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#1
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
There's been a lot of discussion on AGA lately about the Marshall 2204
"Cascaded" preamp circuit and whether or not it sucks and how much. A lot of people really do like the high gain preamp. But what sucks about the cascaded Marshall circuit is that there's no way to switch the high and low gain besides physically moving a cable from one input jack to the other. That circuit was a long time ago, though, these days everyone likes channel switching amps which use relays or other switching logic to push the signal through different tube stages and recombine them at the power amp. Which is complicated and no fun to homebrew. Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. It's built and working, as part of a homemade amplifier, and I really do like the sound so far. I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat. http://img718.imageshack.us/img718/8...edfeedback.png This circuit pretty much looks like a Marshall "cascaded" preamp here. But with the footswitch open, the 47K resistor and 0.44 uF capacitor form a feedback loop between the plate of the second triode (half of a 12AX7) and the cathode of the first triode. The 0.44 uF capacitor really doesn't do anything here but just pass the feedback signal through - the DC is already blocked by the 0.022 uF coupling capacitor between the second triode's plate and the output load, and any high-pass filtering is going to be dominated by the 0.022 uF cap as well. The result of the feedback loop is a stage with a moderate amount of gain, pretty flat frequency response, and low distortion unless driven to the limits where it'll hardclip, which a typical guitar pickup won't be able to do. So it's a typical 'clean' input stage. Now close the footswitch, this grounds the middle of the feedback loop and breaks it. The 0.44 uF capacitor now becomes a cathode bypass capacitor for the first triode, bringing in a gain boost above about 300 Hz. The 47 K resistor becomes just a dummy load hanging off the output. Without feedback, the stage distorts much more readily, gain is much higher, and the frequency response is not as even. Lows are rolled off by the interstage coupling RC network and the treble rolls off due to Miller effect capacitance. It ends up being a typical 'high gain' input stage with appropriate frequency response and distortion. Crunch! Small-signal SPICE simulation of the frequency response and gain of each stage is he http://img20.imageshack.us/img20/379/freqresponse.jpg What makes this really amenable to footswitching is that although the gain at the switching point is high, the impedance is very low, so noise pickup isn't much of an issue. The feedback loop keeps the impedance low to such noise signals, it's sort of a virtual ground. So I just think this circuit is really neat. There's a lot of talk on AGA about the differences in voicing a clean and distorted channel. It's true that you can't totally tweak everything here, but you can probably do quite a bit to change frequency response and gain. The feedback resistor could probably even include a variable pot. |
#2
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.
"Morris Slutsky" wrote .... I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat. http://img718.imageshack.us/img718/8...edfeedback.png I like it! Two thoughts: 1.) If you replace one gain stage with this, you'll invert the signal... this can cause problems sometimes. 2.) Switching the cathode cap out also rolls off lows. I'd like to hear it in action. __ Steve .. |
#3
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
On Jan 24, 10:24*am, "Stephen Cowell"
wrote: "Morris Slutsky" wrote ... I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat. http://img718.imageshack.us/img718/8...edfeedback.png I like it! *Two thoughts: 1.) *If you replace one gain stage with this, you'll * * invert the signal... this can cause problems sometimes. 2.) *Switching the cathode cap out also rolls off lows. I'd like to hear it in action. __ Steve . Looks good! More thoughts... (for a flat frequency response application) -- use a much larger cap than 0.44 uF, e.g. 100 uF, for cathode decouplng (and feedback) over a much wider frequency range -- plate load on the 2nd triode is 100K//47K (at least when switch is closed), or about 32K; this seems a bit low. -- bypass the 2nd triode cathode resistor (and make it smaller, e,g. 2.7K) for more gain, then increase the above 47K for the right amount of feedback This design might serve to switch a single audio input from microphone gain to AUX gain (not tried) Cheers, Roger |
#4
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.
