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#1
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Eico Tone Control Mod
My latest project is to complete some modifications to an Eico 2080
integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Mikkel Simonsen, formerly of R.A.T. 2. Remove the tone control circuitry; 3. Design a new faceplate for the reduced number of controls. In order to achieve goal #1, I used one of the tone control tube positions as a cathode follower, to drive the 50K motorized volume control. Mikkel's circuitry included a driver for a mute relay. I am housing his circuit inside of a metal box installed under the chassis, to reduce interference as much as possible. A schematic of one channel the proposed design is posted on: http://www.yaegeraudio.com/eico_mod.pdf I welcome any comments / criticisms / suggestions on the proposed design. Thank you. Jon |
#2
Posted to rec.audio.tubes
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Eico Tone Control Mod
Yaaaawn. Removing tone controls from a piece of Eico lowest-common-
denominator tin of elcheapo. A more creative act would be to throw it in the skip. A complete and accurate description of your big project would be "snip-snip a coupla wires". The rest is shrinkwrapped waffle. -- AJ On Nov 26, 11:02 pm, Jon Yaeger wrote: My latest project is to complete some modifications to an Eico 2080 integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Mikkel Simonsen, formerly of R.A.T. 2. Remove the tone control circuitry; 3. Design a new faceplate for the reduced number of controls. In order to achieve goal #1, I used one of the tone control tube positions as a cathode follower, to drive the 50K motorized volume control. Mikkel's circuitry included a driver for a mute relay. I am housing his circuit inside of a metal box installed under the chassis, to reduce interference as much as possible. A schematic of one channel the proposed design is posted on: http://www.yaegeraudio.com/eico_mod.pdf I welcome any comments / criticisms / suggestions on the proposed design. Thank you. Jon |
#3
Posted to rec.audio.tubes
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Eico Tone Control Mod
On Nov 26, 7:47 pm, Andre Jute wrote:
Yaaaawn. Removing tone controls from a piece of Eico lowest-common- denominator tin of elcheapo. A more creative act would be to throw it in the skip. A complete and accurate description of your big project would be "snip-snip a coupla wires". rest of the jealous crap snipped Save for the single, irrefutable fact that whatever Jon did to whatever he did it to actually does and will work. This would be unlike any design you have submitted to this NG, past or present. Oh, and we are still waiting for the Lundahl designs - even one (1) of them. That you took the iron (advance against royalties to put it in terms you *might* understand) and did not produce the promised designs makes you a simple thief. Even if you committed theft-by-deception, a likely more venial sin in your dark little mind. It is hardly the habits of an honorable man to criticize the small works of another based on comparison to his own non-existent great works. Peter Wieck Wyncote, PA |
#4
Posted to rec.audio.tubes
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Eico Tone Control Mod
On Nov 26, 7:47 pm, Andre Jute wrote: Yaaaawn. Removing tone controls from a piece of Eico lowest-common- denominator tin of elcheapo. A more creative act would be to throw it in the skip. A complete and accurate description of your big project would be "snip-snip a coupla wires". rest of the jealous crap snipped Ah, I see you folded. Couldn't rise to the challenge . . . really didn't think that you would or could. The Eico stuff . . . being "lowest-common-denominator" as you say . . . is in fact perfect for experimenting. For as much as I like Mikkel's clever implementation, I wouldn't put it on, say, a McIntosh amplifier. You do reveal your ignorance. Some Eico iron, such as the HF-87 outputs, hold their own among the best U.S. iron ever wound. A properly modified Eico ST-70 rivals gear costing 10X as much. It offers an excellent bang for the buck. I'm glad that you are relaxing in the full comfort that a healthy inheritance / trust fund can bring. Enjoy the evening! Jon |
#5
Posted to rec.audio.tubes
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Eico Tone Control Mod
Jon Yaeger wrote:
My latest project is to complete some modifications to an Eico 2080 integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Mikkel Simonsen, formerly of R.A.T. 2. Remove the tone control circuitry; 3. Design a new faceplate for the reduced number of controls. In order to achieve goal #1, I used one of the tone control tube positions as a cathode follower, to drive the 50K motorized volume control. Mikkel's circuitry included a driver for a mute relay. I am housing his circuit inside of a metal box installed under the chassis, to reduce interference as much as possible. A schematic of one channel the proposed design is posted on: http://www.yaegeraudio.com/eico_mod.pdf I welcome any comments / criticisms / suggestions on the proposed design. Thank you. I would be interested to know how you make your decisions. Such as: Choice of valve, values of coupling caps, valve loading and configuration. That's everything, I suppose. And why wouldn't you put Mikkel's remote in a Mac? Is this a quality or a conservation issue? How is the remote powered, BTW? cheers, Ian |
#6
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Eico Tone Control Mod
On Nov 26, 8:35 pm, Jon Yaeger wrote:
I'm glad that you are relaxing in the full comfort that a healthy inheritance / trust fund can bring. Enjoy the evening! Wait a minute!!?? Are you calling Andre a Remittance Man? My, but what a deadly insult. That would be akin to calling westiepoo his batty- boy... Yikes, Jon, you are getting vicious in your old age! Peter Wieck Wyncote, PA |
#8
Posted to rec.audio.tubes
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Eico Tone Control Mod
"Andre Jute" wrote in message ... Yaaaawn. Removing tone controls from a piece of Eico lowest-common- denominator tin of elcheapo. A more creative act would be to throw it in the skip. A more useful act would be to throw you in the skip. |
#9
Posted to rec.audio.tubes
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Eico Tone Control Mod
Jon Yaeger wrote:
My latest project is to complete some modifications to an Eico 2080 integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Take a look at my pages of electronic volume controls using tubes http://pw2.netcom.com/~wa2ise/radios/6be6vol.html and http://pw2.netcom.com/~wa2ise/radios/elvctube.jpg |
#10
Posted to rec.audio.tubes
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Eico Tone Control Mod
Yaaaawn. Removing tone controls from a piece of Eico lowest-common- denominator tin of elcheapo. Save for the single, irrefutable fact that whatever Jon did to whatever he did it to actually does and will work. At least there'd be no serious criticism that he is hacking up a rare and great piece of equipment... If in fact that it is "el cheapo", and that if he can improve it, then more power to him. |
#11
Posted to rec.audio.tubes
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Eico Tone Control Mod
And why wouldn't you put Mikkel's remote in a Mac? Is this a quality or a conservation issue? How is the remote powered, BTW? cheers, Ian Ian, The Mac is considered by many to be a classic piece. There is no chance that my mods will do anything but decrease it's value to almost anyone other than myself. And maybe to me too . . . The Eico is not considered a classic; the one I have is in rough cosmetic shape, and the improvements are solely intended for my ears and convenience. The remote gets its power from the filament supply in the amplifier; the control from a 9V battery. Jon |
#12
Posted to rec.audio.tubes
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Eico Tone Control Mod
"Jon Yaeger" wrote in message ... My latest project is to complete some modifications to an Eico 2080 integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Mikkel Simonsen, formerly of R.A.T. 2. Remove the tone control circuitry; 3. Design a new faceplate for the reduced number of controls. In order to achieve goal #1, I used one of the tone control tube positions as a cathode follower, to drive the 50K motorized volume control. Mikkel's circuitry included a driver for a mute relay. I am housing his circuit inside of a metal box installed under the chassis, to reduce interference as much as possible. A schematic of one channel the proposed design is posted on: http://www.yaegeraudio.com/eico_mod.pdf I welcome any comments / criticisms / suggestions on the proposed design. Hello Jon. Nice project! Some pics would be of interest, and also some info about the remote gain control. With a CF I like to elevate the heater DC so that the cathode to heater differential does not exceed 100V. Nice to see a real project on RAT. Iain |
#13
Posted to rec.audio.tubes
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Eico Tone Control Mod
Hello, John.
