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#1
Posted to rec.audio.tubes
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Amp oscillation
Hi all,
I've been scratch building an amp using a 7199 preamp/phase invert and 6L6GC power tubes. It's a very simple amp - I'm just doing it for the learning process. When I finally got a signal through the circuit (woo hoo) it had a an oscillation (407kHz) at the bottom of every sine wave. I've been going crazy trying to troubleshoot - checking my lead dressing, trying to eliminate ground loops, blah blah - everything I've tried has made it worse to the point that now my signal is almost entirely saturated with the oscillation (407 kHz). Out of desperation I took out the 6L6's and put in a pair of 6F6's that I had laying around...PERFECT signal, no oscillation. What the hell is wrong with the 6L6 tubes? Do they need something specific to dampen oscillations? HELP... |
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#2
Posted to rec.audio.tubes
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Amp oscillation
"htrboy" wrote in message oups.com... Hi all, I've been scratch building an amp using a 7199 preamp/phase invert and 6L6GC power tubes. It's a very simple amp - I'm just doing it for the learning process. When I finally got a signal through the circuit (woo hoo) it had a an oscillation (407kHz) at the bottom of every sine wave. I've been going crazy trying to troubleshoot - checking my lead dressing, trying to eliminate ground loops, blah blah - everything I've tried has made it worse to the point that now my signal is almost entirely saturated with the oscillation (407 kHz). Out of desperation I took out the 6L6's and put in a pair of 6F6's that I had laying around...PERFECT signal, no oscillation. What the hell is wrong with the 6L6 tubes? Do they need something specific to dampen oscillations? HELP... Hi. You have come upon against the spectre of instability, which presents quite a challenge for most hombrew builders. Is the amp stable open loop (with the feedback circuit disconnected?) Also, try adding some series grid resistance to the grids of the 6L6GC pair. Start with a resistor somewhere between 4k7 and 10k. Solder this direct to the grid, as close to the tube base pins as possible, in series with the signal. This helps to dampen high- frequency oscillations caused by capacitance. Circuits driven with cathode followers seem particularly prone to the problem you describe. The "grid stopper" as is it called reduces the HF response of the amp, so once you have stopped the oscillation, reduce the value of the series resistor until you find the threshold of the oscillation. Let us know how you get on. Iain |
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#3
Posted to rec.audio.tubes
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Amp oscillation
On Feb 4, 9:17 pm, "Iain Churches" wrote:
"htrboy" wrote in message oups.com... Hi all, I've been scratch building an amp using a 7199 preamp/phase invert and 6L6GC power tubes. It's a very simple amp - I'm just doing it for the learning process. When I finally got a signal through the circuit (woo hoo) it had a an oscillation (407kHz) at the bottom of every sine wave. I've been going crazy trying to troubleshoot - checking my lead dressing, trying to eliminate ground loops, blah blah - everything I've tried has made it worse to the point that now my signal is almost entirely saturated with the oscillation (407 kHz). Out of desperation I took out the 6L6's and put in a pair of 6F6's that I had laying around...PERFECT signal, no oscillation. What the hell is wrong with the 6L6 tubes? Do they need something specific to dampen oscillations? HELP... Hi. You have come upon against the spectre of instability, which presents quite a challenge for most hombrew builders. Is the amp stable open loop (with the feedback circuit disconnected?) Also, try adding some series grid resistance to the grids of the 6L6GC pair. Start with a resistor somewhere between 4k7 and 10k. Solder this direct to the grid, as close to the tube base pins as possible, in series with the signal. This helps to dampen high- frequency oscillations caused by capacitance. Circuits driven with cathode followers seem particularly prone to the problem you describe. The "grid stopper" as is it called reduces the HF response of the amp, so once you have stopped the oscillation, reduce the value of the series resistor until you find the threshold of the oscillation. Let us know how you get on. Iain Hi lain, Thanks for the input. I tried disconnecting the feedback circuit and the oscillation went away, however the gain seemed to be unstable?? Just for fun I tried switching the output transformer plate connections (maybe positive feedback was the problem?) but no - it made things worse. The original circuit has 1k grid stop resistors at the power tube grids, I'll try upping those as you suggest to 4.7k (and up) and see what happens. I'm glad to have a new direction to try - thanks! |
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#4
Posted to rec.audio.tubes
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Amp oscillation
htrboy wrote: Hi all, I've been scratch building an amp using a 7199 preamp/phase invert and 6L6GC power tubes. It's a very simple amp - I'm just doing it for the learning process. When I finally got a signal through the circuit (woo hoo) it had a an oscillation (407kHz) at the bottom of every sine wave. I've been going crazy trying to troubleshoot - checking my lead dressing, trying to eliminate ground loops, blah blah - everything I've tried has made it worse to the point that now my signal is almost entirely saturated with the oscillation (407 kHz). Out of desperation I took out the 6L6's and put in a pair of 6F6's that I had laying around...PERFECT signal, no oscillation. What the hell is wrong with the 6L6 tubes? Do they need something specific to dampen oscillations? HELP... The 6L6 has been used since about 1935 with little oscillation troubles by thousands of amp makers who knew more than you about amp building. Its not going to get you anywhere to ask what is wrong with a 6L6, because the answer is that there isn't much wrong with a 6L6, and you should be asking many more questions about what you may have done wrong instead. You are not alone, and like you I once was exasperated and frustrated with my early attempts to build amps that then behaved like RF power oscilators. Only at the time, prior to 2000, when I went online, there was nobody to share the problem with online, and I had to damn well work out what i did wrong myself. If I had a $ for every time someone dialed into r.a.t with a problem about amplifiers that oscillate, I'd be rich! I suggest firstly that you give us a URL for the schematic of your amp, so that we may see EXACTLY WHAT SCHEMATIC YOU HAVE RIGHT NOW. Please check all your connections again and draw up the schematic on a large NEAT page in black ink on white and scan and post it somewhere, at your website/home page if you have one, or at a friend's, or at news groups alternate.binaries.pictures.radio-phono or alternate.binaries.schematics.electronic. But for us to empathise with your bothers you have to place a lot of effort into conveying a suitable amount of information on which we may make comment after some rational analysis. Do the scan so it is a black and white format at 400dpi, and convert it to a gif image to keep the file size below about 100kb, and so that it fills a screen nicely, and is clearly legible, ie, we can all see what the numbers and letters are. Once you become proficient with PRODUCING schematics and reading them, you will learn so much more it should dazzle you. There are a pile of schematics at my website, and you are free to use any you like at http://www.turneraudio.com.au Some of the schematics were hand drawn on paper, then scanned, and some created in MS Paint, just sitting there for hours and hours with a mouse..... Patrick Turner. |
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#5
Posted to rec.audio.tubes
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Amp oscillation
On Feb 5, 1:31 am, Patrick Turner wrote:
htrboy wrote: Hi all, I've been scratch building an amp using a 7199 preamp/phase invert and 6L6GC power tubes. It's a very simple amp - I'm just doing it for the learning process. When I finally got a signal through the circuit (woo hoo) it had a an oscillation (407kHz) at the bottom of every sine wave. I've been going crazy trying to troubleshoot - checking my lead dressing, trying to eliminate ground loops, blah blah - everything I've tried has made it worse to the point that now my signal is almost entirely saturated with the oscillation (407 kHz). Out of desperation I took out the 6L6's and put in a pair of 6F6's that I had laying around...PERFECT signal, no oscillation. What the hell is wrong with the 6L6 tubes? Do they need something specific to dampen oscillations? HELP... The 6L6 has been used since about 1935 with little oscillation troubles by thousands of amp makers who knew more than you about amp building. Its not going to get you anywhere to ask what is wrong with a 6L6, because the answer is that there isn't much wrong with a 6L6, and you should be asking many more questions about what you may have done wrong instead. You are not alone, and like you I once was exasperated and frustrated with my early attempts to build amps that then behaved like RF power oscilators. Only at the time, prior to 2000, when I went online, there was nobody to share the problem with online, and I had to damn well work out what i did wrong myself. If I had a $ for every time someone dialed into r.a.t with a problem about amplifiers that oscillate, I'd be rich! I suggest firstly that you give us a URL for the schematic of your amp, so that we may see EXACTLY WHAT SCHEMATIC YOU HAVE RIGHT NOW. Please check all your connections again and draw up the schematic on a large NEAT page in black ink on white and scan and post it somewhere, at your website/home page if you have one, or at a friend's, or at news groups alternate.binaries.pictures.radio-phono or alternate.binaries.schematics.electronic. But for us to empathise with your bothers you have to place a lot of effort into conveying a suitable amount of information on which we may make comment after some rational analysis. Do the scan so it is a black and white format at 400dpi, and convert it to a gif image to keep the file size below about 100kb, and so that it fills a screen nicely, and is clearly legible, ie, we can all see what the numbers and letters are. Once you become proficient with PRODUCING schematics and reading them, you will learn so much more it should dazzle you. There are a pile of schematics at my website, and you are free to use any you like athttp://www.turneraudio.com.au Some of the schematics were hand drawn on paper, then scanned, and some created in MS Paint, just sitting there for hours and hours with a mouse..... Patrick Turner. Hello Patrick, Thanks for the input. My comment about the 6L6 tubes was somewhat tongue-in-cheek which doesn't come across in print as well as it does in person. The schematic I used is adapted out of one of the many RCA Receiving Tube manuals (the specific one escapes me right now). I will try to figure out how to post a schematic as soon as possible. Best regards. |
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#6
Posted to rec.audio.tubes
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Amp oscillation
In article .com,
