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#1
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Using power triode or power pentode wired as a triode as a split-load phase splitter tube?
Ok, I've spent time designing my amp with an experienced designer.
For personal tastes and reasons, I've planned to build an integrated push-pull power triode amp using 300B power tubes. (Thanks for Fabio about his concerns about the 250V driving voltage, as this amp is being designed by a competent designer, I have trust that the desing will be workable, and I'll manage to build it so it'll work :-). Also thanks go for all you good people here in this newsgroup who have shared your experience, and answered my many questions in detail! I'd rather save you from the details why I would like to build this amp, because they're very personal and related to the planned use of the amp, and rather ask you about the following subject. The amp is going to have either a transformer phase splitter, or a split load phase splitter. Well, that's what has been suggested to me. I've searched the net about the subject, and did find some info. Something that caught my attention (especially as my amp will be using power triodes as output tubes), was the mentioning that a power triode tube or a power pentode wired as triode could be used as the split-load phase splitter tube to get lower output impedance. Now, I'm asking you experts, is this idea sensible, and beneficial, as the original author is claiming it might be? Here's the whole piece of text in question: 1.. split load inverter: Arguably the best balanced of all phase splitters (in terms of unloaded voltage balance), it has two serious difficulties, namely that the output impedance of the two voltages is radically different, and it has NO gain. in a split load, the plate and cathode loads are identical resistors, and output is taken from the plate and cathode. the bottom part is functionally a cathode follower, and the top part is a grounded cathode circuit with a large unbypassed cathode resistor. i.e., low z out in the K circuit and high z out of the P circuit. If the high Z signal is looking at the starving mouth a power triode grid represents, you can easily imagine that it will be really easy to load it down. A typical power triode might have 15pf of grid to plate C, plus the socket stray (5pf), times the Miller number (mu times 20pf times the driver Miller, which might easily be 20pf by the time you're swinging 30-60 vrms, if you are not using a cathode follower...). All in all, it could typically shift from 60pf at 0 signal to about 400 pf at full output. If the driving Z is 50k (approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will load down by 8khz at full swing. The bottom tube will not load down until 80khz or there-abouts. Additionally, you will need about 400 volts B+ to swing 40 top and bottom, not to mention that the tube in front will have to provide 40 volts to the phase inverter! Weird! This is why Williamson used a combo cathodyne and a diff amp. It's a good solution. The cathodyne splits phase with near perfect balance, but it drives the much easier grids of a small dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in turn have the same output z and easily drive pp grids. One further consideration that few have considered is using a power triode as a P/K inverter. An EL-34 wired in triode, with 4k P+K resistors could have an output z low enough to avoid problems in the audio range. The same 400v B+ would still apply... I'm not pretending I did understand all the technical factors involved. That's why I would appreciate your expertise on the matter. So, for my application, would there be any benefit for using a power triode/pentode wired as a triode as the phase splitter tube, to get that lower output impedance? I've got spare EL-34 tubes and other pentodes lying around so I could use them, or buy more 300B tubes. Of course, if I decide to go the transformer way this question isn't needed, but I doubt it, based on the high price tags associated with quality phase splitter transformers. If I'm wrong, and you know some good and "inexpensive" transformers for this use, please correct me. If you feel this question was complete spamm, my apologies! Thanks! -at |
#2
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I'll suggest You a couple of books:
- Brian Jones, "Valve Amplifiers" - the RCA-Radiotron Designer's Handbook (only used, printed in the 50s, cost a lot but it's worth every penny) - the books with the schematics developed by Mullard or General Electric Co. of England (GEC) You can get them from Antique Electronics Supply or from Amazon or from many sources. Ciao Fabio "at" ha scritto nel messaggio ... Ok, I've spent time designing my amp with an experienced designer. For personal tastes and reasons, I've planned to build an integrated push-pull power triode amp using 300B power tubes. (Thanks for Fabio about his concerns about the 250V driving voltage, as this amp is being designed by a competent designer, I have trust that the desing will be workable, and I'll manage to build it so it'll work :-). Also thanks go for all you good people here in this newsgroup who have shared your experience, and answered my many questions in detail! I'd rather save you from the details why I would like to build this amp, because they're very personal and related to the planned use of the amp, and rather ask you about the following subject. The amp is going to have either a transformer phase splitter, or a split load phase splitter. Well, that's what has been suggested to me. I've searched the net about the subject, and did find some info. Something that caught my attention (especially as my amp will be using power triodes as output tubes), was the mentioning that a power triode tube or a power pentode wired as triode could be used as the split-load phase splitter tube to get lower output impedance. Now, I'm asking you experts, is this idea sensible, and beneficial, as the original author is claiming it might be? Here's the whole piece of text in question: 1.. split load inverter: Arguably the best balanced of all phase splitters (in terms of unloaded voltage balance), it has two serious difficulties, namely that the output impedance of the two voltages is radically