1. Current through the valve 6CG7 will be lower than 4mA, since a lot will drop across the cathode resistor 39K. Have you considered to reduce the cathode resistor to, say, 10...15K to get higher transconductance oand hence lower output impedance on "tape" output? 2. Beware of the cathode-to-heater voltage. It shall be less than 100V. For that reason also why not reduce the cathode resistor. 3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu of this tube is low (about 20) and it will not give good power supply noise rejection ratio. 4. If your taperecorder connected to "tape" output happens to have relatively low input impedance (say, 10K) you will lose bass, because your interstage capacitor 0.47uF will not be sufficient. Why not then increase it to 2.2uF? (Otherwise make sure to connect only high impedance loads to your "tape" output. 5. [Warning!] Connect a 100R...1K resistor in series with the grid close to the socket. It will guarantee against parasitic oscillation (in Kolpitz mode) on the Ls and Cs of the circuit, particularly when your input "Balance" pot happens to go to ground. 6. Good thing is that your choice tube has a very common pinout. You can plug almost any twin triode in there and compare results. I believe, ECC85 is a better option. And if you decide to play with high transconductance ones, like 6ES8 (to get very low output impedance on "tape"), you will indeed appreciate advice #5. Regards, Alex "Jon Yaeger" wrote in message ... My latest project is to complete some modifications to an Eico 2080 integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Mikkel Simonsen, formerly of R.A.T. 2. Remove the tone control circuitry; 3. Design a new faceplate for the reduced number of controls. In order to achieve goal #1, I used one of the tone control tube positions as a cathode follower, to drive the 50K motorized volume control. Mikkel's circuitry included a driver for a mute relay. I am housing his circuit inside of a metal box installed under the chassis, to reduce interference as much as possible. A schematic of one channel the proposed design is posted on: http://www.yaegeraudio.com/eico_mod.pdf I welcome any comments / criticisms / suggestions on the proposed design. Thank you. Jon |
#14
Posted to rec.audio.tubes
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Eico Tone Control Mod
Hello, John.
1. Current through the valve 6CG7 will be lower than 4mA, since a lot will drop across the cathode resistor 39K. Have you considered to reduce the cathode resistor to, say, 10...15K to get higher transconductance oand hence lower output impedance on "tape" output? 2. Beware of the cathode-to-heater voltage. It shall be less than 100V. For that reason also why not reduce the cathode resistor. 3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu of this tube is low (about 20) and it will not give good power supply noise rejection ratio. 4. If your taperecorder connected to "tape" output happens to have relatively low input impedance (say, 10K) you will lose bass, because your interstage capacitor 0.47uF will not be sufficient. Why not then increase it to 2.2uF? (Otherwise make sure to connect only high impedance loads to your "tape" output. 5. [Warning!] Connect a 100R...1K resistor in series with the grid close to the socket. It will guarantee against parasitic oscillation (in Kolpitz mode) on the Ls and Cs of the circuit, particularly when your input "Balance" pot happens to go to ground. 6. Good thing is that your choice tube has a very common pinout. You can plug almost any twin triode in there and compare results. I believe, ECC85 is a better option. And if you decide to play with high transconductance ones, like 6ES8 (to get very low output impedance on "tape"), you will indeed appreciate advice #5. Regards, Alex "Jon Yaeger" wrote in message ... My latest project is to complete some modifications to an Eico 2080 integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Mikkel Simonsen, formerly of R.A.T. 2. Remove the tone control circuitry; 3. Design a new faceplate for the reduced number of controls. In order to achieve goal #1, I used one of the tone control tube positions as a cathode follower, to drive the 50K motorized volume control. Mikkel's circuitry included a driver for a mute relay. I am housing his circuit inside of a metal box installed under the chassis, to reduce interference as much as possible. A schematic of one channel the proposed design is posted on: http://www.yaegeraudio.com/eico_mod.pdf I welcome any comments / criticisms / suggestions on the proposed design. Thank you. Jon |
#15
Posted to rec.audio.tubes
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Working on Classic amps: butchery or necessary updating?