"htrboy" wrote: Hi lain, Thanks for the input. I tried disconnecting the feedback circuit and the oscillation went away, however the gain seemed to be unstable?? Just for fun I tried switching the output transformer plate connections (maybe positive feedback was the problem?) but no - it made things worse. The original circuit has 1k grid stop resistors at the power tube grids, I'll try upping those as you suggest to 4.7k (and up) and see what happens. I'm glad to have a new direction to try - thanks! It sounds like you don't have the correct high frequency compensation in your negative feedback loop, or you just have way too much feedback in the first place, or both. I believe Patrick has explained before here in this group how to do high frequency compensation and add a HF gain stepping network, check the group archives on Google. Also I believe Patrick has said he has this information on his web pages, although I haven't checked and can't vouch for that. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
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#7
Posted to rec.audio.tubes
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Amp oscillation
"htrboy" wrote in message ups.com... On Feb 5, 1:31 am, Patrick Turner wrote: htrboy wrote: Hi all, I've been scratch building an amp using a 7199 preamp/phase invert and 6L6GC power tubes. It's a very simple amp - I'm just doing it for the learning process. When I finally got a signal through the circuit (woo hoo) it had a an oscillation (407kHz) at the bottom of every sine wave. I've been going crazy trying to troubleshoot - checking my lead dressing, trying to eliminate ground loops, blah blah - everything I've tried has made it worse to the point that now my signal is almost entirely saturated with the oscillation (407 kHz). Out of desperation I took out the 6L6's and put in a pair of 6F6's that I had laying around...PERFECT signal, no oscillation. What the hell is wrong with the 6L6 tubes? Do they need something specific to dampen oscillations? HELP... The 6L6 has been used since about 1935 with little oscillation troubles by thousands of amp makers who knew more than you about amp building. Its not going to get you anywhere to ask what is wrong with a 6L6, because the answer is that there isn't much wrong with a 6L6, and you should be asking many more questions about what you may have done wrong instead. You are not alone, and like you I once was exasperated and frustrated with my early attempts to build amps that then behaved like RF power oscilators. Only at the time, prior to 2000, when I went online, there was nobody to share the problem with online, and I had to damn well work out what i did wrong myself. If I had a $ for every time someone dialed into r.a.t with a problem about amplifiers that oscillate, I'd be rich! I suggest firstly that you give us a URL for the schematic of your amp, so that we may see EXACTLY WHAT SCHEMATIC YOU HAVE RIGHT NOW. Please check all your connections again and draw up the schematic on a large NEAT page in black ink on white and scan and post it somewhere, at your website/home page if you have one, or at a friend's, or at news groups alternate.binaries.pictures.radio-phono or alternate.binaries.schematics.electronic. But for us to empathise with your bothers you have to place a lot of effort into conveying a suitable amount of information on which we may make comment after some rational analysis. Do the scan so it is a black and white format at 400dpi, and convert it to a gif image to keep the file size below about 100kb, and so that it fills a screen nicely, and is clearly legible, ie, we can all see what the numbers and letters are. Once you become proficient with PRODUCING schematics and reading them, you will learn so much more it should dazzle you. There are a pile of schematics at my website, and you are free to use any you like athttp://www.turneraudio.com.au Some of the schematics were hand drawn on paper, then scanned, and some created in MS Paint, just sitting there for hours and hours with a mouse..... Thanks for the input. My comment about the 6L6 tubes was somewhat tongue-in-cheek which doesn't come across in print as well as it does in person. The schematic I used is adapted out of one of the many RCA Receiving Tube manuals (the specific one escapes me right now). I will try to figure out how to post a schematic as soon as possible. Best regards. I will be happy to host the schematic for you while this thread is being discussed, if it is of any help to you. Contact me by e-mail. This is an interesting topic, and relevant to many many tube amp projects. Regards -- Iain www.kolumbus.fi/iain.churches |
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#8
Posted to rec.audio.tubes
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Amp oscillation
"htrboy" wrote in message ups.com... On Feb 5, 1:31 am, Patrick Turner wrote: htrboy wrote: Hi all, I've been scratch building an amp using a 7199 preamp/phase invert and 6L6GC power tubes. It's a very simple amp - I'm just doing it for the learning process. When I finally got a signal through the circuit (woo hoo) it had a an oscillation (407kHz) at the bottom of every sine wave. I've been going crazy trying to troubleshoot - checking my lead dressing, trying to eliminate ground loops, blah blah - everything I've tried has made it worse to the point that now my signal is almost entirely saturated with the oscillation (407 kHz). Out of desperation I took out the 6L6's and put in a pair of 6F6's that I had laying around...PERFECT signal, no oscillation. What the hell is wrong with the 6L6 tubes? Do they need something specific to dampen oscillations? HELP... The 6L6 has been used since about 1935 with little oscillation troubles by thousands of amp makers who knew more than you about amp building. Its not going to get you anywhere to ask what is wrong with a 6L6, because the answer is that there isn't much wrong with a 6L6, and you should be asking many more questions about what you may have done wrong instead. You are not alone, and like you I once was exasperated and frustrated with my early attempts to build amps that then behaved like RF power oscilators. Only at the time, prior to 2000, when I went online, there was nobody to share the problem with online, and I had to damn well work out what i did wrong myself. If I had a $ for every time someone dialed into r.a.t with a problem about amplifiers that oscillate, I'd be rich! I suggest firstly that you give us a URL for the schematic of your amp, so that we may see EXACTLY WHAT SCHEMATIC YOU HAVE RIGHT NOW. Please check all your connections again and draw up the schematic on a large NEAT page in black ink on white and scan and post it somewhere, at your website/home page if you have one, or at a friend's, or at news groups alternate.binaries.pictures.radio-phono or alternate.binaries.schematics.electronic. But for us to empathise with your bothers you have to place a lot of effort into conveying a suitable amount of information on which we may make comment after some rational analysis. Do the scan so it is a black and white format at 400dpi, and convert it to a gif image to keep the file size below about 100kb, and so that it fills a screen nicely, and is clearly legible, ie, we can all see what the numbers and letters are. Once you become proficient with PRODUCING schematics and reading them, you will learn so much more it should dazzle you. There are a pile of schematics at my website, and you are free to use any you like athttp://www.turneraudio.com.au Some of the schematics were hand drawn on paper, then scanned, and some created in MS Paint, just sitting there for hours and hours with a mouse..... Patrick Turner. Hello Patrick, Thanks for the input. My comment about the 6L6 tubes was somewhat tongue-in-cheek which doesn't come across in print as well as it does in person. The schematic I used is adapted out of one of the many RCA Receiving Tube manuals (the specific one escapes me right now). I will try to figure out how to post a schematic as soon as possible. Best regards. With 6L6's, sounds like the early 30 watt circuit. Initally, this circuit specified 6L6 w/ a 5K pp output transformer; later, the design migrated to 7868's and 6.6K pp OPT. Is this the amp circuit? MarkS |
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#9
Posted to rec.audio.tubes
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Amp oscillation
On Feb 5, 4:04 pm, "MarkS" wrote:
"htrboy" wrote in message ups.com... On Feb 5, 1:31 am, Patrick Turner wrote: htrboy wrote: Hi all, I've been scratch building an amp using a 7199 preamp/phase invert and 6L6GC power tubes. It's a very simple amp - I'm just doing it for the learning process. When I finally got a signal through the circuit (woo hoo) it had a an oscillation (407kHz) at the bottom of every sine wave. I've been going crazy trying to troubleshoot - checking my lead dressing, trying to eliminate ground loops, blah blah - everything I've tried has made it worse to the point that now my signal is almost entirely saturated with the oscillation (407 kHz). Out of desperation I took out the 6L6's and put in a pair of 6F6's that I had laying around...PERFECT signal, no oscillation. What the hell is wrong with the 6L6 tubes? Do they need something specific to dampen oscillations? HELP... The 6L6 has been used since about 1935 with little oscillation troubles by thousands of amp makers who knew more than you about amp building. Its not going to get you anywhere to ask what is wrong with a 6L6, because the answer is that there isn't much wrong with a 6L6, and you should be asking many more questions about what you may have done wrong instead. You are not alone, and like you I once was exasperated and frustrated with my early attempts to build amps that then behaved like RF power oscilators. Only at the time, prior to 2000, when I went online, there was nobody to share the problem with online, and I had to damn well work out what i did wrong myself. If I had a $ for every time someone dialed into r.a.t with a problem about amplifiers that oscillate, I'd be rich! I suggest firstly that you give us a URL for the schematic of your amp, so that we may see EXACTLY WHAT SCHEMATIC YOU HAVE RIGHT NOW. Please check all your connections again and draw up the schematic on a large NEAT page in black ink on white and scan and post it somewhere, at your website/home page if you have one, or at a friend's, or at news groups alternate.binaries.pictures.radio-phono or alternate.binaries.schematics.electronic. But for us to empathise with your bothers you have to place a lot of effort into conveying a suitable amount of information on which we may make comment after some rational analysis. Do the scan so it is a black and white format at 400dpi, and convert it to a gif image to keep the file size below about 100kb, and so that it fills a screen nicely, and is clearly legible, ie, we can all see what the numbers and letters are. Once you become proficient with PRODUCING schematics and reading them, you will learn so much more it should dazzle you. There are a pile of schematics at my website, and you are free to use any you like athttp://www.turneraudio.com.au Some of the schematics were hand drawn on paper, then scanned, and some created in MS Paint, just sitting there for hours and hours with a mouse..... Patrick Turner. Hello Patrick, Thanks for the input. My comment about the 6L6 tubes was somewhat tongue-in-cheek which doesn't come across in print as well as it does in person. The schematic I used is adapted out of one of the many RCA Receiving Tube manuals (the specific one escapes me right now). I will try to figure out how to post a schematic as soon as possible. Best regards. With 6L6's, sounds like the early 30 watt circuit. Initally, this circuit specified 6L6 w/ a 5K pp output transformer; later, the design migrated to 7868's and 6.6K pp OPT. Is this the amp circuit? MarkS Yes - thats the one. It's out of the RC-20 manual. I will soon have the schematic posted (thanks to Iain) with the alterations I had to make in the power supply circuit. |