different, and it has NO gain. in a split load, the plate and cathode loads are identical resistors, and output is taken from the plate and cathode. the bottom part is functionally a cathode follower, and the top part is a grounded cathode circuit with a large unbypassed cathode resistor. i.e., low z out in the K circuit and high z out of the P circuit. If the high Z signal is looking at the starving mouth a power triode grid represents, you can easily imagine that it will be really easy to load it down. A typical power triode might have 15pf of grid to plate C, plus the socket stray (5pf), times the Miller number (mu times 20pf times the driver Miller, which might easily be 20pf by the time you're swinging 30-60 vrms, if you are not using a cathode follower...). All in all, it could typically shift from 60pf at 0 signal to about 400 pf at full output. If the driving Z is 50k (approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will load down by 8khz at full swing. The bottom tube will not load down until 80khz or there-abouts. Additionally, you will need about 400 volts B+ to swing 40 top and bottom, not to mention that the tube in front will have to provide 40 volts to the phase inverter! Weird! This is why Williamson used a combo cathodyne and a diff amp. It's a good solution. The cathodyne splits phase with near perfect balance, but it drives the much easier grids of a small dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in turn have the same output z and easily drive pp grids. One further consideration that few have considered is using a power triode as a P/K inverter. An EL-34 wired in triode, with 4k P+K resistors could have an output z low enough to avoid problems in the audio range. The same 400v B+ would still apply... I'm not pretending I did understand all the technical factors involved. That's why I would appreciate your expertise on the matter. So, for my application, would there be any benefit for using a power triode/pentode wired as a triode as the phase splitter tube, to get that lower output impedance? I've got spare EL-34 tubes and other pentodes lying around so I could use them, or buy more 300B tubes. Of course, if I decide to go the transformer way this question isn't needed, but I doubt it, based on the high price tags associated with quality phase splitter transformers. If I'm wrong, and you know some good and "inexpensive" transformers for this use, please correct me. If you feel this question was complete spamm, my apologies! Thanks! -at |
#3
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"Fabio Berutti" wrote in message ... I'll suggest You a couple of books: - Brian Jones, "Valve Amplifiers" - the RCA-Radiotron Designer's Handbook (only used, printed in the 50s, cost a lot but it's worth every penny) - the books with the schematics developed by Mullard or General Electric Co. of England (GEC) You can get them from Antique Electronics Supply or from Amazon or from many sources. Ciao Fabio Thanks, I'm gonna buy as many books as I can find. But reading them takes many months and years, and right now I have this one question of importance to my present situation, what kind of phase splitter tube to use? So, can anybody give me a simple answer, or, must I wait until I already have the answer, before I can finish this amp desing, and start to build it? Your answers are greatly appreciated. Thanks. -at |
#4
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More to the point:
- I'm not concerned about the 250V p-p required to drive the 300B, I just say it's not easy to provide them (BTW 180V can be enough) - the cathodyne or concertina or distributed load phase splitter is OK for driving pentodes, but You'd need 800V for driving a 300B out of it - the interstage transformer is very difficult to wind correctly, often You end up with a narrow bandwidth and with phase shifts leading to FB instability. See the GEC design for a DA42 125W amp, if You want to use an IT. Even the Tamuras (1K$ each) only guarantee 30Hz-15K, due to stray inductance. - You can use the classic scheme of the Williamson amp (gain-cathodyne-driver-300B) just using a higher B+ voltage and/or a stronger driver tube (say a 6BX7 or 6BL7 or a triode-strapped EL84). Plain RC coupling is the more reliable in terms of phase shift and does not require using transformers made of Unobtainium. A fixed-bias 300B PP using a 450V B+ and a 5K a-a load should provide about 30W at the onset of grid current with a low distortion even using no feedback. At the good end You'll have one of the most beautiful amps that ever played music over the Finnish Tundra... As per the power supply You'll need 2x 5U4G to feed the power tubes with the 320 mA (80x4) they need. I suggest You to make 2x mono frames to avoid spinal injuries while moving it around. A feasible line-up is therefore 6SN7 / 6BX7 / 300B for a 1V sensitivity with no feedback, or 6SJ7 / 6J5 / 6BL7 / 300B if You want to add feedback (a pentode is needed to provide the extra gain, see the classic Leak schemes). I keep on suggesting Octal types because they're MUCH easier to weld in. TRhe 6SN7 can drive a 300B, but it would be better using the GT type (higher B+) which is scarce and expensive; the 6BX or 6BL are TV tubes, cheap and lots of. I'd keep with the "no-feedback" philosophy, to make a PP with triodes. I'm designing a similar unit with Svetlana's SV811, but I use a long-tail splitter (6SL7) in front of the 6BL7 driving the 811s, and I did it this way to avoid FB while keeping about 1% 2nd h. and getting some 50W. Ciao and.. mail us a picture of the working Beast. Fabio "at" ha scritto nel messaggio ... Ok, I've spent time designing my amp with an experienced designer. For personal tastes and reasons, I've planned to build an integrated push-pull power triode amp using 300B power tubes. (Thanks for Fabio about his concerns about the 250V driving voltage, as this amp is being designed by a competent designer, I have trust that the desing will be workable, and I'll manage to build it so it'll work :-). Also thanks go for all you good people here in this newsgroup who have shared your experience, and answered my many questions in detail! I'd rather save you from the details why I would like to build this amp, because they're very personal and related to the planned use of