Morris Slutsky wrote:
There's been a lot of discussion on AGA lately about the Marshall 2204 "Cascaded" preamp circuit and whether or not it sucks and how much. A lot of people really do like the high gain preamp. But what sucks about the cascaded Marshall circuit is that there's no way to switch the high and low gain besides physically moving a cable from one input jack to the other. That circuit was a long time ago, though, these days everyone likes channel switching amps which use relays or other switching logic to push the signal through different tube stages and recombine them at the power amp. Which is complicated and no fun to homebrew. Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. It's built and working, as part of a homemade amplifier, and I really do like the sound so far. I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat. http://img718.imageshack.us/img718/8...edfeedback.png This circuit pretty much looks like a Marshall "cascaded" preamp here. But with the footswitch open, the 47K resistor and 0.44 uF capacitor form a feedback loop between the plate of the second triode (half of a 12AX7) and the cathode of the first triode. The 0.44 uF capacitor really doesn't do anything here but just pass the feedback signal through - the DC is already blocked by the 0.022 uF coupling capacitor between the second triode's plate and the output load, and any high-pass filtering is going to be dominated by the 0.022 uF cap as well. The result of the feedback loop is a stage with a moderate amount of gain, pretty flat frequency response, and low distortion unless driven to the limits where it'll hardclip, which a typical guitar pickup won't be able to do. So it's a typical 'clean' input stage. Now close the footswitch, this grounds the middle of the feedback loop and breaks it. The 0.44 uF capacitor now becomes a cathode bypass capacitor for the first triode, bringing in a gain boost above about 300 Hz. The 47 K resistor becomes just a dummy load hanging off the output. Without feedback, the stage distorts much more readily, gain is much higher, and the frequency response is not as even. Lows are rolled off by the interstage coupling RC network and the treble rolls off due to Miller effect capacitance. It ends up being a typical 'high gain' input stage with appropriate frequency response and distortion. Crunch! Small-signal SPICE simulation of the frequency response and gain of each stage is he http://img20.imageshack.us/img20/379/freqresponse.jpg What makes this really amenable to footswitching is that although the gain at the switching point is high, the impedance is very low, so noise pickup isn't much of an issue. The feedback loop keeps the impedance low to such noise signals, it's sort of a virtual ground. So I just think this circuit is really neat. There's a lot of talk on AGA about the differences in voicing a clean and distorted channel. It's true that you can't totally tweak everything here, but you can probably do quite a bit to change frequency response and gain. The feedback resistor could probably even include a variable pot. Did you notice clicks when you press the foot switch, caused by the 0.44uF cap being charged and discharged at the cathode bias potential? You can avoid that by paralleling the foot switch with a resistor. The value wouldn't be critical - say 47-100k. This will also keep the output terminal at ground potential even when the foot switch is open. This is desireable to avoid a 'thump' if the unit is a standalone piece that may be hot-plugged to the main amp. |
#5
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
On Jan 24, 8:51*am, Engineer wrote:
On Jan 24, 10:24*am, "Stephen Cowell" wrote: "Morris Slutsky" wrote ... I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat. http://img718.imageshack.us/img718/8...edfeedback.png I like it! *Two thoughts: 1.) *If you replace one gain stage with this, you'll * * invert the signal... this can cause problems sometimes. 2.) *Switching the cathode cap out also rolls off lows. I'd like to hear it in action. __ Steve . Looks good! *More thoughts... (for a flat frequency response application) -- use a much larger cap than 0.44 uF, e.g. 100 uF, for cathode decouplng (and feedback) over a much wider frequency range Beware of leakage currents in the 100uF if you go this way. You will hear the click as the leakage current gets sent to the output node. Since the plate resistor of the second stage is 100K, this is the impedance you need to assume for the short term effect. IIRC the 12AX7 has a higher plate impedance than this. A two device amplifier with the feedback like shown is not a novel idea. It has been used quite a bit. The wiring of the switched to ground will add some capacitance etc to that node when the switch is open. This will cause peaking of the high frequencies. It is better to run wiring at the low impedance points of the circuit. With the switch closed, the first section is running the tube at its full gain. This means that unit to unit variations and the nonlinear effects of this part will show up in the results. -- plate load on the 2nd triode is 100K//47K (at least when switch is closed), or about 32K; this seems a bit low. -- bypass the 2nd triode cathode resistor (and make it smaller, e,g. 2.7K) for more gain, then increase the above 47K for the right amount of feedback This design might serve to switch a single audio input from microphone gain to AUX gain (not tried) Cheers, Roger |
#6
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.
Stephen Cowell wrote:
2.) Switching the cathode cap out also rolls off lows. But also provides improved sound staging, reduced midrange granularity, and better bass speed. Full details at http://www.tinaja.com/glib/marcia.pdf -- Many thanks, Don Lancaster voice phone: (928)428-4073 Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552 rss: http://www.tinaja.com/whtnu.xml email: Please visit my GURU's LAIR web site at http://www.tinaja.com |
#7
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.
Morris Slutsky wrote:
Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. It's built and working, as part of a homemade amplifier, and I really do like the sound so far. Did you try to simply by-pass the second tube ? |
#8
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.