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#10
Posted to rec.audio.tubes
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Amp oscillation
snip for brevity
Hello Patrick, Thanks for the input. My comment about the 6L6 tubes was somewhat tongue-in-cheek which doesn't come across in print as well as it does in person. The schematic I used is adapted out of one of the many RCA Receiving Tube manuals (the specific one escapes me right now). I will try to figure out how to post a schematic as soon as possible. Best regards. With 6L6's, sounds like the early 30 watt circuit. Initally, this circuit specified 6L6 w/ a 5K pp output transformer; later, the design migrated to 7868's and 6.6K pp OPT. Is this the amp circuit? MarkS Yes - thats the one. It's out of the RC-20 manual. I will soon have the schematic posted (thanks to Iain) with the alterations I had to make in the power supply circuit. Wow, that's the first circuit I built! 30 years ago...*sigh*. I first saw it in an RC-19 manual. In that version, they had PS electrolytic C10 polarity backwards! Did that transfer over to RC-20? In any case, I ended up building the later version with the 7868's. I did not have any high frequency oscillation problems but low frequency stability took awhile to figure out. What type output transformer are you using? This circuit feeds the pentode voltage amplifier screen grid from the phase splitter cathode through a resistor for a local feedback arrangement, that is somewhat unusual. MarkS |
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#11
Posted to rec.audio.tubes
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Amp oscillation
On Feb 5, 5:30 pm, "Mark S" wrote:
snip for brevity Hello Patrick, Thanks for the input. My comment about the 6L6 tubes was somewhat tongue-in-cheek which doesn't come across in print as well as it does in person. The schematic I used is adapted out of one of the many RCA Receiving Tube manuals (the specific one escapes me right now). I will try to figure out how to post a schematic as soon as possible. Best regards. With 6L6's, sounds like the early 30 watt circuit. Initally, this circuit specified 6L6 w/ a 5K pp output transformer; later, the design migrated to 7868's and 6.6K pp OPT. Is this the amp circuit? MarkS Yes - thats the one. It's out of the RC-20 manual. I will soon have the schematic posted (thanks to Iain) with the alterations I had to make in the power supply circuit. Wow, that's the first circuit I built! 30 years ago...*sigh*. I first saw it in an RC-19 manual. In that version, they had PS electrolytic C10 polarity backwards! Did that transfer over to RC-20? In any case, I ended up building the later version with the 7868's. I did not have any high frequency oscillation problems but low frequency stability took awhile to figure out. What type output transformer are you using? This circuit feeds the pentode voltage amplifier screen grid from the phase splitter cathode through a resistor for a local feedback arrangement, that is somewhat unusual. MarkS Hi Mark, C10's polarity is correct in this schematic...I actually omitted that cap, thinking I didn't need it (I'm using solid-state rectification), but maybe I need it after all? It looks like it bypasses R20, the large bias resistor. My output transformer is from an old Fisher and is supposedly 5k input impedance. I bought it through Ebay so I don't know for sure what its out of. It looks like most of Fisher's recievers use 7591's which are 6.6k impedance (like the 7868's). It'll be much easier to ask questions once I get the schematic up, but I have a disturbing voltage reading on the screen grid of one of the 6L6 tubes. It's reading 75 volts compared to 278 volts for its mate. Have I destroyed the tube? --Scott |
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#12
Posted to rec.audio.tubes
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Amp oscillation
htrboy wrote: On Feb 5, 1:31 am, Patrick Turner wrote: htrboy wrote: Hi all, I've been scratch building an amp using a 7199 preamp/phase invert and 6L6GC power tubes. It's a very simple amp - I'm just doing it for the learning process. When I finally got a signal through the circuit (woo hoo) it had a an oscillation (407kHz) at the bottom of every sine wave. I've been going crazy trying to troubleshoot - checking my lead dressing, trying to eliminate ground loops, blah blah - everything I've tried has made it worse to the point that now my signal is almost entirely saturated with the oscillation (407 kHz). Out of desperation I took out the 6L6's and put in a pair of 6F6's that I had laying around...PERFECT signal, no oscillation. What the hell is wrong with the 6L6 tubes? Do they need something specific to dampen oscillations? HELP... The 6L6 has been used since about 1935 with little oscillation troubles by thousands of amp makers who knew more than you about amp building. Its not going to get you anywhere to ask what is wrong with a 6L6, because the answer is that there isn't much wrong with a 6L6, and you should be asking many more questions about what you may have done wrong instead. You are not alone, and like you I once was exasperated and frustrated with my early attempts to build amps that then behaved like RF power oscilators. Only at the time, prior to 2000, when I went online, there was nobody to share the problem with online, and I had to damn well work out what i did wrong myself. If I had a $ for every time someone dialed into r.a.t with a problem about amplifiers that oscillate, I'd be rich! I suggest firstly that you give us a URL for the schematic of your amp, so that we may see EXACTLY WHAT SCHEMATIC YOU HAVE RIGHT NOW. Please check all your connections again and draw up the schematic on a large NEAT page in black ink on white and scan and post it somewhere, at your website/home page if you have one, or at a friend's, or at news groups alternate.binaries.pictures.radio-phono or alternate.binaries.schematics.electronic. But for us to empathise with your bothers you have to place a lot of effort into conveying a suitable amount of information on which we may make comment after some rational analysis. Do the scan so it is a black and white format at 400dpi, and convert it to a gif image to keep the file size below about 100kb, and so that it fills a screen nicely, and is clearly legible, ie, we can all see what the numbers and letters are. Once you become proficient with PRODUCING schematics and reading them, you will learn so much more it should dazzle you. There are a pile of schematics at my website, and you are free to use any you like athttp://www.turneraudio.com.au Some of the schematics were hand drawn on paper, then scanned, and some created in MS Paint, just sitting there for hours and hours with a mouse..... Patrick Turner. Hello Patrick, Thanks for the input. My comment about the 6L6 tubes was somewhat tongue-in-cheek which doesn't come across in print as well as it does in person. The schematic I used is adapted out of one of the many RCA Receiving Tube manuals (the specific one escapes me right now). I will try to figure out how to post a schematic as soon as possible. Best regards. I have a few RCA books with schematics within. What is the exact number/description of the schematic? Patrick Turner. |
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#13
Posted to rec.audio.tubes
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Amp oscillation
"htrboy" wrote in message oups.com... On Feb 5, 5:30 pm, "Mark S" wrote: snip for brevity Hello Patrick, Thanks for the input. My comment about the 6L6 tubes was somewhat tongue-in-cheek which doesn't come across in print as well as it does in person. The schematic I used is adapted out of one of the many RCA Receiving Tube manuals (the specific one escapes me right now). I will try to figure out how to post a schematic as soon as possible. Best regards. With 6L6's, sounds like the early 30 watt circuit. Initally, this circuit specified 6L6 w/ a 5K pp output transformer; later, the design migrated to 7868's and 6.6K pp OPT. Is this the amp circuit? MarkS Yes - thats the one. It's out of the RC-20 manual. I will soon have the schematic posted (thanks to Iain) with the alterations I had to make in the power supply circuit. Wow, that's the first circuit I built! 30 years ago...*sigh*. I first saw it in an RC-19 manual. In that version, they had PS electrolytic C10 polarity backwards! Did that transfer over to RC-20? In any case, I ended up building the later version with the 7868's. I did not have any high frequency oscillation problems but low frequency stability took awhile to figure out. What type output transformer are you using? This circuit feeds the pentode voltage amplifier screen grid from the phase splitter cathode through a resistor for a local feedback arrangement, that is somewhat unusual. MarkS Hi Mark, C10's polarity is correct in this schematic...I actually omitted that cap, thinking I didn't need it (I'm using solid-state rectification), but maybe I need it after all? It looks like it bypasses R20, the large bias resistor. My output transformer is from an old Fisher and is supposedly 5k input impedance. I bought it through Ebay so I don't know for sure what its out of. It looks like most of Fisher's recievers use 7591's which are 6.6k impedance (like the 7868's). It'll be much easier to ask questions once I get the schematic up, but I have a disturbing voltage reading on the screen grid of one of the 6L6 tubes. It's reading 75 volts compared to 278 volts for its mate. Have I destroyed the tube? --Scott Scott, Check the screen dropping resistors R16 & R17. As I remember, 1/2w resistors may not hold up too well and you may have fried one of them. MarkS |
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#14
Posted to rec.audio.tubes
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Amp oscillation
"Patrick Turner" wrote in message ... I have a few RCA books with schematics within. What is the exact number/description of the schematic? Patrick Turner. The OP has promised to send me a scan of the schematic which I will host and post a link as soon as it can be seen. I think it would be to the benefit of everyone if we could all see the schematic as it is being discussed. This is a very interesting thread from which much can be learned. Iain |
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#15
Posted to rec.audio.tubes
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Amp oscillation
On Feb 6, 8:07 am, "Iain Churches" wrote:
"Patrick Turner" wrote in message ... I have a few RCA books with schematics within. What is the exact number/description of the schematic? Patrick Turner. The OP has promised to send me a scan of the schematic which I will host and post a link as soon as it can be seen. I think it would be to the benefit of everyone if we could all see the schematic as it is being discussed. This is a very interesting thread from which much can be learned. Iain My apologies for the delay - the scanner I use is at my place of work. Yesterday I got caught up in business as usual and couldn't get it done...This afternoon for sure!! Thanks for all the help and attention so far... Scott |
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#16
Posted to rec.audio.tubes
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Amp oscillation
In article k.net,