the amp, and rather ask you about the following subject. The amp is going to have either a transformer phase splitter, or a split load phase splitter. Well, that's what has been suggested to me. I've searched the net about the subject, and did find some info. Something that caught my attention (especially as my amp will be using power triodes as output tubes), was the mentioning that a power triode tube or a power pentode wired as triode could be used as the split-load phase splitter tube to get lower output impedance. Now, I'm asking you experts, is this idea sensible, and beneficial, as the original author is claiming it might be? Here's the whole piece of text in question: 1.. split load inverter: Arguably the best balanced of all phase splitters (in terms of unloaded voltage balance), it has two serious difficulties, namely that the output impedance of the two voltages is radically different, and it has NO gain. in a split load, the plate and cathode loads are identical resistors, and output is taken from the plate and cathode. the bottom part is functionally a cathode follower, and the top part is a grounded cathode circuit with a large unbypassed cathode resistor. i.e., low z out in the K circuit and high z out of the P circuit. If the high Z signal is looking at the starving mouth a power triode grid represents, you can easily imagine that it will be really easy to load it down. A typical power triode might have 15pf of grid to plate C, plus the socket stray (5pf), times the Miller number (mu times 20pf times the driver Miller, which might easily be 20pf by the time you're swinging 30-60 vrms, if you are not using a cathode follower...). All in all, it could typically shift from 60pf at 0 signal to about 400 pf at full output. If the driving Z is 50k (approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will load down by 8khz at full swing. The bottom tube will not load down until 80khz or there-abouts. Additionally, you will need about 400 volts B+ to swing 40 top and bottom, not to mention that the tube in front will have to provide 40 volts to the phase inverter! Weird! This is why Williamson used a combo cathodyne and a diff amp. It's a good solution. The cathodyne splits phase with near perfect balance, but it drives the much easier grids of a small dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in turn have the same output z and easily drive pp grids. One further consideration that few have considered is using a power triode as a P/K inverter. An EL-34 wired in triode, with 4k P+K resistors could have an output z low enough to avoid problems in the audio range. The same 400v B+ would still apply... I'm not pretending I did understand all the technical factors involved. That's why I would appreciate your expertise on the matter. So, for my application, would there be any benefit for using a power triode/pentode wired as a triode as the phase splitter tube, to get that lower output impedance? I've got spare EL-34 tubes and other pentodes lying around so I could use them, or buy more 300B tubes. Of course, if I decide to go the transformer way this question isn't needed, but I doubt it, based on the high price tags associated with quality phase splitter transformers. If I'm wrong, and you know some good and "inexpensive" transformers for this use, please correct me. If you feel this question was complete spamm, my apologies! Thanks! -at |
#5
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In 2 words, I prefer the long-tail. I built both cathodynes and longtails,
but I personally hear that the cathodyne has a bit less vivid sound. The longtail is more brilliant (IMHO), this is why I chose it for my will-be flagship amp (6SL7 longtail + 6BL7 driver +SV811-10 power). The same arrangement can easily drive a 300B. I understood You already had chosen the cathodyne. To make it even shorter: a 300B PP will ALMOST ALWAYS sing like the angels, unless You make a gross mistake somewhere. Differences among components and circuits (as long as they are good) are only a matter of taste. Ciao Fabio "at" ha scritto nel messaggio ... "Fabio Berutti" wrote in message ... I'll suggest You a couple of books: - Brian Jones, "Valve Amplifiers" - the RCA-Radiotron Designer's Handbook (only used, printed in the 50s, cost a lot but it's worth every penny) - the books with the schematics developed by Mullard or General Electric Co. of England (GEC) You can get them from Antique Electronics Supply or from Amazon or from many sources. Ciao Fabio Thanks, I'm gonna buy as many books as I can find. But reading them takes many months and years, and right now I have this one question of importance to my present situation, what kind of phase splitter tube to use? So, can anybody give me a simple answer, or, must I wait until I already have the answer, before I can finish this amp desing, and start to build it? Your answers are greatly appreciated. Thanks. -at |
#6
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at wrote: "Fabio Berutti" wrote in message ... I'll suggest You a couple of books: - Brian Jones, "Valve Amplifiers" - the RCA-Radiotron Designer's Handbook (only used, printed in the 50s, cost a lot but it's worth every penny) - the books with the schematics developed by Mullard or General Electric Co. of England (GEC) You can get them from Antique Electronics Supply or from Amazon or from many sources. Ciao Fabio Thanks, I'm gonna buy as many books as I can find. But reading them takes many months and years, and right now I have this one question of importance to my present situation, what kind of phase splitter tube to use? So, can anybody give me a simple answer, or, must I wait until I already have the answer, before I can finish this amp desing, and start to build it? Your answers are greatly appreciated. Thanks. -at To learn how to design amps which make smoke, stray RF signals, lousy sound, and which blow fuses and cost a fortune to repair takes 3 days. The troubles you get are reduced with time spent reading, and applying what is learnt as you learn it, so when building an amp stage, you experience the electronic principles along the way. There is no easy fast instant route to wisdom or experience. Patrick Turner. |
#7
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"Fabio Berutti" wrote in message ...