"Don Lancaster" wrote in message ... Stephen Cowell wrote: 2.) Switching the cathode cap out also rolls off lows. But also provides improved sound staging, reduced midrange granularity, and better bass speed. Full details at http://www.tinaja.com/glib/marcia.pdf I can smell the cork from here! __ Steve .. |
#9
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.
"WB" wrote Morris Slutsky wrote: Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. It's built and working, as part of a homemade amplifier, and I really do like the sound so far. Did you try to simply by-pass the second tube ? 1.) You need a voicing change for guitar 'crunch'... lifting the cathode cap gives this (necessary) change. 2.) 'Bypassing' a stage is not easy... pops, thumps, DC states, inversion of phase... all apply. Your suggestion would result in 180degree phase change when you stomp... not a good idea, in and of itself. __ Steve .. |
#10
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
Stephen Cowell wrote:
"WB" wrote Morris Slutsky wrote: Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. It's built and working, as part of a homemade amplifier, and I really do like the sound so far. Did you try to simply by-pass the second tube ? 1.) You need a voicing change for guitar 'crunch'... lifting the cathode cap gives this (necessary) change. 2.) 'Bypassing' a stage is not easy... pops, thumps, DC states, inversion of phase... all apply. Your suggestion would result in 180degree phase change when you stomp... not a good idea, in and of itself. __ Steve . Since this is crossposted to rec.audio.tubes, there'll be a disconnect between the dudes who handle their glass with white gloves and keep their gear on the shelf and the dudes who pound the crap out of it and pour it fulla beer... LV |
#11
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
On Jan 24, 12:54*pm, Don Lancaster wrote:
Stephen Cowell wrote: 2.) *Switching the cathode cap out also rolls off lows. But also provides improved sound staging, reduced midrange granularity, and better bass speed. Full details athttp://www.tinaja.com/glib/marcia.pdf -- Many thanks, Don Lancaster * * * * * * * * * * * * *voice phone: (928)428-4073 Synergetics * 3860 West First Street * Box 809 Thatcher, AZ 85552 rss:http://www.tinaja.com/whtnu.xml* email: Please visit my GURU's LAIR web site athttp://www.tinaja.com I think I get better bass speed in the high-gain mode, actually. Because it's much easier to play Megadeth that way. It's an honor to have you here, by the way, your writing does in fact rule. |
#12
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.
"Lord Valve" wrote in message
... Spam. Tacky spam at that. Audio tends to be that way. Don't read it, asshole. ... Ah, I see RAT is still just as ****ty as ever... The smart posters left just in time. All that remained since then are trolls and trollfeeders. Sad, really. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |
#13
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.
Lord Valve wrote:
Since this is crossposted to rec.audio.tubes, there'll be a disconnect between the dudes who handle their glass with white gloves and keep their gear on the shelf and the dudes who pound the crap out of it and pour it fulla beer... I doubt if they drink beer with their pinkies extended. |
#14
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
A two device amplifier with the feedback like shown is not a novel idea. *It has been used quite a bit. The wiring of the switched to ground will add some capacitance etc to that node when the switch is open. *This will cause peaking of the high frequencies. *It is better to run wiring at the low impedance points of the circuit. With the switch closed, the first section is running the tube at its full gain. *This means that unit to unit variations and the nonlinear effects of this part will show up in the results. Moose, I just checked SPICE about capacitance to ground at the footswitch point. There is a small peak at about 100 KHz from a 100 pF capacitance there, and a really huge one from 1000 pF. I'm hoping that my actual footswitch is closer to 300 pF - it probably is, assuming that most of this is due to the capacitance of the footswitch cable itself (about 6 feet) - which SPICE expects to produce a 4 dB boost peaking at about 165 pF. I suspect that the actual boost will be less than this, due to stray capacitances (wiring to ground plane?) lowering the overall high frequency response in ways that SPICE cannot readily take into account. But yes, you're right, I should expect peaky behavior here. It's outside the audio band hopefully, which is good because you won't hear it, bad because it could cause inaudible parasitics, mitigated by the fact that this peak is still less than the response at this frequency in high-gain mode and if it doesn't oscillate there why would it oscillate in low gain mode? Thanks for your thoughts and advice. |
#15
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
On Jan 24, 6:39*pm, Morris Slutsky wrote:
A two device amplifier with the feedback like shown is not a novel idea. *It has been used quite a bit. The wiring of the switched to ground will add some capacitance etc to that node when the switch is open. *This will cause peaking of the high frequencies. *It is better to run wiring at the low impedance points of the circuit. With the switch closed, the first section is running the tube at its full gain. *This means that unit to unit variations and the nonlinear effects of this part will show up in the results. Moose, I just checked SPICE about capacitance to ground at the footswitch point. *There is a small peak at about 100 KHz from a 100 pF capacitance there, and a really huge one from 1000 pF. *I'm hoping that my actual footswitch is closer to 300 pF *- it probably is, assuming that most of this is due to the capacitance of the footswitch cable itself (about 6 feet) - which SPICE expects to produce a 4 dB boost peaking at about 165 pF. *I suspect that the actual boost will be less than this, due to stray capacitances (wiring to ground plane?) lowering the overall high frequency response in ways that SPICE cannot readily take into account. * But yes, you're right, I should expect peaky behavior here. *It's outside the audio band hopefully, which is good because you won't hear it, bad because it could cause inaudible parasitics, mitigated by the fact that this peak is still less than the response at this frequency in high-gain mode and if it doesn't oscillate there why would it oscillate in low gain mode? Thanks for your thoughts and advice. peaking at about 165 KHz I mean. |
#16
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.