"Mark S" wrote: Wow, that's the first circuit I built! 30 years ago...*sigh*. I first saw it in an RC-19 manual. In that version, they had PS electrolytic C10 polarity backwards! Did that transfer over to RC-20? Yes, the polarity of C10 is backwards in my copy of the RC-20 manual. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
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#17
Posted to rec.audio.tubes
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Amp oscillation
On Feb 6, 8:07 am, "Iain Churches" wrote:
"Patrick Turner" wrote in message ... I have a few RCA books with schematics within. What is the exact number/description of the schematic? Patrick Turner. The OP has promised to send me a scan of the schematic which I will host and post a link as soon as it can be seen. I think it would be to the benefit of everyone if we could all see the schematic as it is being discussed. This is a very interesting thread from which much can be learned. Iain I emailed it this afternoon - please let me know if the quality is acceptable, I can probably darken it up if needed. Thanks Scott |
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#18
Posted to rec.audio.tubes
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Amp oscillation
On Feb 6, 4:25 am, "MarkS" wrote:
"htrboy" wrote in message oups.com... On Feb 5, 5:30 pm, "Mark S" wrote: snip for brevity Hello Patrick, Thanks for the input. My comment about the 6L6 tubes was somewhat tongue-in-cheek which doesn't come across in print as well as it does in person. The schematic I used is adapted out of one of the many RCA Receiving Tube manuals (the specific one escapes me right now). I will try to figure out how to post a schematic as soon as possible. Best regards. With 6L6's, sounds like the early 30 watt circuit. Initally, this circuit specified 6L6 w/ a 5K pp output transformer; later, the design migrated to 7868's and 6.6K pp OPT. Is this the amp circuit? MarkS Yes - thats the one. It's out of the RC-20 manual. I will soon have the schematic posted (thanks to Iain) with the alterations I had to make in the power supply circuit. Wow, that's the first circuit I built! 30 years ago...*sigh*. I first saw it in an RC-19 manual. In that version, they had PS electrolytic C10 polarity backwards! Did that transfer over to RC-20? In any case, I ended up building the later version with the 7868's. I did not have any high frequency oscillation problems but low frequency stability took awhile to figure out. What type output transformer are you using? This circuit feeds the pentode voltage amplifier screen grid from the phase splitter cathode through a resistor for a local feedback arrangement, that is somewhat unusual. MarkS Hi Mark, C10's polarity is correct in this schematic...I actually omitted that cap, thinking I didn't need it (I'm using solid-state rectification), but maybe I need it after all? It looks like it bypasses R20, the large bias resistor. My output transformer is from an old Fisher and is supposedly 5k input impedance. I bought it through Ebay so I don't know for sure what its out of. It looks like most of Fisher's recievers use 7591's which are 6.6k impedance (like the 7868's). It'll be much easier to ask questions once I get the schematic up, but I have a disturbing voltage reading on the screen grid of one of the 6L6 tubes. It's reading 75 volts compared to 278 volts for its mate. Have I destroyed the tube? --Scott Scott, Check the screen dropping resistors R16 & R17. As I remember, 1/2w resistors may not hold up too well and you may have fried one of them. MarkS You nailed this one - one was open and the other one shorted. Now where was that 24 hour electronics shop...  |
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#19
Posted to rec.audio.tubes
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Amp oscillation
"htrboy" wrote in message oups.com... On Feb 6, 8:07 am, "Iain Churches" wrote: "Patrick Turner" wrote in message ... I have a few RCA books with schematics within. What is the exact number/description of the schematic? Patrick Turner. The OP has promised to send me a scan of the schematic which I will host and post a link as soon as it can be seen. I think it would be to the benefit of everyone if we could all see the schematic as it is being discussed. This is a very interesting thread from which much can be learned. Iain I emailed it this afternoon - please let me know if the quality is acceptable, I can probably darken it up if needed. Thanks Scott Not too good:-) I will post it, and wait for an replacement from you. Iain |
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#20
Posted to rec.audio.tubes
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Amp oscillation
"htrboy" wrote in message oups.com... On Feb 6, 8:07 am, "Iain Churches" wrote: "Patrick Turner" wrote in message ... I have a few RCA books with schematics within. What is the exact number/description of the schematic? Patrick Turner. The OP has promised to send me a scan of the schematic which I will host and post a link as soon as it can be seen. I think it would be to the benefit of everyone if we could all see the schematic as it is being discussed. This is a very interesting thread from which much can be learned. Iain I emailed it this afternoon - please let me know if the quality is acceptable, I can probably darken it up if needed. The contrast is not ideal:-) The schematic is at: http://www.kolumbus.fi/iain.churches/Pics/Schematics/Scott'sAmp Iain Thanks Scott |
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#21
Posted to rec.audio.tubes
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Amp oscillation
Iain Churches wrote: "htrboy" wrote in message oups.com... On Feb 6, 8:07 am, "Iain Churches" wrote: "Patrick Turner" wrote in message ... I have a few RCA books with schematics within. What is the exact number/description of the schematic? Patrick Turner. The OP has promised to send me a scan of the schematic which I will host and post a link as soon as it can be seen. I think it would be to the benefit of everyone if we could all see the schematic as it is being discussed. This is a very interesting thread from which much can be learned. Iain I emailed it this afternoon - please let me know if the quality is acceptable, I can probably darken it up if needed. The contrast is not ideal:-) The schematic is at: http://www.kolumbus.fi/iain.churches/Pics/Schematics/Scott'sAmp Iain There are a few things wrong with the schematic which has low contrast but is otherwise quite readable and understandable. Next time anyone submits a schematic like this they should use *black ink pen* on white pape. The best way is to draw it up lightly in pencil and triple check for mistakes before over drawing in ink, and then erazing the pencil away with a rubber. The amount of paper space on the schematic is quite large enough to have the C&R numbers *plus* ohm/uF values so that reading it is easier, and size could be trimmed to tit *one screenful*. Even though you have a triode-pentode tube for the input-driver-phase inverter, consider drawing the pentode away from the triode which makes a mistake less likely when tracing the circuit and makes it all easier to understand. My RCA Receiving Tube Manual copywrite 1966 has the same schematic on page 549 but with 7868 output tubes and 7199 tri-pent input tube. This shematic has a couple of things which are queer. The screen of V1 gets its dc feed from the cathode of V2 triode via 180k, and so if the dc rises on V2k, Va lowers on V1, and rises more on V2k so it looks like dc positive FB to me, and could cause LF oscillation so I would have instead about 1M to Vi screen from the anode B+ supply to this tube. Then R9, 1k, can be omitted since for equal +/- phase amplitudes the anode RL and cathode RL should be equal. R16&17 are screen stoppers and shown 56 ohms, and these won't stop much. Increase these R to 470 ohms, 1 watt at least. Stoppers to output grids, R14,15, should be 2k7 to 4k7, not 1k as shown, which won't stop much, because Cin to the pentode or beam tetrode output tubes is very low. The 820 ohm biasing resistance to V1 pentode may work better with 220uF across it which would allow R18, 270 ohm GNFB resistance to be increased to about 470 ohms and C6 to be lower, see below. Vitally important and difficult to judge is the value for C6 HF phase tweaking FB cap across GNB resistor of 270 ohms; its shown as 0.012uF, but might better be 0.0022uF. C2 HF gain stepping cap in series with R10, 15k, is shown as 22pF, and is also difficult to verify, but up to about 100pF may be right. The best values for R10 and C2 must be ascertained experimentally with a 50k pot mounted on variable radio tuning gang, and when stablity with 0.22 uF across the output is achieved along with maximal bandwidth into a resistance load is achieved then the varied R and C values are measured and fixed values soldered in. That's the part that 120% of novices always get wrong, unless they learn about the functions of such R&C compensation networks, and why they are necessary, which is better explained elswhere, and in some detail to confuse the novice even more at http://www.turneraudio.com.au The reason why the values are shown as they are by RCA is to provide a **guide only** to good practices. Exact values *WILL* have to be different according to the characteristics of the OPT chosen in terms of its primary inductance, leakage inductance, and shunt capacitances. RCA were not in any position in 1965 to provide enough data on what YOU may do some 40 years later. Hmm, rockets to the moon occured 3 years after 1966. Methinks the then old dudes who wrote the RCA tube book are all now dead, and cannot comment on *THEIR WONDERFUL CONTRIBUTIONS* to tube craft. The RCA schematic shown was a forerunner of the Dynaco ST70 schematic which also used a triode + pentode input tube but which supercharged the circuit by bootsrapping of the pentode anode load to the phase inverter cathode circuit thus providing a much higher load value to the pentode anode and thus much higher pentode gain. It was a not so obnoxious use of internal positive FB. The ST70 had lower gain UL output circuitry, so with bootstrapping the circuit could be made to be sensitive despite the GNFB and lack of gain in the output stage. The RCA circuit has more gain in the output circuit so there seems little need to increase the pentode gain by bootstrapping, and so the RCA circuit is simpler, and easier to get right. Having C2 too large is a sure recipe for having oscillations at 400kHz or thereabouts, so if anything it should only be large enough to prevent overshoot on square waves with a pure R load. The C2 value across R 10 should only be large enough to prevent oscillations on any value of pure C across the output without any R present. Its impossible to get a a perfect square wave with such "compensation" capacitors connected, but a perfect square wave isn't what you want. You want full power bandwidth from 20Hz to 20kHz, that is enough, and into a solely resistance only load. There will be some overshoot on square waves when you have the circuit working OK when a pure C load of typically 0.22 uF is used but that is just a natural function of the circuit you effectively have created, because every amplifier is a *BAND PASS FILTER* whether you like it or not, and dc can't pass, and nor can much 1Mhz. the roll off at HF may be slightly more than 6dB/octave above say 50kHz. Filters with roll off slopes outside the -3dB point, or "pole" may have a slope exceeding first order, or 6dB/octave, ie, perhaps 9 or 12dB/octave, and such filters produce ringing or overshoot on square waves. The ringing frequency should be well above the audio band, and 60kHz is OK. Maximal overshoot should be 6dB more than the level part of the square wave when the wave has "settled" when using a 5 kHz square wave. What one is always looking for is widest bandwidth between -3dB LF and HF poles, and at the highest output voltage equal to 1 bD below the 1 kHz sine wave clipping level. Then you want minimal freedom from square wave overshoot, with any C value betaeen 0.05uF and 5uF. Also wanted is freedom from peaks in the sine wave response outside the pass band of 20Hz to 20kHz. Such response peaks should not exceed 1dB at LF below 20Hz, nore exceed 6dB at any F above 20kHz even with any C value used as the load. Using combined R and C loads in parallel will make the amp look a lot better than it may be, but if capacitances of 0.22uF can be placed across the output without making the amp oscillate at say 150kH or any HF, then it will be stable into any other combination of R&C in parallel, or in series. A test load for ESL driving is usually about 1.5ohms in series with say 16ohms & 2uF in parallel. So even at 100kHz, the load has reverted to being mainly resistive with 1.5ohms, because the 2uF Z has become 0.8ohms. You need to have basic knowledge about R&C and filter behaviour and about relative impedance effects in filters to more fully understand what you are doing if you are to easily furnish your sound lounge with a DIYNSPA, a do it yourself non smoke producing amp. Some ESL C value is a lot greater, and may load the amp more adversly than the above which is a a simplistic simulated load for Quad ESL57. Martin Logan are not so easy on an amp. Patrick Turner. Scott |