More to the point: - I'm not concerned about the 250V p-p required to drive the 300B, I just say it's not easy to provide them (BTW 180V can be enough) - the cathodyne or concertina or distributed load phase splitter is OK for driving pentodes, but You'd need 800V for driving a 300B out of it - the interstage transformer is very difficult to wind correctly, often You end up with a narrow bandwidth and with phase shifts leading to FB instability. See the GEC design for a DA42 125W amp, if You want to use an IT. Even the Tamuras (1K$ each) only guarantee 30Hz-15K, due to stray inductance. - You can use the classic scheme of the Williamson amp (gain-cathodyne-driver-300B) just using a higher B+ voltage and/or a stronger driver tube (say a 6BX7 or 6BL7 or a triode-strapped EL84). Plain RC coupling is the more reliable in terms of phase shift and does not require using transformers made of Unobtainium. A fixed-bias 300B PP using a 450V B+ and a 5K a-a load should provide about 30W at the onset of grid current with a low distortion even using no feedback. At the good end You'll have one of the most beautiful amps that ever played music over the Finnish Tundra... As per the power supply You'll need 2x 5U4G to feed the power tubes with the 320 mA (80x4) they need. I suggest You to make 2x mono frames to avoid spinal injuries while moving it around. A feasible line-up is therefore 6SN7 / 6BX7 / 300B for a 1V sensitivity with no feedback, or 6SJ7 / 6J5 / 6BL7 / 300B if You want to add feedback (a pentode is needed to provide the extra gain, see the classic Leak schemes). I keep on suggesting Octal types because they're MUCH easier to weld in. TRhe 6SN7 can drive a 300B, but it would be better using the GT type (higher B+) which is scarce and expensive; the 6BX or 6BL are TV tubes, cheap and lots of. I'd keep with the "no-feedback" philosophy, to make a PP with triodes. I'm designing a similar unit with Svetlana's SV811, but I use a long-tail splitter (6SL7) in front of the 6BL7 driving the 811s, and I did it this way to avoid FB while keeping about 1% 2nd h. and getting some 50W. Ciao and.. mail us a picture of the working Beast. Fabio I read that the full B+ of 450 volts on a 300B in p-p is 40 watts. You may be right with 30 watts. Since I have one source of information, that is what I will have to use a guide for B+ and power output. I would think that running them at 400 volts would be just as good with about 35 watts output and longer life. The 5 watts will not be missed. I agree with you about no feedback. I prefer to not use it. Calculating it is very difficult because of the number of variables to getting the right figures. It is easier to use an amplifier and put feedback in and then determine the rest. Feedback is mainly used to lower the effective plate resistance thus higher damping. Triodes do not need feedback because they have high damping (low plate resistance). What B+ do you use and the resistor values for the cathode and plate in the 6BL7/6BX7 drivers for a 300B or other similiar high drive outputs? I have tested a 6SN7 and similiar types, e.g. 6F8G, and found they will output the voltage needed for high drive outputs, 70 volts, but are close to the maximum output voltage with 300 volts as B+, 51,000 ohm 2 watt load resistor condenser coupled to a 100,000 ohm load. Driver transformers will be costly for quality. The 6C5 works a bit better than a 6SN7 because it will output a bit more voltage before distortion sets in with the same load resistors. I do hope to build myself a p-p 300B stereo amplifier when I can afford it. I will use the 5AR4s to have B+ delay and fixed bias will be used on the outputs. "at" ha scritto nel messaggio ... Ok, I've spent time designing my amp with an experienced designer. For personal tastes and reasons, I've planned to build an integrated push-pull power triode amp using 300B power tubes. (Thanks for Fabio about his concerns about the 250V driving voltage, as this amp is being designed by a competent designer, I have trust that the desing will be workable, and I'll manage to build it so it'll work :-). Also thanks go for all you good people here in this newsgroup who have shared your experience, and answered my many questions in detail! I'd rather save you from the details why I would like to build this amp, because they're very personal and related to the planned use of the amp, and rather ask you about the following subject. The amp is going to have either a transformer phase splitter, or a split load phase splitter. Well, that's what has been suggested to me. I've searched the net about the subject, and did find some info. Something that caught my attention (especially as my amp will be using power triodes as output tubes), was the mentioning that a power triode tube or a power pentode wired as triode could be used as the split-load phase splitter tube to get lower output impedance. Now, I'm asking you experts, is this idea sensible, and beneficial, as the original author is claiming it might be? Here's the whole piece of text in question: 1.. split load inverter: Arguably the best balanced of all phase splitters (in terms of unloaded voltage balance), it has two serious difficulties, namely that the output impedance of the two voltages is radically different, and it has NO gain. in a split load, the plate and cathode loads are identical resistors, and output is taken from the plate and cathode. the bottom part is functionally a cathode follower, and the top part is a grounded cathode circuit with a large unbypassed cathode resistor. i.e., low z out in the K circuit and high z out of the P circuit. If the high Z signal is looking at the starving mouth a power triode grid represents, you can easily imagine that it will be really easy to load it down. A typical power triode might have 15pf of grid to plate C, plus the socket stray (5pf), times the Miller number (mu times 20pf times the driver Miller, which might easily be 20pf by the time you're swinging 