On Sun, 24 Jan 2010 01:35:38 -0800 (PST), Morris Slutsky
wrote: There's been a lot of discussion on AGA lately about the Marshall 2204 "Cascaded" preamp circuit and whether or not it sucks and how much. A lot of people really do like the high gain preamp. But what sucks about the cascaded Marshall circuit is that there's no way to switch the high and low gain besides physically moving a cable from one input jack to the other. That circuit was a long time ago, though, these days everyone likes channel switching amps which use relays or other switching logic to push the signal through different tube stages and recombine them at the power amp. Which is complicated and no fun to homebrew. Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. It's built and working, as part of a homemade amplifier, and I really do like the sound so far. I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat. http://img718.imageshack.us/img718/8...edfeedback.png Reminds me of the "GE Circuit" which was used a lot as a mic/phono/tape head front-end in the early transistor days: ftp://jjlarkin.lmi.net/GEcircuit.jpg John |
#17
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.
"Lord Valve" wrote Don Lancaster wrote: Stephen Cowell wrote: 2.) Switching the cathode cap out also rolls off lows. But also provides improved sound staging, reduced midrange granularity, and better bass speed. Full details at http://www.tinaja.com/glib/marcia.pdf THE Don Lancaster? Of "Active Filter Cookbook" (etc.) fame? Hey, **** off... he replied to *me*... and I *damn* sure know who 'the' Don L. is. Gotta be cool, dude! __ Steve .. |
#18
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
On Jan 24, 3:39*pm, Morris Slutsky wrote:
A two device amplifier with the feedback like shown is not a novel idea. *It has been used quite a bit. The wiring of the switched to ground will add some capacitance etc to that node when the switch is open. *This will cause peaking of the high frequencies. *It is better to run wiring at the low impedance points of the circuit. With the switch closed, the first section is running the tube at its full gain. *This means that unit to unit variations and the nonlinear effects of this part will show up in the results. Moose, I just checked SPICE about capacitance to ground at the footswitch point. *There is a small peak at about 100 KHz from a 100 pF capacitance there, and a really huge one from 1000 pF. *I'm hoping that my actual footswitch is closer to 300 pF *- it probably is, Did you measure it with a low frequency capacitance meter or use a high frequency one. I would expect that the capacitance may be higher at higher frequencies and that there could be a large increase in the damping factor. assuming that most of this is due to the capacitance of the footswitch cable itself (about 6 feet) - which SPICE expects to produce a 4 dB boost peaking at about 165 pF. *I suspect that the actual boost will be less than this, due to stray capacitances (wiring to ground plane?) Any extra capacitance to ground at that point will increase the peak. Capacitances at other points may make it get very ugly. There is a phase shift as you go around the loop. lowering the overall high frequency response in ways that SPICE cannot readily take into account. * But yes, you're right, I should expect peaky behavior here. *It's outside the audio band hopefully, which is good because you won't hear it, bad because it could cause inaudible parasitics, mitigated by the fact that this peak is still less than the response at this frequency in high-gain mode and if it doesn't oscillate there why would it oscillate in low gain mode? In the low gain mode, you have a feed back path that doesn't exist in the high gain mode. This means that as far as the tubes are concerned, the system gain (closed loop gain) is higher in the low gain case. Thanks for your thoughts and advice. |
#19
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchableusing"folded" feedback loop.