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#22
Posted to rec.audio.tubes
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Amp oscillation
"Iain Churches" wrote in message i... "htrboy" wrote in message oups.com... On Feb 6, 8:07 am, "Iain Churches" wrote: "Patrick Turner" wrote in message ... I have a few RCA books with schematics within. What is the exact number/description of the schematic? Patrick Turner. The OP has promised to send me a scan of the schematic which I will host and post a link as soon as it can be seen. I think it would be to the benefit of everyone if we could all see the schematic as it is being discussed. This is a very interesting thread from which much can be learned. Iain I emailed it this afternoon - please let me know if the quality is acceptable, I can probably darken it up if needed. The contrast is not ideal:-) The schematic is at: http://www.kolumbus.fi/iain.churches/Pics/Schematics/Scott'sAmp Iain Thanks Scott Scott, Looks like the 220 ohm 10w resistor is tied to the grids not tbe cathodes of the 6L6's, is that a miss print? If not, that needs to be corrected before doing anything else. Otherwise, you're running with 0 bias. Mark |
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#23
Posted to rec.audio.tubes
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Amp oscillation
snipped
Patrick, thank you for all the great tips and feedback (so to speak). This is the first tube amp I have attempted to build, and I expected to make mistakes. I am an electronics technician by trade and training and in this day and age, that means I deal mostly with 1's and 0's. While I have a basic knowledge of this stuff, I needed / wanted to build this thing to try and work through the problems and better understand what I learned in school about the fundamentals (that, and at heart I'm an analog nut). So, I'll try to take what I translate to be shots at my ignorance to heart and *really* try to work through the details of your post. I REALLY DO appreciate the knowledge being shared! best regards, Scott |
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#24
Posted to rec.audio.tubes
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Amp oscillation
On Feb 7, 3:24 am, "MarkS" wrote:
"Iain Churches" wrote in message i... "htrboy" wrote in message roups.com... On Feb 6, 8:07 am, "Iain Churches" wrote: "Patrick Turner" wrote in message ... I have a few RCA books with schematics within. What is the exact number/description of the schematic? Patrick Turner. The OP has promised to send me a scan of the schematic which I will host and post a link as soon as it can be seen. I think it would be to the benefit of everyone if we could all see the schematic as it is being discussed. This is a very interesting thread from which much can be learned. Iain I emailed it this afternoon - please let me know if the quality is acceptable, I can probably darken it up if needed. The contrast is not ideal:-) The schematic is at: http://www.kolumbus.fi/iain.churches/Pics/Schematics/Scott'sAmp Iain Thanks Scott Scott, Looks like the 220 ohm 10w resistor is tied to the grids not tbe cathodes of the 6L6's, is that a miss print? If not, that needs to be corrected before doing anything else. Otherwise, you're running with 0 bias. Mark Hi Mark, That part of the circuit is a point of confusion for me since the original circuit was tube rectified. I am going to construct a negative voltage circuit using one of the PT taps and run it to the grids. The design isn't supposed to be cathode biased. Best regards, Scott |
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#25
Posted to rec.audio.tubes
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Amp oscillation
This shematic has a couple of things which are queer. The best values for R10 and C2 must be ascertained experimentally with a 50k pot mounted on variable radio tuning gang, and when stablity with 0.22 uF across the output is achieved along with maximal bandwidth into a resistance load is achieved then the varied R and C values are measured and fixed values soldered in. Could you elaborate on how to do this? I have a large tuning cap and 50K pot that I can use... That's the part that 120% of novices always get wrong, unless they learn about the functions of such R&C compensation networks, and why they are necessary, which is better explained elswhere, and in some detail to confuse the novice even more athttp://www.turneraudio.com.au I'll be digesting the info on your site for a while... The RCA schematic shown was a forerunner of the Dynaco ST70 schematic which also used a triode + pentode input tube but which supercharged the circuit by bootsrapping of the pentode anode load to the phase inverter cathode circuit thus providing a much higher load value to the pentode anode and thus much higher pentode gain. It was a not so obnoxious use of internal positive FB. The ST70 had lower gain UL output circuitry, so with bootstrapping the circuit could be made to be sensitive despite the GNFB and lack of gain in the output stage. Could you explain what you mean by Bootstrapping - my definition has to do with starting up a PC Please see above for my questions inserted ^ Best regards, Scott |
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#26
Posted to rec.audio.tubes
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Amp oscillation
In article ,
Patrick Turner wrote: This shematic has a couple of things which are queer. The screen of V1 gets its dc feed from the cathode of V2 triode via 180k, and so if the dc rises on V2k, Va lowers on V1, and rises more on V2k so it looks like dc positive FB to me, and could cause LF oscillation so I would have instead about 1M to Vi screen from the anode B+ supply to this tube. This feedback is actually negative, when Va lowers on V1 then V2k also lowers, lowering the voltage on the screen grid which raises Va on V1 off setting the original lowering of Va on V1, this is negative feedback. Did you notice with respect to the C10 polarity issue that there is no negative bias at all on the grids of the 6L6s in this modified schematic? The circuit should either be restored to its original backbias configuration, or C10 and the 220 Ohm resistor should be placed in the cathode circuit of the 6L6s. Regards, John Byrns Then R9, 1k, can be omitted since for equal +/- phase amplitudes the anode RL and cathode RL should be equal. R16&17 are screen stoppers and shown 56 ohms, and these won't stop much. Increase these R to 470 ohms, 1 watt at least. Stoppers to output grids, R14,15, should be 2k7 to 4k7, not 1k as shown, which won't stop much, because Cin to the pentode or beam tetrode output tubes is very low. The 820 ohm biasing resistance to V1 pentode may work better with 220uF across it which would allow R18, 270 ohm GNFB resistance to be increased to about 470 ohms and C6 to be lower, see below. Vitally important and difficult to judge is the value for C6 HF phase tweaking FB cap across GNB resistor of 270 ohms; its shown as 0.012uF, but might better be 0.0022uF. C2 HF gain stepping cap in series with R10, 15k, is shown as 22pF, and is also difficult to verify, but up to about 100pF may be right. The best values for R10 and C2 must be ascertained experimentally with a 50k pot mounted on variable radio tuning gang, and when stablity with 0.22 uF across the output is achieved along with maximal bandwidth into a resistance load is achieved then the varied R and C values are measured and fixed values soldered in. That's the part that 120% of novices always get wrong, unless they learn about the functions of such R&C compensation networks, and why they are necessary, which is better explained elswhere, and in some detail to confuse the novice even more at http://www.turneraudio.com.au The reason why the values are shown as they are by RCA is to provide a **guide only** to good practices. Exact values *WILL* have to be different according to the characteristics of the OPT chosen in terms of its primary inductance, leakage inductance, and shunt capacitances. RCA were not in any position in 1965 to provide enough data on what YOU may do some 40 years later. Hmm, rockets to the moon occured 3 years after 1966. Methinks the then old dudes who wrote the RCA tube book are all now dead, and cannot comment on *THEIR WONDERFUL CONTRIBUTIONS* to tube craft. The RCA schematic shown was a forerunner of the Dynaco ST70 schematic which also used a triode + pentode input tube but which supercharged the circuit by bootsrapping of the pentode anode load to the phase inverter cathode circuit thus providing a much higher load value to the pentode anode and thus much higher pentode gain. It was a not so obnoxious use of internal positive FB. The ST70 had lower gain UL output circuitry, so with bootstrapping the circuit could be made to be sensitive despite the GNFB and lack of gain in the output stage. The RCA circuit has more gain in the output circuit so there seems little need to increase the pentode gain by bootstrapping, and so the RCA circuit is simpler, and easier to get right. Having C2 too large is a sure recipe for having oscillations at 400kHz or thereabouts, so if anything it should only be large enough to prevent overshoot on square waves with a pure R load. The C2 value across R 10 should only be large enough to prevent oscillations on any value of pure C across the output without any R present. Its impossible to get a a perfect square wave with such "compensation" capacitors connected, but a perfect square wave isn't what you want. You want full power bandwidth from 20Hz to 20kHz, that is enough, and into a solely resistance only load. There will be some overshoot on square waves when you have the circuit working OK when a pure C load of typically 0.22 uF is used but that is just a natural function of the circuit you effectively have created, because every amplifier is a *BAND PASS FILTER* whether you like it or not, and dc can't pass, and nor can much 1Mhz. the roll off at HF may be slightly more than 6dB/octave above say 50kHz. Filters with roll off slopes outside the -3dB point, or "pole" may have a slope exceeding first order, or 6dB/octave, ie, perhaps 9 or 12dB/octave, and such filters produce ringing or