30-60 vrms, if you are not using a cathode follower...). All in all, it could typically shift from 60pf at 0 signal to about 400 pf at full output. If the driving Z is 50k (approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will load down by 8khz at full swing. The bottom tube will not load down until 80khz or there-abouts. Additionally, you will need about 400 volts B+ to swing 40 top and bottom, not to mention that the tube in front will have to provide 40 volts to the phase inverter! Weird! This is why Williamson used a combo cathodyne and a diff amp. It's a good solution. The cathodyne splits phase with near perfect balance, but it drives the much easier grids of a small dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in turn have the same output z and easily drive pp grids. One further consideration that few have considered is using a power triode as a P/K inverter. An EL-34 wired in triode, with 4k P+K resistors could have an output z low enough to avoid problems in the audio range. The same 400v B+ would still apply... I'm not pretending I did understand all the technical factors involved. That's why I would appreciate your expertise on the matter. So, for my application, would there be any benefit for using a power triode/pentode wired as a triode as the phase splitter tube, to get that lower output impedance? I've got spare EL-34 tubes and other pentodes lying around so I could use them, or buy more 300B tubes. Of course, if I decide to go the transformer way this question isn't needed, but I doubt it, based on the high price tags associated with quality phase splitter transformers. If I'm wrong, and you know some good and "inexpensive" transformers for this use, please correct me. If you feel this question was complete spamm, my apologies! Thanks! -at |
#8
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In fact in my project the 6BL7s should be choke-loaded and work at about
300V, 30 mA. Vout can be up to 500Vpp. Nevertheless, due to choke stray inductance, I'm considering plain RC loading. A separated PS for the driving stage is already provided, and it can go as high as 400V. I'll fiddle a bit with Tubecad. I suppose a 10-15K load is OK, plate resistance is only1K7 for the 6BL7. Ciao Fabio "Mark" ha scritto nel messaggio om... "Fabio Berutti" wrote in message ... More to the point: - I'm not concerned about the 250V p-p required to drive the 300B, I just say it's not easy to provide them (BTW 180V can be enough) - the cathodyne or concertina or distributed load phase splitter is OK for driving pentodes, but You'd need 800V for driving a 300B out of it - the interstage transformer is very difficult to wind correctly, often You end up with a narrow bandwidth and with phase shifts leading to FB instability. See the GEC design for a DA42 125W amp, if You want to use an IT. Even the Tamuras (1K$ each) only guarantee 30Hz-15K, due to stray inductance. - You can use the classic scheme of the Williamson amp (gain-cathodyne-driver-300B) just using a higher B+ voltage and/or a stronger driver tube (say a 6BX7 or 6BL7 or a triode-strapped EL84). Plain RC coupling is the more reliable in terms of phase shift and does not require using transformers made of Unobtainium. A fixed-bias 300B PP using a 450V B+ and a 5K a-a load should provide about 30W at the onset of grid current with a low distortion even using no feedback. At the good end You'll have one of the most beautiful amps that ever played music over the Finnish Tundra... As per the power supply You'll need 2x 5U4G to feed the power tubes with the 320 mA (80x4) they need. I suggest You to make 2x mono frames to avoid spinal injuries while moving it around. A feasible line-up is therefore 6SN7 / 6BX7 / 300B for a 1V sensitivity with no feedback, or 6SJ7 / 6J5 / 6BL7 / 300B if You want to add feedback (a pentode is needed to provide the extra gain, see the classic Leak schemes). I keep on suggesting Octal types because they're MUCH easier to weld in. TRhe 6SN7 can drive a 300B, but it would be better using the GT type (higher B+) which is scarce and expensive; the 6BX or 6BL are TV tubes, cheap and lots of. I'd keep with the "no-feedback" philosophy, to make a PP with triodes. I'm designing a similar unit with Svetlana's SV811, but I use a long-tail splitter (6SL7) in front of the 6BL7 driving the 811s, and I did it this way to avoid FB while keeping about 1% 2nd h. and getting some 50W. Ciao and.. mail us a picture of the working Beast. Fabio I read that the full B+ of 450 volts on a 300B in p-p is 40 watts. You may be right with 30 watts. Since I have one source of information, that is what I will have to use a guide for B+ and power output. I would think that running them at 400 volts would be just as good with about 35 watts output and longer life. The 5 watts will not be missed. I agree with you about no feedback. I prefer to not use it. Calculating it is very difficult because of the number of variables to getting the right figures. It is easier to use an amplifier and put feedback in and then determine the rest. Feedback is mainly used to lower the effective plate resistance thus higher damping. Triodes do not need feedback because they have high damping (low plate resistance). What B+ do you use and the resistor values for the cathode and plate in the 6BL7/6BX7 drivers for a 300B or other similiar high drive outputs? I have tested a 6SN7 and similiar types, e.g. 6F8G, and found they will output the voltage needed for high drive outputs, 70 volts, but are close to the maximum output voltage with 300 volts as B+, 51,000 ohm 2 watt load resistor condenser coupled to a 100,000 ohm load. Driver transformers will be costly for quality. The 6C5 works a bit better than a 6SN7 because it will output a bit more voltage before distortion sets in with the same load resistors. I do hope to build myself a p-p 300B stereo amplifier when I can afford it. I will use the 5AR4s to have B+ delay and fixed bias will be used on