Stephen Cowell wrote:
"Lord Valve" wrote Don Lancaster wrote: Stephen Cowell wrote: 2.) Switching the cathode cap out also rolls off lows. But also provides improved sound staging, reduced midrange granularity, and better bass speed. Full details at http://www.tinaja.com/glib/marcia.pdf THE Don Lancaster? Of "Active Filter Cookbook" (etc.) fame? Hey, **** off... he replied to *me*... and I *damn* sure know who 'the' Don L. is. Gotta be cool, dude! __ Steve . Here ya go: http://tinyurl.com/ygwosee LV |
#20
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
Lord Valve wrote: Stephen Cowell wrote: "WB" wrote Morris Slutsky wrote: Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. It's built and working, as part of a homemade amplifier, and I really do like the sound so far. Did you try to simply by-pass the second tube ? 1.) You need a voicing change for guitar 'crunch'... lifting the cathode cap gives this (necessary) change. 2.) 'Bypassing' a stage is not easy... pops, thumps, DC states, inversion of phase... all apply. Your suggestion would result in 180degree phase change when you stomp... not a good idea, in and of itself. __ Steve . Since this is crossposted to rec.audio.tubes, there'll be a disconnect between the dudes who handle their glass with white gloves and keep their gear on the shelf and the dudes who pound the crap out of it and pour it fulla beer... Real tubes handle 50+ KW. -- Greed is the root of all eBay. |
#21
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
On Jan 24, 8:24*pm, MooseFET wrote:
On Jan 24, 3:39*pm, Morris Slutsky wrote: A two device amplifier with the feedback like shown is not a novel idea. *It has been used quite a bit. The wiring of the switched to ground will add some capacitance etc to that node when the switch is open. *This will cause peaking of the high frequencies. *It is better to run wiring at the low impedance points of the circuit. With the switch closed, the first section is running the tube at its full gain. *This means that unit to unit variations and the nonlinear effects of this part will show up in the results. Moose, I just checked SPICE about capacitance to ground at the footswitch point. *There is a small peak at about 100 KHz from a 100 pF capacitance there, and a really huge one from 1000 pF. *I'm hoping that my actual footswitch is closer to 300 pF *- it probably is, Did you measure it with a low frequency capacitance meter or use a high frequency one. *I would expect that the capacitance may be higher at higher frequencies and that there could be a large increase in the damping factor. assuming that most of this is due to the capacitance of the footswitch cable itself (about 6 feet) - which SPICE expects to produce a 4 dB boost peaking at about 165 pF. *I suspect that the actual boost will be less than this, due to stray capacitances (wiring to ground plane?) Any extra capacitance to ground at that point will increase the peak. Capacitances at other points may make it get very ugly. *There is a phase shift as you go around the loop. lowering the overall high frequency response in ways that SPICE cannot readily take into account. * But yes, you're right, I should expect peaky behavior here. *It's outside the audio band hopefully, which is good because you won't hear it, bad because it could cause inaudible parasitics, mitigated by the fact that this peak is still less than the response at this frequency in high-gain mode and if it doesn't oscillate there why would it oscillate in low gain mode? In the low gain mode, you have a feed back path that doesn't exist in the high gain mode. *This means that as far as the tubes are concerned, the system gain (closed loop gain) is higher in the low gain case. Thanks for your thoughts and advice. Hear you, Moose, so I SPICEd it again. I've been using LT-SPICE with Duncan Munro's tube models, it's easy to use and the price is right! Anyhow, modeling an open switch as a 300 pF capacitor, which is reasonable from what's known about guitar cables, I can snub out the ultrasonic peak pretty much completely with a 30 pF cap across the 680 K grid resistor on the second tube. The cost is the loss of 1 dB or so, at 20 KHz in high-gain mode. Probably I could live with that. Although some guitar amp purists would complain about loss of 'sparkle', guitar amp lovers have some hatred towards snubber caps due to their abuse by vendors in the past to compensate for really bad wiring layouts. This ain't a huge snubber though. I could put one in. The thing is, though, it's such a small capacitance value that I have no idea how much stray capacitance is currently there and what it's doing, I don't have a spectrum analyzer setup here, I make amps in my kitchen mostly. But it's a reasonable thing to do. If I get it open again, I might put a small snubber there. |
#22
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.