overshoot on square waves. The ringing frequency should be well above the audio band, and 60kHz is OK. Maximal overshoot should be 6dB more than the level part of the square wave when the wave has "settled" when using a 5 kHz square wave. What one is always looking for is widest bandwidth between -3dB LF and HF poles, and at the highest output voltage equal to 1 bD below the 1 kHz sine wave clipping level. Then you want minimal freedom from square wave overshoot, with any C value betaeen 0.05uF and 5uF. Also wanted is freedom from peaks in the sine wave response outside the pass band of 20Hz to 20kHz. Such response peaks should not exceed 1dB at LF below 20Hz, nore exceed 6dB at any F above 20kHz even with any C value used as the load. Using combined R and C loads in parallel will make the amp look a lot better than it may be, but if capacitances of 0.22uF can be placed across the output without making the amp oscillate at say 150kH or any HF, then it will be stable into any other combination of R&C in parallel, or in series. A test load for ESL driving is usually about 1.5ohms in series with say 16ohms & 2uF in parallel. So even at 100kHz, the load has reverted to being mainly resistive with 1.5ohms, because the 2uF Z has become 0.8ohms. You need to have basic knowledge about R&C and filter behaviour and about relative impedance effects in filters to more fully understand what you are doing if you are to easily furnish your sound lounge with a DIYNSPA, a do it yourself non smoke producing amp. Some ESL C value is a lot greater, and may load the amp more adversly than the above which is a a simplistic simulated load for Quad ESL57. Martin Logan are not so easy on an amp. Patrick Turner. Scott -- Surf my web pages at, http://fmamradios.com/ |
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#27
Posted to rec.audio.tubes
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Amp oscillation
In article om,
"htrboy" wrote: On Feb 7, 3:24 am, "MarkS" wrote: Scott, Looks like the 220 ohm 10w resistor is tied to the grids not tbe cathodes of the 6L6's, is that a miss print? If not, that needs to be corrected before doing anything else. Otherwise, you're running with 0 bias. Mark Hi Mark, That part of the circuit is a point of confusion for me since the original circuit was tube rectified. I am going to construct a negative voltage circuit using one of the PT taps and run it to the grids. The design isn't supposed to be cathode biased. The original RCA design (20-13) used what is called "backbias". In a simple amplifier like this where almost all of the current drain comes from the output stage this is essentially equivalent to cathode bias, at least as far as the DC conditions go. I would recommend restoring R20 and C10 to their original positions in the circuit, note the the negative lead of C10 should connect to the power transformer center tap which connects to the 6L6 grid resistors. The use of the backbias circuit has nothing to do with the use of a rectifier tube, it will work just fine with your solid state rectifiers. The back bias circuit will be more forgiving to get operating initially, if you want to go to fixed bias do that later after you get the amplifier up and running as originally designed. It will be better to initially concentrate on issues like getting the NFB properly stabilized before tackling changes to the design. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ |
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#28
Posted to rec.audio.tubes
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Amp oscillation
"Patrick Turner" wrote in message ... There are a few things wrong with the schematic which has low contrast but is otherwise quite readable and understandable. Next time anyone submits a schematic like this they should use *black ink pen* on white pape. The best way is to draw it up lightly in pencil and triple check for mistakes before over drawing in ink, and then erazing the pencil away with a rubber. Scott has now sent me a better scan of the schematic which is at: http://www.kolumbus.fi/iain.churches/Pics/Schematics/Scott'sAmp2.gif Iain |
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#29
Posted to rec.audio.tubes
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Amp oscillation
"Htrboy scribbled;
"htrboy" wrote in message ups.com... snip snip snip This shematic has a couple of things which are queer Best regards, Scott Scott, why don't you stuff the 7199 tube and build a Williamson driver with octal 6SN7 or minature 6CG7 or 12AU7 tubes ????...............GC |
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#30
Posted to rec.audio.tubes
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Amp oscillation
"John Byrns" wrote in message ... In article , Patrick Turner wrote: This shematic has a couple of things which are queer. The screen of V1 gets its dc feed from the cathode of V2 triode via 180k, and so if the dc rises on V2k, Va lowers on V1, and rises more on V2k so it looks like dc positive FB to me, and could cause LF oscillation so I would have instead about 1M to Vi screen from the anode B+ supply to this tube. This feedback is actually negative, when Va lowers on V1 then V2k also lowers, lowering the voltage on the screen grid which raises Va on V1 off setting the original lowering of Va on V1, this is negative feedback. Did you notice with respect to the C10 polarity issue that there is no negative bias at all on the grids of the 6L6s in this modified schematic? The circuit should either be restored to its original backbias configuration, or C10 and the 220 Ohm resistor should be placed in the cathode circuit of the 6L6s. Regards, John Byrns Scott, Returning the circuit to its original form as John suggests is a good idea and starting point if you will. In fact, RCA refering to the circuit as "fixed bias" is a bit of a stretch. Mark Then R9, 1k, can be omitted since for equal +/- phase amplitudes the anode RL and cathode RL should be equal. R16&17 are screen stoppers and shown 56 ohms, and these won't stop much. Increase these R to 470 ohms, 1 watt at least. Stoppers to output grids, R14,15, should be 2k7 to 4k7, not 1k as shown, which won't stop much, because Cin to the pentode or beam tetrode output tubes is very low. The 820 ohm biasing resistance to V1 pentode may work better with 220uF across it which would allow R18, 270 ohm GNFB resistance to be increased to about 470 ohms and C6 to be lower, see below. Vitally important and difficult to judge is the value for C6 HF phase tweaking FB cap across GNB resistor of 270 ohms; its shown as 0.012uF, but might better be 0.0022uF. C2 HF gain stepping cap in series with R10, 15k, is shown as 22pF, and is also difficult to verify, but up to about 100pF may be right. The best values for R10 and C2 must be ascertained experimentally with a 50k pot mounted on variable radio tuning gang, and when stablity with 0.22 uF across the output is achieved along with maximal bandwidth into a resistance load is achieved then the varied R and C values are measured and fixed values soldered in. That's the part that 120% of novices always get wrong, unless they learn about the functions of such R&C compensation networks, and why they are necessary, which is better explained elswhere, and in some detail to confuse the novice even more at http://www.turneraudio.com.au The reason why the values are shown as they are by RCA is to provide a **guide only** to good practices. Exact values *WILL* have to be different according to the characteristics of the OPT chosen in terms of its primary inductance, leakage inductance, and shunt capacitances. RCA were not in any position in 1965 to provide enough data on what YOU may do some 40 years later. Hmm, rockets to the moon occured 3 years after 1966. Methinks the then old dudes who wrote the RCA tube book are all now dead, and cannot comment on *THEIR WONDERFUL CONTRIBUTIONS* to tube craft. The RCA schematic shown was a forerunner of the Dynaco ST70 schematic which also used a triode + pentode input tube but which supercharged the circuit by bootsrapping of the pentode anode load to the phase inverter cathode circuit thus providing a much higher load value to the pentode anode and thus much higher pentode gain. It was a not so obnoxious use of internal positive FB. The ST70 had lower gain UL output circuitry, so with bootstrapping the circuit could be made to be sensitive despite the GNFB and lack of gain in the output stage. The RCA circuit has more gain in the output circuit so there seems little need to increase the pentode gain by bootstrapping, and so the RCA circuit is simpler, and easier to get right. Having C2 too large is a sure recipe for having oscillations at 400kHz or thereabouts, so if anything it should only be large enough to prevent overshoot on square waves with a pure R load. The C2 value across R 10 should only be large enough to prevent oscillations on any value of pure C across the output without any R present. Its impossible to get a a perfect square wave with such "compensation" capacitors connected, but a perfect square wave isn't what you want. You want full power bandwidth from 20Hz to 20kHz, that is enough, and into a solely resistance only load. There will be some overshoot on square waves when you have the circuit working OK when a pure C load of typically 0.22 uF is used but that is just a natural function of the circuit you effectively have created, because every amplifier is a *BAND PASS FILTER* whether you like it or not, and dc can't pass, and nor can much 1Mhz. the roll off at HF may be slightly more than 6dB/octave above say 50kHz. Filters with roll off slopes outside the -3dB point, or "pole" may have a slope exceeding first order, or 6dB/octave, ie, perhaps 9 or 12dB/octave, and such filters produce ringing or overshoot on square waves. The ringing frequency should be well above the audio band, and 60kHz is OK. Maximal overshoot should be 6dB more than the level part of the square wave when the wave has "settled" when using a 5 kHz square wave. What one is always looking for is widest bandwidth between -3dB LF and HF poles, and at the highest output voltage equal to 1 bD below the 1 kHz sine wave clipping level. Then you want minimal freedom from square wave overshoot, with any C value betaeen 0.05uF and 5uF. Also wanted is freedom from peaks in the sine wave response outside the pass band of 20Hz to 20kHz. Such response peaks should not exceed 1dB at LF below 20Hz, nore exceed 6dB at any F above 20kHz even with any C value used as the load. Using combined R and C loads in parallel will make the amp look a lot better than it may be, but if capacitances of 0.22uF can be placed across the output without making the amp oscillate at say 150kH or any HF, then it will be stable into any other combination of R&C in parallel, or in series. A test load for ESL driving is usually about 1.5ohms in series with say 16ohms & 2uF in parallel. So even at 100kHz, the load has reverted to being mainly resistive with 1.5ohms, because the 2uF Z has become 0.8ohms. You need to have basic knowledge about R&C and filter behaviour and about relative impedance effects in filters to more fully understand what you are doing if you are to easily furnish your sound lounge with a DIYNSPA, a do it yourself non smoke producing amp. Some ESL C value is a lot greater, and may load the amp more adversly than the above which is a a simplistic simulated load for Quad ESL57. Martin Logan are not so easy on an amp. Patrick Turner. Scott -- Surf my web pages at, http://fmamradios.com/ |
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#31
Posted to rec.audio.tubes
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Amp oscillation
Hi RATs!