the outputs. "at" ha scritto nel messaggio ... Ok, I've spent time designing my amp with an experienced designer. For personal tastes and reasons, I've planned to build an integrated push-pull power triode amp using 300B power tubes. (Thanks for Fabio about his concerns about the 250V driving voltage, as this amp is being designed by a competent designer, I have trust that the desing will be workable, and I'll manage to build it so it'll work :-). Also thanks go for all you good people here in this newsgroup who have shared your experience, and answered my many questions in detail! I'd rather save you from the details why I would like to build this amp, because they're very personal and related to the planned use of the amp, and rather ask you about the following subject. The amp is going to have either a transformer phase splitter, or a split load phase splitter. Well, that's what has been suggested to me. I've searched the net about the subject, and did find some info. Something that caught my attention (especially as my amp will be using power triodes as output tubes), was the mentioning that a power triode tube or a power pentode wired as triode could be used as the split-load phase splitter tube to get lower output impedance. Now, I'm asking you experts, is this idea sensible, and beneficial, as the original author is claiming it might be? Here's the whole piece of text in question: 1.. split load inverter: Arguably the best balanced of all phase splitters (in terms of unloaded voltage balance), it has two serious difficulties, namely that the output impedance of the two voltages is radically different, and it has NO gain. in a split load, the plate and cathode loads are identical resistors, and output is taken from the plate and cathode. the bottom part is functionally a cathode follower, and the top part is a grounded cathode circuit with a large unbypassed cathode resistor. i.e., low z out in the K circuit and high z out of the P circuit. If the high Z signal is looking at the starving mouth a power triode grid represents, you can easily imagine that it will be really easy to load it down. A typical power triode might have 15pf of grid to plate C, plus the socket stray (5pf), times the Miller number (mu times 20pf times the driver Miller, which might easily be 20pf by the time you're swinging 30-60 vrms, if you are not using a cathode follower...). All in all, it could typically shift from 60pf at 0 signal to about 400 pf at full output. If the driving Z is 50k (approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will load down by 8khz at full swing. The bottom tube will not load down until 80khz or there-abouts. Additionally, you will need about 400 volts B+ to swing 40 top and bottom, not to mention that the tube in front will have to provide 40 volts to the phase inverter! Weird! This is why Williamson used a combo cathodyne and a diff amp. It's a good solution. The cathodyne splits phase with near perfect balance, but it drives the much easier grids of a small dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in turn have the same output z and easily drive pp grids. One further consideration that few have considered is using a power triode as a P/K inverter. An EL-34 wired in triode, with 4k P+K resistors could have an output z low enough to avoid problems in the audio range. The same 400v B+ would still apply... I'm not pretending I did understand all the technical factors involved. That's why I would appreciate your expertise on the matter. So, for my application, would there be any benefit for using a power triode/pentode wired as a triode as the phase splitter tube, to get that lower output impedance? I've got spare EL-34 tubes and other pentodes lying around so I could use them, or buy more 300B tubes. Of course, if I decide to go the transformer way this question isn't needed, but I doubt it, based on the high price tags associated with quality phase splitter transformers. If I'm wrong, and you know some good and "inexpensive" transformers for this use, please correct me. If you feel this question was complete spamm, my apologies! Thanks! -at |
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Mark wrote: "Fabio Berutti" wrote in message ... More to the point: - I'm not concerned about the 250V p-p required to drive the 300B, I just say it's not easy to provide them (BTW 180V can be enough) - the cathodyne or concertina or distributed load phase splitter is OK for driving pentodes, but You'd need 800V for driving a 300B out of it - the interstage transformer is very difficult to wind correctly, often You end up with a narrow bandwidth and with phase shifts leading to FB instability. See the GEC design for a DA42 125W amp, if You want to use an IT. Even the Tamuras (1K$ each) only guarantee 30Hz-15K, due to stray inductance. - You can use the classic scheme of the Williamson amp (gain-cathodyne-driver-300B) just using a higher B+ voltage and/or a stronger driver tube (say a 6BX7 or 6BL7 or a triode-strapped EL84). Plain RC coupling is the more reliable in terms of phase shift and does not require using transformers made of Unobtainium. A fixed-bias 300B PP using a 450V B+ and a 5K a-a load should provide about 30W at the onset of grid current with a low distortion even using no feedback. At the good end You'll have one of the most beautiful amps that ever played music over the Finnish Tundra... As per the power supply You'll need 2x 5U4G to feed the power tubes with the 320 mA (80x4) they need. I suggest You to make 2x mono frames to avoid spinal injuries while moving it around. A feasible line-up is therefore 6SN7 / 6BX7 / 300B for a 1V sensitivity with no feedback, or 6SJ7 / 6J5 / 6BL7 / 300B if You want to add feedback (a pentode is needed to provide the extra gain, see the classic Leak schemes). I keep on suggesting Octal types because they're MUCH easier to weld in. TRhe 6SN7 can drive a 300B, but it would be better using the GT type (higher B+) which is scarce and expensive; the 6BX or 6BL are TV tubes, cheap and lots of. I'd keep with the "no-feedback" philosophy, to make a PP with triodes. I'm designing a similar unit with Svetlana's SV811, but I use a long-tail splitter (6SL7) in front of the 6BL7 driving the 811s, and I did it this way to avoid FB while keeping about 1% 2nd h. and getting some 50W. Ciao and.. mail us a picture of the working Beast. Fabio I read that the full B+ of 450 volts on a 300B in p-p is 40 watts. You may be right with 30 watts. Since I have one source of information, that is what I will have to use a guide for B+ and power output. I would think that running them at 400 volts would be just as good with about 35 watts output and longer life. The 5 watts will not be missed. 