John Larkin wrote:
On Sun, 24 Jan 2010 01:35:38 -0800 (PST), Morris Slutsky wrote: There's been a lot of discussion on AGA lately about the Marshall 2204 "Cascaded" preamp circuit and whether or not it sucks and how much. A lot of people really do like the high gain preamp. But what sucks about the cascaded Marshall circuit is that there's no way to switch the high and low gain besides physically moving a cable from one input jack to the other. That circuit was a long time ago, though, these days everyone likes channel switching amps which use relays or other switching logic to push the signal through different tube stages and recombine them at the power amp. Which is complicated and no fun to homebrew. Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. It's built and working, as part of a homemade amplifier, and I really do like the sound so far. I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat. http://img718.imageshack.us/img718/8...edfeedback.png Reminds me of the "GE Circuit" which was used a lot as a mic/phono/tape head front-end in the early transistor days: ftp://jjlarkin.lmi.net/GEcircuit.jpg John I have seen similar circuits somewhere on the 'net where I think there is positive feedback as well as negative feedback used. I guess the positive feedback was used to kick up the gain of the crappy early transistors. Here's a two transistor circuit from 1964 that uses "DC positive feedback and AC-DC negative feedback": http://www.freepatentsonline.com/3267386.html |
#23
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Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.
On Mon, 25 Jan 2010 04:20:42 -0800 (PST), Morris Slutsky
wrote: On Jan 25, 4:02*am, Fred Abse wrote: On Sun, 24 Jan 2010 18:46:38 -0700, Lord Valve wrote: we're just ignorant guitar pickers So why crosspost to sci.electronics.design? sci - the hint's in the name. -- "Electricity is of two kinds, positive and negative. The difference is, I presume, that one comes a little more expensive, but is more durable; the other is a cheaper thing, but the moths get into it." * * * * * * * * * * * * * * * * * * * * * * *(Stephen Leacock) I'm the one who crossposted. Guilty! I just wanted to hear from these 3 different groups. It's a guitar amp circuit so it belongs on AGA. It's a tube circuit so it belongs on RAT. And since it's design, feedback, all that complicated stuff, I wanted to hear from SED. Fair enough I guess! I understand that the perspectives here are all going to be different. As to the Lord, he is well known to dislike "3 legged fuses" and anything digital - but oddly enough he does know quite a bit about the old-fashioned FETs of the 70s so go figure, it's his personal opinion and not his lack of knowledge, it's an informed opinion even if we may disagree. Me, I don't always dislike solid state guitar amps, and nobody has ever had a problem with transistorized guitar effects such as the classic Fuzz Face, although I do dislike digital guitar gear probably because I first saw the stuff in the early 90s when it had latency so bad that you could hit a note and go make a sandwich before coming back and hearing it come out of the speaker. So that's where I'm coming from and why I crossposted. As to the Lord's client list - never heard of ANY of them? Really? Not even Meatloaf or Fleetwood Mac or Huey Louis? I can understand that most people wouldn't like Yo La Tengo but they are cool I think. And definitely the Dirty Dozen Brass Band rules - check out "Brooklyn" I think that's their main popular tune. As to the Flobots - I love the Flobots. My band actually covers "Handlebars" and they let me play the horn solo (I only play guitar but I use an octave fuzz there. It's kind of close. Kinda.) I mean, it's not like he has done work for the Beatles but hey you know the Beatles are half-dead already, life is like that, there's plenty of good music out there today. Electrical engineers tend to be suspicious of the subjectivity of audio, not to mention the multitude of bad, bad circuits. Our world involves measuring things, not asking drugged-out, hearing-impaired rockers how they like the sound of our gear. I designed one guitar amp. and that was plenty. Not only did some goofy music store owners decide how it should work, but they wanted to make all the money, too. John |
#24
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
On Jan 25, 7:31*am, Bitrex wrote:
John Larkin wrote: On Sun, 24 Jan 2010 01:35:38 -0800 (PST), Morris Slutsky wrote: There's been a lot of discussion on AGA lately about the Marshall 2204 "Cascaded" preamp circuit and whether or not it sucks and how much. *A lot of people really do like the high gain preamp. *But what sucks about the cascaded Marshall circuit is that there's no way to switch the high and low gain besides physically moving a cable from one input jack to the other. *That circuit was a long time ago, though, these days everyone likes channel switching amps which use relays or other switching logic to push the signal through different tube stages and recombine them at the power amp. *Which is complicated and no fun to homebrew. Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. *It's built and working, as part of a homemade amplifier, and I really do like the sound so far. I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat. http://img718.imageshack.us/img718/8...edfeedback.png Reminds me of the "GE Circuit" which was used a lot as a mic/phono/tape head front-end in the early transistor days: ftp://jjlarkin.lmi.net/GEcircuit.jpg John I have seen similar circuits somewhere on the 'net where I think there is positive feedback as well as negative feedback used. *I guess the positive feedback was used to kick up the gain of the crappy early transistors. *Here's a two transistor circuit from 1964 that uses "DC positive feedback and AC-DC negative feedback": http://www.freepatentsonline.com/3267386.html Very interesting! I looked at that circuit and definitely that's DC coupled positive feedback and AC coupled negative feedback. The positive feedback increases input impedance (so I guess that's what some people call 'bootstrapping') while the negative feedback counteracts it to provide linearity. Neat idea. However I'd probably not want to use that circuit myself - having positive feedback at DC is just begging for thermal drift once the transistors heat up, isn't it? It would worry me. And in these times of high-impedance input devices like FETs, there's no need to use this circuit I don't think. Nice idea, though, I did check it out. |
#25
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Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.