OK, I do not fully comprehend the schematic Not your problem ...I would put a fuse in the primary of the power transformer circuit. They are not expensive, and a bit simpler to replace than some other bits which may serve a similar function, under duress  What value cap do you have across that primary? I like 6F6G tubes. What lead you to them? Enjoy this process. Later failures will be just as maddening, but, perhaps a bit less interesting ... Happy Ears! Al |
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#32
Posted to rec.audio.tubes
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Amp oscillation
htrboy wrote: snipped Patrick, thank you for all the great tips and feedback (so to speak). This is the first tube amp I have attempted to build, and I expected to make mistakes. I am an electronics technician by trade and training and in this day and age, that means I deal mostly with 1's and 0's. While I have a basic knowledge of this stuff, I needed / wanted to build this thing to try and work through the problems and better understand what I learned in school about the fundamentals (that, and at heart I'm an analog nut). So, I'll try to take what I translate to be shots at my ignorance to heart and *really* try to work through the details of your post. I REALLY DO appreciate the knowledge being shared! best regards, Scott Its always amazing how ppl who deal with digital have trouble with analog and vice versa. Patrick Turner. |
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#33
Posted to rec.audio.tubes
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Amp oscillation
htrboy wrote: This shematic has a couple of things which are queer. The best values for R10 and C2 must be ascertained experimentally with a 50k pot mounted on variable radio tuning gang, and when stablity with 0.22 uF across the output is achieved along with maximal bandwidth into a resistance load is achieved then the varied R and C values are measured and fixed values soldered in. Could you elaborate on how to do this? I have a large tuning cap and 50K pot that I can use... Connect the pot in series with one of the tuning gangs to make an "R+C series network" where both the R and C are variable across a range wide enough for this application. Solder up some short say 200mm temporary leads to the tuning gang, and pot to allow easy connection across the anode load resistor of V1. Altering the tuning cap will be dangerous because it may be at the B+ voltage potential so you'll need to fir a plastic knowb to pot and tuning gang, and make sure everything remains well insulated from anything else it should not touch, including you. But you could have a dc blocking cap of 0.1uF off the anode and have the R&C network taken to 0V, with the tuning gang body being grounded; the V1 anode circuit will still "see" the same HF load. With the output taken to a CRO to monitor everything you do, and a 5 kHz square wave, watch the effect of varying the tuning gang and pot when you have a 0.22uF across the output. Keep the output voltage level less than 1Vrms assumung it isn't oscillating, but its common for amps to oscillate with no R&C network and you may find as soon as you connect the pot set at about 15k and the tuner set at about 50pF, that the full screen oscillations may stop, and there will just be some over shoot on the square wave with a declining oscillation. The square wave tends to shock the circuit into temporary oscillations at each Up and down stroke, since these abrupt voltage changes are what are known as transients, and the transient behaviour of any amp should be "damped", ie, slowed, but not excessively, rather like a shock absorber works on a car wheel when you drive over rough sharp bumps. That's the part that 120% of novices always get wrong, unless they learn about the functions of such R&C compensation networks, and why they are necessary, which is better explained elswhere, and in some detail to confuse the novice even more athttp://www.turneraudio.com.au I'll be digesting the info on your site for a while... The RCA schematic shown was a forerunner of the Dynaco ST70 schematic which also used a triode + pentode input tube but which supercharged the circuit by bootsrapping of the pentode anode load to the phase inverter cathode circuit thus providing a much higher load value to the pentode anode and thus much higher pentode gain. It was a not so obnoxious use of internal positive FB. The ST70 had lower gain UL output circuitry, so with bootstrapping the circuit could be made to be sensitive despite the GNFB and lack of gain in the output stage. Could you explain what you mean by Bootstrapping - my definition has to do with starting up a PC Booting up a PC is either starting it, or getting so darn fed up with the bloody thing you boot it out of the house. Bootstrapping is where an attempt is made to pick yourself up off the floor by tugging at your boot straps. Its obviously impossible to elevate yourself this way unless you are extremely well connected to GOD. I'm not. However in electronics, a circuit that causes a voltage change in an RL can be set up so that change is transfered back to a point in the circuit so that the load seems like a much higher value than its actual ohms value if you measured it with an ohm meter. Some of my preamps have bootstrapped follower gain stages, otherwise known as µ-followers, because the gain available is nearly equal to µ, the amplification factor data number for a tube. Build a µ follower and measure all the working voltages at every connection point, then ask yourself "what is going on here and why?" and you should be able to analyse and become familiar with what is happening. This is how I taught myself, and I want you to teach yourself the same way. Reading about it here, or me just explaining it is too difficult and time consuming and we don't have easy access to posting schematics here so I can't illustrate as I go. I have already written a website of 18MB, and I don't like repeating here what is at my site. There are quite a few sites with bootstrapped folowers on them, but a simple line gain stage I have at my site is a very fine starting point for building a decent simple triode amplifier, so that you learn, and remember. You will never understand tubes or transistors et all unless you become utterly familiar with the behaviour of the basic devices, and the basics about L,C, and R behaviour with signal frequencies. Patrick Turner. Please see above for my questions inserted ^ Best regards, Scott |
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#34
Posted to rec.audio.tubes
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Amp oscillation
John Byrns wrote: In article , Patrick Turner wrote: This shematic has a couple of things which are queer. The screen of V1 gets its dc feed from the cathode of V2 triode via 180k, and so if the dc rises on V2k, Va lowers on V1, and rises more on V2k so it looks like dc positive FB to me, and could cause LF oscillation so I would have instead about 1M to Vi screen from the anode B+ supply to this tube. This feedback is actually negative, when Va lowers on V1 then V2k also lowers, lowering the voltage on the screen grid which raises Va on V1 off setting the original lowering of Va on V1, this is negative feedback. Yes you are quite correct. I woke up this morning thinking I bet someone says I was wrong about the FB between V1 and V2, and I expected a Mr Byrns to point out the error, and sure enough, it happened. The FB basically is arranged to give dc NEGATIVE FB so that the circuit is better stabilised and has reducing gain at LF. Did you notice with respect to the C10 polarity issue that there is no negative bias at all on the grids of the 6L6s in this modified schematic? The circuit should either be restored to its original backbias configuration, or C10 and the 220 Ohm resistor should be placed in the cathode circuit of the 6L6s. The RCA book schematic I have that seems about the same as the one the guy drew up has the 7868 grids with bias R = 100k, and the junction of two of them is taken to the CT ot the HT winding. Then there is a 120 ohm R between this CT and and the anode current develops a negative biasing voltage across the 120 ohms. Its known as back-bias, and commonly used in radio circuits. Anyway, the biasing looked suspect on the schematic the guy drew, but hey, he's a starter only and such things as back bias may have not been noticed or understood at first. Its easy to miss that sort of thing completely. Patrick Turner. Regards, John Byrns Then R9, 1k, can be omitted since for equal +/- phase amplitudes the anode RL and cathode RL should be equal. R16&17 are screen stoppers and shown 56 ohms, and these won't stop much. Increase these R to 470 ohms, 1 watt at least. Stoppers to output grids, R14,15, should be 2k7 to 4k7, not 1k as shown, which won't stop much, because Cin to the pentode or beam tetrode output tubes is very low. The 820 ohm biasing resistance to V1 pentode may work better with 220uF across it which would allow R18, 270 ohm GNFB resistance to be increased to about 470 ohms and C6 to be lower, see below. Vitally important and difficult to judge is the value for C6 HF phase tweaking FB cap across GNB resistor of 270 ohms; its shown as 0.012uF, but might better be 0.0022uF. C2 HF gain stepping cap in series with R10, 15k, is shown as 22pF, and is also difficult to verify, but up to about 100pF may be right. The best values for R10 and C2 must be ascertained experimentally with a 50k pot mounted on variable radio tuning gang, and when stablity with 0.22 uF across the output is achieved along with maximal bandwidth into a resistance load is achieved then the varied R and C values are measured and fixed values soldered in. That's the part that 120% of novices always get wrong, unless they learn about the functions of such R&C compensation networks, and why they are necessary, which is better explained elswhere, and in some detail to confuse the novice even more at http://www.turneraudio.com.au The reason why the values are shown as they are by RCA is to provide a **guide only** to good practices. Exact values *WILL* have to be different according to the characteristics of the OPT chosen in terms of its primary inductance, leakage inductance, and shunt capacitances. RCA were not in any position in 1965 to provide enough data on what YOU may do some 40 years later. Hmm, rockets to the moon occured 3 years after 1966. Methinks the then old dudes who wrote the RCA tube book are all now dead, and cannot comment on *THEIR WONDERFUL CONTRIBUTIONS* to tube craft. The RCA schematic shown was a forerunner of the Dynaco ST70 schematic which also used a triode + pentode input tube but which supercharged the circuit by bootsrapping of the pentode anode load to the phase inverter cathode circuit thus providing a much higher load value to the pentode anode and thus much higher pentode gain. It was a not so obnoxious use of internal positive FB. The ST70 had lower gain UL output circuitry, so with bootstrapping the circuit could be made to be sensitive despite the GNFB and lack of gain in the output stage. The RCA circuit has more gain in the output circuit so there seems little need to increase the pentode gain by bootstrapping, and so the RCA circuit is simpler, and easier to get right. Having C2 too large is a sure recipe for having oscillations at 400kHz or thereabouts, so if anything it should only be large enough to prevent overshoot on square waves with a pure R load. The C2 value across R 10 should only be large enough to prevent oscillations on any value of pure C across the output without any R present. Its impossible to get a a perfect square wave with such "compensation" capacitors connected, but a perfect square wave isn't what you want. You want full power bandwidth from 20Hz to 20kHz, that is enough, and into a solely resistance only load. There will be some overshoot on square waves when you have the circuit working OK when a pure C load of typically 0.22 uF is used but that is just a natural function of the circuit you effectively have created, because every amplifier is a *BAND PASS FILTER* whether you like it or not, and dc can't pass, and nor can much 1Mhz. the roll off at HF may be slightly more than 6dB/octave above say 50kHz. Filters with roll off slopes outside the -3dB point, or "pole" may have a slope exceeding first order, or 6dB/octave, ie, perhaps 9 or 12dB/octave, and such filters produce ringing or overshoot on square waves. The ringing frequency should be well above the audio band, and 60kHz is OK. Maximal overshoot should be 6dB more than the level part of the square wave when the wave has "settled" when using a 5 kHz square wave. What one is always looking for is widest bandwidth between -3dB LF and HF poles, and at the highest output voltage equal to 1 bD below the 1 kHz sine wave clipping level. Then you want minimal freedom from square wave overshoot, with any C value betaeen 0.05uF and 5uF. Also wanted is freedom from peaks in the sine wave response outside the pass band of 20Hz to 20kHz. Such response peaks should not exceed 1dB at LF below 20Hz, nore exceed 6dB at any F above 20kHz even with any C value used as the load. Using combined R and C loads in parallel will make the amp look a lot better than it may be, but if capacitances of 0.22uF can be placed across the output without making the amp oscillate at say 150kH or any HF, then it will be stable into any other combination of R&C in parallel, or in series. A test load for ESL driving is usually about 1.5ohms in series with say 16ohms & 2uF in parallel. So even at 100kHz, the load has reverted to being mainly resistive with 1.5ohms, because the 2uF Z has become 0.8ohms. You need to have basic knowledge about R&C and filter behaviour and about relative impedance effects in filters to more fully understand what you are doing if you are to easily furnish your sound lounge with a DIYNSPA, a do it yourself non smoke producing amp. Some ESL C value is a lot greater, and may load the amp more adversly than the above which is a a simplistic simulated load for Quad ESL57. Martin Logan are not so easy on an amp. Patrick Turner. Scott -- Surf my web pages at, http://fmamradios.com/ |