8 watts from an SE 300B which idles at 30 watts of dissipation is about all one should expect, unless you use beefed up versions of the 300B available from emission labs, at ungodly prices. So a pair in PP can only give 16 watts in class A, and I always thought about 28 watts was about right for class AB into say 4 ohms, so that with 8 ohms, its all class A and you get the 16 watts. I agree with you about no feedback. I prefer to not use it. Calculating it is very difficult because of the number of variables to getting the right figures. It is easier to use an amplifier and put feedback in and then determine the rest. Feedback is mainly used to lower the effective plate resistance thus higher damping. Triodes do not need feedback because they have high damping (low plate resistance). If you have a pair of 300B in PP, the anode to anode Ra is about 1.6k, and if the OPt is 10k to 6 ohms, the Z ratio is 1,666 :1, so the 1,600 ohms of Ra-a appears as 0.96 ohms, but if the OPT winding losses total 10%, or 0.6 ohms, then Ro = 1.6 ohms approx, which could ideally be lower. 12 dB of NFB would reduce this 1.6 ohms to less than 0.5 ohms. What B+ do you use and the resistor values for the cathode and plate in the 6BL7/6BX7 drivers for a 300B or other similiar high drive outputs? I have tested a 6SN7 and similiar types, e.g. 6F8G, and found they will output the voltage needed for high drive outputs, 70 volts, but are close to the maximum output voltage with 300 volts as B+, 51,000 ohm 2 watt load resistor condenser coupled to a 100,000 ohm load. Driver transformers will be costly for quality. The 6C5 works a bit better than a 6SN7 because it will output a bit more voltage before distortion sets in with the same load resistors. You don't need a costly IST, use a choke with a CT, and a pair of additional R to isolate the stray shunt C of the choke, and the inductance shunt at LF, and you have a splendid driver, which can be CR coupled to the 300B grids, see:- http://www.turneraudio.com.au/websch...ma550w335h.gif Patrick Turner. |
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Fabio Berutti wrote: In fact in my project the 6BL7s should be choke-loaded and work at about 300V, 30 mA. Vout can be up to 500Vpp. Nevertheless, due to choke stray inductance, I'm considering plain RC loading. Using the choke *as well as the L* is the answer. The choke has a high impedance for most of the band, and if the R = 4 x Ra at least, then even at DC or at some HF, the gain won't roll off to nothing. The load seen by the triode with such a DC delivery path is then a much higher value, and the following grid bias R for 300B can be a lower value which still won't load down the driver triode/s so the driver thd is *much* less, as loadline analysis will show when you draw all this up before building anything. A separated PS for the driving stage is already provided, and it can go as high as 400V. I'll fiddle a bit with Tubecad. I suppose a 10-15K load is OK, plate resistance is only1K7 for the 6BL7. If the choke has 400 k impedance at 1 kHz, which ie easy to get with a non gapped choke with CT, then the tube never sees the 15 k load. The load line is dramatically flatter, thd down about 10 dB at least, and voltage swing a lot wider. Patrick Turner. Ciao Fabio "Mark" ha scritto nel messaggio om... "Fabio Berutti" wrote in message ... More to the point: - I'm not concerned about the 250V p-p required to drive the 300B, I just say it's not easy to provide them (BTW 180V can be enough) - the cathodyne or concertina or distributed load phase splitter is OK for driving pentodes, but You'd need 800V for driving a 300B out of it - the interstage transformer is very difficult to wind correctly, often You end up with a narrow bandwidth and with phase shifts leading to FB instability. See the GEC design for a DA42 125W amp, if You want to use an IT. Even the Tamuras (1K$ each) only guarantee 30Hz-15K, due to stray inductance. - You can use the classic scheme of the Williamson amp (gain-cathodyne-driver-300B) just using a higher B+ voltage and/or a stronger driver tube (say a 6BX7 or 6BL7 or a triode-strapped EL84). Plain RC coupling is the more reliable in terms of phase shift and does not require using transformers made of Unobtainium. A fixed-bias 300B PP using a 450V B+ and a 5K a-a load should provide about 30W at the onset of grid current with a low distortion even using no feedback. At the good end You'll have one of the most beautiful amps that ever played music over the Finnish Tundra... As per the power supply You'll need 2x 5U4G to feed the power tubes with the 320 mA (80x4) they need. I suggest You to make 2x mono frames to avoid spinal injuries while moving it around. A feasible line-up is therefore 6SN7 / 6BX7 / 300B for a 1V sensitivity with no feedback, or 6SJ7 / 6J5 / 6BL7 / 300B if You want to add feedback (a pentode is needed to provide the extra gain, see the classic Leak schemes). I keep on suggesting Octal types because they're MUCH easier to weld in. TRhe 6SN7 can drive a 300B, but it would be better using the GT type (higher B+) which is scarce and expensive; the 6BX or 6BL are TV tubes, cheap and lots of. I'd keep with the "no-feedback" philosophy, to make a PP