On Mon, 25 Jan 2010 07:31:38 -0500, Bitrex
wrote: John Larkin wrote: On Sun, 24 Jan 2010 01:35:38 -0800 (PST), Morris Slutsky wrote: There's been a lot of discussion on AGA lately about the Marshall 2204 "Cascaded" preamp circuit and whether or not it sucks and how much. A lot of people really do like the high gain preamp. But what sucks about the cascaded Marshall circuit is that there's no way to switch the high and low gain besides physically moving a cable from one input jack to the other. That circuit was a long time ago, though, these days everyone likes channel switching amps which use relays or other switching logic to push the signal through different tube stages and recombine them at the power amp. Which is complicated and no fun to homebrew. Anyway I came up with this circuit, which allows easy footswitching with no relays or anything between a 'clean' input stage and a 'high gain' input stage, with appropriate frequency response voicing. It's built and working, as part of a homemade amplifier, and I really do like the sound so far. I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat. http://img718.imageshack.us/img718/8...edfeedback.png Reminds me of the "GE Circuit" which was used a lot as a mic/phono/tape head front-end in the early transistor days: ftp://jjlarkin.lmi.net/GEcircuit.jpg John I have seen similar circuits somewhere on the 'net where I think there is positive feedback as well as negative feedback used. I guess the positive feedback was used to kick up the gain of the crappy early transistors. Here's a two transistor circuit from 1964 that uses "DC positive feedback and AC-DC negative feedback": http://www.freepatentsonline.com/3267386.html Both the feedbacks are negative in the GE thing. The lower one is DC feedback to bias the first transistor. The one through Zf is negative feedback to set the gain and do equalization. John |
#26
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
On Jan 24, 11:51*am, Engineer wrote:
On Jan 24, 10:24*am, "Stephen Cowell" wrote: "Morris Slutsky" wrote ... I don't know if it's original or not, in the context of a guitar amp, but I think it's pretty neat. http://img718.imageshack.us/img718/8...edfeedback.png I like it! *Two thoughts: 1.) *If you replace one gain stage with this, you'll * * invert the signal... this can cause problems sometimes. 2.) *Switching the cathode cap out also rolls off lows. I'd like to hear it in action. __ Steve . Looks good! *More thoughts... (for a flat frequency response application) -- use a much larger cap than 0.44 uF, e.g. 100 uF, for cathode decouplng (and feedback) over a much wider frequency range -- plate load on the 2nd triode is 100K//47K (at least when switch is closed), or about 32K; this seems a bit low. -- bypass the 2nd triode cathode resistor (and make it smaller, e,g. 2.7K) for more gain, then increase the above 47K for the right amount of feedback This design might serve to switch a single audio input from microphone gain to AUX gain (not tried) Cheers, Roger Hi Roger, If I was to design a circuit for hi-fi work, I'd never want the feedback loop to be broken at all. Perhaps I'd be swapping resistors within the feedback loop, but I wouldn't want to break the loop altogether. Only guitarists want the gain and distortion to go up at the same time, along with a band-limited frequency response - that really is just for this one specific application. Yeah, I load that triode heavy. It's a tradeoff. The 12AX7 has a pretty high rP, I think about 68K. But it has a ton of mu! The other 12A*7 tubes have lower rP but a lot less mu. It probably all works out the same in the end, except for bias levels. But the 12AX7 is the tube that I can conveniently buy, so I'm using it. Output impedance would be expected to be 68K || 100K = 40K, so my 47K load does eat about half the signal, about 3dB of gain. There's still plenty gain there to go around, you just may have to turn the pot for the next stage to 10 instead of 9, no big deal. And it is a 47K load under all circumstances - with the loop open it's 47K to ground, with the loop closed it's 47K to what amounts to a virtual ground. Probably something very similar could work for hi-fi gain control, I think it's probably pretty common in that context actually to do gain by switching feedback resistors. Thanks for your advice |
#27
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Novel? Guitar amp input stage, single 12AX7, footswitchableusing "folded" feedback loop.
