 375 watts carrier output. Yes, you're right. *Thank you. You're welcome! *Hopefully if I were doing this for real I would have thought of that before it's too late (in fact I have, while designing fantasy rigs). Works for me! To make the math simple, let's assume our Class C plate modulated final stage has 75% efficiency. Then we will need to run 500 W DC plate input to get 375 watts carrier output, and 1500 watts peak output. The audio power needed to plate-modulate that 500 watt DC input stage is 250 watts - exactly half the DC input. More modulator power is OK - it's a good idea to figure on 300 watts or so, to allow for various losses, and so the modulator tubes aren't running flat out to give you 100% modulation. Right. *And that's still modest for a pair of 813s, at least used as pentodes in class AB-2. Agreed - in fact, maybe even in AB1! A pair of 813s in full-yellow pushpull Class B audio service can deliver 650 watts of audio, according to my 1948 ARRL Handbook. With just 0.35 watts of driving power. I think the main reason the Ancient Ones used triodes in their modulators so much was to avoid the problems of a screen supply and a bias supply. Since they were building a separate speech amplifier anyway, building one that delivered 10 watts or so wasn't much different than building one that delivered a watt or so. Two articles of interest: In QST for November, 1958, is "Medium to High Power Audio from 813s". A modulator using 813s in pushpull AB is described. It is really quite simple, too. The bias supply is just a halfwave rectifier feeding a 0B3/VR90. Screen supply is obtained from the plate supply with good regulation and without a humongous power resistor by using a 211 triode as a series regulator, and a string of NE-2s for reference. This series regulator setup has a lot of advantages. The screen supply is regulated, there's no enormous dropping resistor, and most component failures result in a loss of screen voltage rather than excessive screen voltage or screen voltage without plate voltage. But you need a big triode for the pass tube, with a well-insulated transformer to light it. According to the article, a pair of 813s in AB1 will provide 260 watts at 1500 volts B+, or 335 watts at 2000 volts B+ - with *no* driving power! In AB2, it says you can get 455 watts at 2000 volts and 650 watts at 2500 volts, with less than a watt of audio drive. There is also a "peak extension circuit", which is a form of supermodulation by limiting the negative modulation percentage. Such a system was a big deal in 1958, but if you use one today, the carrier power has to be *decreased* to stay legal! Hardly seems worth the trouble. Another interesting article is from QST for March, 1948. "500 Watts of Audio from AB1" shows a modulator that ends up with a pair of 4-250As - driven by a pair of 6SJ7s in a phase inverter! Speech amplifier, mod iron, screen regulator VR string, bias and low voltage supplies are all on the one 17x13x3 chassis...all you need is 2500 volts for the 4-250As. The only transformer in the audio chain is the modulation transformer - all other stages are resistance-coupled. The speech amp is just four tubes - 6SJ7 mike amp, 6J5 2nd audio, pair of 6SJ7s phase inverter. You can easily see how the design could get complicated in a hurry. Suppose your Class C modulated stage is only 50% efficient, instead of 75%. Then you would need 750 watts DC input to the stage to get 375 watts carrier output. And you'd need at least 375 watts of audio! I think at that point I'd decide I needed a better modulated stage, unless I were operating at UHF. I chose 50% to make the math simple and to illustrate the point. And it's easy to get less than 75% efficiency in real life when a rig has certain compromises, like trying to cover 160 through 10 with tubes that have lots of output C. -- When reading old Handbooks and other amateur radio publications, it's important to remember that in the past hams rated transmitters by DC input, not output. If you build a "100 watt" transmitter from those days and expect to get 100 watts of RF from it, you will either be sadly disappointed, or will run it way over design ratings in order to get those 100 watts. In the bad old days before 1984, Part 97 simply limited US hams to 1000 watts DC plate input to the final. Which meant you simply needed 500 or so watts of audio to plate modulate a legal-limit AM rig, regardless of efficiency.' The old rules are one big reason plate modulation was the AM method of choice for the Ancient Ones. The carrier efficiency of most other AM techniques (grid modulation, screen modulation, suppressor modulation, cathode modulation, conventional AM linear, etc.) is only 30-35%. So for a kilowatt DC input, you could only get about 300 watts of carrier output, and you'd need a final that could dissipate 700 W! Under today's rules, the DC plate input doesn't matter - only the peak RF output matters. btw, there's all sorts of tube data info online. No need to buy tube manuals anymore unless you are really into paper! The tube manual I quoted is good for two things that you don't get as easily from the web -- the front pages are a very good text on tube amplifier design, and you can read it while you're eating your cereal in the morning. *I'm just old enough that I need things on paper -- were I ten years younger I'd probably not need it at all. As one who is "really into paper", I agree. Google my call to see part of the library... 73 de Jim, N2EY |
| All times are GMT +1. The time now is 02:33 PM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com