with triodes. I'm designing a similar unit with Svetlana's SV811, but I use a long-tail splitter (6SL7) in front of the 6BL7 driving the 811s, and I did it this way to avoid FB while keeping about 1% 2nd h. and getting some 50W. Ciao and.. mail us a picture of the working Beast. Fabio I read that the full B+ of 450 volts on a 300B in p-p is 40 watts. You may be right with 30 watts. Since I have one source of information, that is what I will have to use a guide for B+ and power output. I would think that running them at 400 volts would be just as good with about 35 watts output and longer life. The 5 watts will not be missed. I agree with you about no feedback. I prefer to not use it. Calculating it is very difficult because of the number of variables to getting the right figures. It is easier to use an amplifier and put feedback in and then determine the rest. Feedback is mainly used to lower the effective plate resistance thus higher damping. Triodes do not need feedback because they have high damping (low plate resistance). What B+ do you use and the resistor values for the cathode and plate in the 6BL7/6BX7 drivers for a 300B or other similiar high drive outputs? I have tested a 6SN7 and similiar types, e.g. 6F8G, and found they will output the voltage needed for high drive outputs, 70 volts, but are close to the maximum output voltage with 300 volts as B+, 51,000 ohm 2 watt load resistor condenser coupled to a 100,000 ohm load. Driver transformers will be costly for quality. The 6C5 works a bit better than a 6SN7 because it will output a bit more voltage before distortion sets in with the same load resistors. I do hope to build myself a p-p 300B stereo amplifier when I can afford it. I will use the 5AR4s to have B+ delay and fixed bias will be used on the outputs. "at" ha scritto nel messaggio ... Ok, I've spent time designing my amp with an experienced designer. For personal tastes and reasons, I've planned to build an integrated push-pull power triode amp using 300B power tubes. (Thanks for Fabio about his concerns about the 250V driving voltage, as this amp is being designed by a competent designer, I have trust that the desing will be workable, and I'll manage to build it so it'll work :-). Also thanks go for all you good people here in this newsgroup who have shared your experience, and answered my many questions in detail! I'd rather save you from the details why I would like to build this amp, because they're very personal and related to the planned use of the amp, and rather ask you about the following subject. The amp is going to have either a transformer phase splitter, or a split load phase splitter. Well, that's what has been suggested to me. I've searched the net about the subject, and did find some info. Something that caught my attention (especially as my amp will be using power triodes as output tubes), was the mentioning that a power triode tube or a power pentode wired as triode could be used as the split-load phase splitter tube to get lower output impedance. Now, I'm asking you experts, is this idea sensible, and beneficial, as the original author is claiming it might be? Here's the whole piece of text in question: 1.. split load inverter: Arguably the best balanced of all phase splitters (in terms of unloaded voltage balance), it has two serious difficulties, namely that the output impedance of the two voltages is radically different, and it has NO gain. in a split load, the plate and cathode loads are identical resistors, and output is taken from the plate and cathode. the bottom part is functionally a cathode follower, and the top part is a grounded cathode circuit with a large unbypassed cathode resistor. i.e., low z out in the K circuit and high z out of the P circuit. If the high Z signal is looking at the starving mouth a power triode grid represents, you can easily imagine that it will be really easy to load it down. A typical power triode might have 15pf of grid to plate C, plus the socket stray (5pf), times the Miller number (mu times 20pf times the driver Miller, which might easily be 20pf by the time you're swinging 30-60 vrms, if you are not using a cathode follower...). All in all, it could typically shift from 60pf at 0 signal to about 400 pf at full output. If the driving Z is 50k (approximately 1/2 a 6SN7 with 20k p+k resistors), the top tube will load down by 8khz at full swing. The bottom tube will not load down until 80khz or there-abouts. Additionally, you will need about 400 volts B+ to swing 40 top and bottom, not to mention that the tube in front will have to provide 40 volts to the phase inverter! Weird! This is why Williamson used a combo cathodyne and a diff amp. It's a good solution. The cathodyne splits phase with near perfect balance, but it drives the much easier grids of a small dual medium mu voltage amp (2 or 3 pf instead of 15 or 20), which in turn have the same output z and easily drive pp grids. One further consideration that few have considered is using a power triode as a P/K inverter. An EL-34 wired in triode, with 4k P+K resistors could have an output z low enough to avoid problems in the audio range. The same 400v B+ would still apply... I'm not pretending I did understand all the technical factors involved. That's why I would appreciate your expertise on the matter. So, for my application, would there be any benefit for using a power triode/pentode wired as a triode as the phase splitter tube, to get that lower output impedance? I've got spare EL-34 tubes and other pentodes lying around so I could use them, or buy more 300B tubes. Of course, if I decide to go the transformer way this question isn't needed, but I doubt it, based on the high price tags associated with quality phase splitter transformers. If I'm wrong, and you know some good and "inexpensive" transformers for this use, please correct me. If you feel this question was complete spamm, my apologies! Thanks! -at |
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