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#35
Posted to rec.audio.tubes
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Amp oscillation
John Byrns wrote: In article om, "htrboy" wrote: On Feb 7, 3:24 am, "MarkS" wrote: Scott, Looks like the 220 ohm 10w resistor is tied to the grids not tbe cathodes of the 6L6's, is that a miss print? If not, that needs to be corrected before doing anything else. Otherwise, you're running with 0 bias. Mark Hi Mark, That part of the circuit is a point of confusion for me since the original circuit was tube rectified. I am going to construct a negative voltage circuit using one of the PT taps and run it to the grids. The design isn't supposed to be cathode biased. The original RCA design (20-13) used what is called "backbias". In a simple amplifier like this where almost all of the current drain comes from the output stage this is essentially equivalent to cathode bias, at least as far as the DC conditions go. I would recommend restoring R20 and C10 to their original positions in the circuit, note the the negative lead of C10 should connect to the power transformer center tap which connects to the 6L6 grid resistors. The use of the backbias circuit has nothing to do with the use of a rectifier tube, it will work just fine with your solid state rectifiers. The back bias circuit will be more forgiving to get operating initially, if you want to go to fixed bias do that later after you get the amplifier up and running as originally designed. It will be better to initially concentrate on issues like getting the NFB properly stabilized before tackling changes to the design. Regards, John Byrns The main trouble with back bias is that it offers no fixed biasing for the class AB tube working, and as the signal increases, so does the bias, and the distortion, like trying to use class AB with cathode biasing. There is a slight benefit that the anode suply voltage does not have to be so great as it would with cathode bias where the Ea Plus bias voltage must supplied by the anode B+ supply. Nevertheless, back bias works OK where one wants only a few AB watts from an amp capable of 30 watts AB. But also there is no individual bias current regulation for each output tube. I have lost count of the number of amps, especially Quad-II, with a common biasing resistance and where one output tube has a red hot anode and the other is too cool. Where fixed bias is used, there should be a separate biasing pot FOR EACH OUTPUT GRID. If cathode bias is used, there should always be a separate RC biasing circuit for each output tube. Typical value si use in Quad-II mod is 470 ohms plus 1,000 uF rated at 63V, and I have active protection, to stop ppl blowing up bias circuits, tubes, and OPTs and PTs, all of which are very expensive to fix because new spares are hard to find. Patrick Turner. -- Surf my web pages at, http://fmamradios.com/ |
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#36
Posted to rec.audio.tubes
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Amp oscillation
Iain Churches wrote: "Patrick Turner" wrote in message ... There are a few things wrong with the schematic which has low contrast but is otherwise quite readable and understandable. Next time anyone submits a schematic like this they should use *black ink pen* on white pape. The best way is to draw it up lightly in pencil and triple check for mistakes before over drawing in ink, and then erazing the pencil away with a rubber. Scott has now sent me a better scan of the schematic which is at: http://www.kolumbus.fi/iain.churches/Pics/Schematics/Scott'sAmp2.gif Iain The scan seems to be pencil drawn, so contrast is still awful, and the size is 830KB and V1 occupies my whole screen when I open the file. The best drawn schematics are done with a special old fashioned drawing board black ink pen. I used then a lot when i did house plans for people, but now nearly all drafting is done on the PC. Last night I spent 8 hours draing ONE revised Quad-II amp circuit using MS-Paint, slow, but it looks better than the hand drawn on paper + scan method which takes much less time. The MS Paint schematic is saved as a monochrome image, then as a GIF for web display and a typical schematic is never more than about 50KB, much less than the 830KB file at the above address. People could to worse than copy my techniques at my website. Perhaps ppl could not only try to learn about tubes, but learn about the basics of presenting schematics so the viewer only has to double click to open it, and nothing more. The sender MUST take the trouble with the schematic if he expects us to give professional advice about them in public for free, and for the benefit of everyone visiting the NG or for those that search the archives which are slowly building up a huge knowledge base. Patrick Turner. |
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#37
Posted to rec.audio.tubes
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Amp oscillation
On Feb 7, 7:15 am, John Byrns wrote:
In article om, "htrboy" wrote: On Feb 7, 3:24 am, "MarkS" wrote: Scott, Looks like the 220 ohm 10w resistor is tied to the grids not tbe cathodes of the 6L6's, is that a miss print? If not, that needs to be corrected before doing anything else. Otherwise, you're running with 0 bias. Mark Hi Mark, That part of the circuit is a point of confusion for me since the original circuit was tube rectified. I am going to construct a negative voltage circuit using one of the PT taps and run it to the grids. The design isn't supposed to be cathode biased. The original RCA design (20-13) used what is called "backbias". In a simple amplifier like this where almost all of the current drain comes from the output stage this is essentially equivalent to cathode bias, at least as far as the DC conditions go. I would recommend restoring R20 and C10 to their original positions in the circuit, note the the negative lead of C10 should connect to the power transformer center tap which connects to the 6L6 grid resistors. The use of the backbias circuit has nothing to do with the use of a rectifier tube, it will work just fine with your solid state rectifiers. The back bias circuit will be more forgiving to get operating initially, if you want to go to fixed bias do that later after you get the amplifier up and running as originally designed. It will be better to initially concentrate on issues like getting the NFB properly stabilized before tackling changes to the design. Regards, John Byrns -- Surf my web pages at, http://fmamradios.com/ Hi John, I will wire up the original circuit this weekend! Thanks for explaining what the bias was called I couldn't find anything similar on the net. I have to say that I am VERY much encouraged now - I spent a little time last night replacing the power tube resistors with the values suggested and just those changes made a big difference. The HF oscillation is now only at minimum and maximum volumes, though now there is quite a bit of LF oscillation. After struggling with this, any change for the better is great! More questions to follow... |
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#38
Posted to rec.audio.tubes
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Amp oscillation
On Feb 7, 4:10 pm, "tubegarden" wrote:
Hi RATs! OK, I do not fully comprehend the schematic Not your problem ...I would put a fuse in the primary of the power transformer circuit. They are not expensive, and a bit simpler to replace than some other bits which may serve a similar function, under duress What value cap do you have across that primary? I like 6F6G tubes. What lead you to them? Enjoy this process. Later failures will be just as maddening, but, perhaps a bit less interesting ... Happy Ears! Al Crap - There is actually a fuse in the circuit, it escaped my attention when drawing the schematic sorry. The cap is an X2 safety cap, .068mF I think. I pulled the 6F6's out of a short wave radio chassis that was otherwise trashed. The owner had pulled the chassis out to turn the cabinet into a wet bar (to each his own I guess). I also pulled out the tuning eye tube which I want to do something with down the road. |
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#39
Posted to rec.audio.tubes
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Amp oscillation
On Feb 7, 9:40 pm, Patrick Turner wrote:
htrboy wrote: snipped Patrick, thank you for all the great tips and feedback (so to speak). This is the first tube amp I have attempted to build, and I expected to make mistakes. I am an electronics technician by trade and training and in this day and age, that means I deal mostly with 1's and 0's. While I have a basic knowledge of this stuff, I needed / wanted to build this thing to try and work through the problems and better understand what I learned in school about the fundamentals (that, and at heart I'm an analog nut). So, I'll try to take what I translate to be shots at my ignorance to heart and *really* try to work through the details of your post. I REALLY DO appreciate the knowledge being shared! best regards, Scott Its always amazing how ppl who deal with digital have trouble with analog and vice versa. Patrick Turner. I work with a few research and development EE's, brilliant digital guys (and gals) all. They can help explain the basic theory behind the current flow etc. but not a clue when it comes to the ins and outs of what has been discussed here. Mostly they remember their dad's working on this stuff |
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#40
Posted to rec.audio.tubes
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Amp oscillation
On Feb 7, 10:20 pm, Patrick Turner wrote:
John Byrns wrote: In article , Patrick Turner wrote: This shematic has a couple of things which are queer. The screen of V1 gets its dc feed from the cathode of V2 triode via 180k, and so if the dc rises on V2k, Va lowers on V1, and rises more on V2k so it looks like dc positive FB to me, and could cause LF oscillation so I would have instead about 1M to Vi screen from the anode B+ supply to this tube. This feedback is actually negative, when Va lowers on V1 then V2k also lowers, lowering the voltage on the screen grid which raises Va on V1 off setting the original lowering of Va on V1, this is negative feedback. This feedback is actually negative, when Va lowers on V1 then V2k also lowers, lowering the voltage on the screen grid which raises Va on V1 off setting the original lowering of Va on V1, this is negative feedback. Yes you are quite correct. I woke up this morning thinking I bet someone says I was wrong about the FB between V1 and V2, and I expected a Mr Byrns to point out the error, and sure enough, it happened. The FB basically is arranged to give dc NEGATIVE FB so that the circuit is better stabilised and has reducing gain at LF. Did you notice with respect to the C10 polarity issue that there is no negative bias at all on the grids of the 6L6s in this modified schematic? The circuit should either be restored to its original backbias configuration, or C10 and the 220 Ohm resistor should be placed in the cathode circuit of the 6L6s. The RCA book schematic I have that seems about the same as the one the guy drew up has the 7868 grids with bias R = 100k, and the junction of two of them is taken to the CT ot the HT winding. Then there is a 120 ohm R between this CT and and the anode current develops a negative biasing voltage across the 120 ohms. Its known as back-bias, and commonly used in radio circuits. Anyway, the biasing looked suspect on the schematic the guy drew, but hey, he's a starter only and such things as back bias may have not been noticed or understood at first. Its easy to miss that sort of thing completely. Patrick Turner. With regards to the comments about V1, should the values in the circuit be left alone then? Should I concentrate for now on getting the bias right and then working with the feedback circuit? |
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Linear Mode