"Lord Valve" wrote Stephen Cowell wrote: "Lord Valve" wrote Don Lancaster wrote: Stephen Cowell wrote: 2.) Switching the cathode cap out also rolls off lows. But also provides improved sound staging, reduced midrange granularity, and better bass speed. Full details at http://www.tinaja.com/glib/marcia.pdf THE Don Lancaster? Of "Active Filter Cookbook" (etc.) fame? Hey, **** off... he replied to *me*... and I *damn* sure know who 'the' Don L. is. Gotta be cool, dude! Here ya go: http://tinyurl.com/ygwosee Everyone knows that Ex-Lax destroys the 'air' in the stage space... Dulco-Lax maintains the phase resilience in the trans-uranic highs. However, true hardcore aficianados work it out with a pencil... No. 2 Eberhard, pre-Sanford NOS... no mechanical sharpening! __ Steve .. |
#28
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Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.
"Michael A. Terrell" wrote Lord Valve wrote: Since this is crossposted to rec.audio.tubes, there'll be a disconnect between the dudes who handle their glass with white gloves and keep their gear on the shelf and the dudes who pound the crap out of it and pour it fulla beer... Real tubes handle 50+ KW. And you'd *damn* sure better handle *them* with white cloth gloves... or clean them in solvent before installation! __ Steve .. |
#29
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Novel? Guitar amp input stage, single 12AX7, footswitchable using "folded" feedback loop.
On Mon, 25 Jan 2010 09:44:18 -0600, "Stephen Cowell"
wrote: "Michael A. Terrell" wrote Lord Valve wrote: Since this is crossposted to rec.audio.tubes, there'll be a disconnect between the dudes who handle their glass with white gloves and keep their gear on the shelf and the dudes who pound the crap out of it and pour it fulla beer... Real tubes handle 50+ KW. And you'd *damn* sure better handle *them* with white cloth gloves... or clean them in solvent before installation! __ Steve . I use only liquid-nitrogen cured National Union 6SN7s. Be careful about the direction of the filament current. John |
#30
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchableusing"folded" feedback loop.
Stephen Cowell wrote:
"Lord Valve" wrote Stephen Cowell wrote: "Lord Valve" wrote Don Lancaster wrote: Stephen Cowell wrote: 2.) Switching the cathode cap out also rolls off lows. But also provides improved sound staging, reduced midrange granularity, and better bass speed. Full details at http://www.tinaja.com/glib/marcia.pdf THE Don Lancaster? Of "Active Filter Cookbook" (etc.) fame? Hey, **** off... he replied to *me*... and I *damn* sure know who 'the' Don L. is. Gotta be cool, dude! Here ya go: http://tinyurl.com/ygwosee Everyone knows that Ex-Lax destroys the 'air' in the stage space... Dulco-Lax maintains the phase resilience in the trans-uranic highs. However, true hardcore aficianados work it out with a pencil... No. 2 Eberhard, pre-Sanford NOS... no mechanical sharpening! __ Steve . Well, you'd know more about that **** than I would. LV |
#31
Posted to alt.guitar.amps,sci.electronics.design,rec.audio.tubes
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Novel? Guitar amp input stage, single 12AX7, footswitchable using"folded" feedback loop.
Stephen Cowell wrote: "Michael A. Terrell" wrote Lord Valve wrote: Since this is crossposted to rec.audio.tubes, there'll be a disconnect between the dudes who handle their glass with white gloves and keep their gear on the shelf and the dudes who pound the crap out of it and pour it fulla beer... Real tubes handle 50+ KW. And you'd *damn* sure better handle *them* with white cloth gloves... or clean them in solvent before installation! No need, unless you don't like fingerprints on the stainless steel body, but you absolutely have to make sure that there is enough water flow to keep them from melting down. One system i worked with had three 65 kW tubes, for a 195 kW output. -- Greed is the root of all eBay. |
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