Robert Casey wrote about Yaeger's Eico wiresnips:
At least there'd be no serious criticism that he is hacking up a rare and great piece of equipment... If in fact that it is "el cheapo", and that if he can improve it, then more power to him. I agree. He could learn something. Up to fifteen years ago I thought nothing of rebuilding a QUAD II in my own image but eventually I decided that Peter Walker & Co knew more than I do and restored all mine to original, and over a period of years called in those that had left here modded and did the same restoration to original on them. In my defense I point out that Patrick still blithely alters Quad Ii quite extensively. So now I think how much work you can justifiably do on a "classic" is relevant to the time period in which you do it, and to the original quality of the "classic". It would be a crime to hack up a Quad II or a Radford, but Dynaco and Eico and so on are pretty common and therefore fair game. The Quad II case is confused by the fact that there is a couple of modern factory reissues that include many of the updates I did, and that Patrick now does, so in fact throwing the sanction of official approval over our humble endeavours. Andre Jute Visit Jute on Amps at http://members.lycos.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review |
#16
Posted to rec.audio.tubes
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Eico Tone Control Mod
in article , robert casey at
wrote on 11/27/07 12:24 AM: Jon Yaeger wrote: My latest project is to complete some modifications to an Eico 2080 integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Take a look at my pages of electronic volume controls using tubes http://pw2.netcom.com/~wa2ise/radios/6be6vol.html and http://pw2.netcom.com/~wa2ise/radios/elvctube.jpg Robert, Thanks for the interesting info. I guess the pentode provides a nice degree of isolation. There is still the "problem" that a motorized pot would have to be installed. I'm trying to stay with triodes as much as I can in this project. I appreciate your help. Jon |
#17
Posted to rec.audio.tubes
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Eico Tone Control Mod
Great advice! Thank you.
When I get some free time I'll incorporate many of these suggestions and revise the design. I'll then post the circuit. Jon in article , Alex at wrote on 11/27/07 6:48 AM: Hello, John. 1. Current through the valve 6CG7 will be lower than 4mA, since a lot will drop across the cathode resistor 39K. Have you considered to reduce the cathode resistor to, say, 10...15K to get higher transconductance oand hence lower output impedance on "tape" output? 2. Beware of the cathode-to-heater voltage. It shall be less than 100V. For that reason also why not reduce the cathode resistor. 3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu of this tube is low (about 20) and it will not give good power supply noise rejection ratio. 4. If your taperecorder connected to "tape" output happens to have relatively low input impedance (say, 10K) you will lose bass, because your interstage capacitor 0.47uF will not be sufficient. Why not then increase it to 2.2uF? (Otherwise make sure to connect only high impedance loads to your "tape" output. 5. [Warning!] Connect a 100R...1K resistor in series with the grid close to the socket. It will guarantee against parasitic oscillation (in Kolpitz mode) on the Ls and Cs of the circuit, particularly when your input "Balance" pot happens to go to ground. 6. Good thing is that your choice tube has a very common pinout. You can plug almost any twin triode in there and compare results. I believe, ECC85 is a better option. And if you decide to play with high transconductance ones, like 6ES8 (to get very low output impedance on "tape"), you will indeed appreciate advice #5. Regards, Alex "Jon Yaeger" wrote in message ... My latest project is to complete some modifications to an Eico 2080 integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Mikkel Simonsen, formerly of R.A.T. 2. Remove the tone control circuitry; 3. Design a new faceplate for the reduced number of controls. In order to achieve goal #1, I used one of the tone control tube positions as a cathode follower, to drive the 50K motorized volume control. Mikkel's circuitry included a driver for a mute relay. I am housing his circuit inside of a metal box installed under the chassis, to reduce interference as much as possible. A schematic of one channel the proposed design is posted on: http://www.yaegeraudio.com/eico_mod.pdf I welcome any comments / criticisms / suggestions on the proposed design. Thank you. Jon |
#18
Posted to rec.audio.tubes
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Eico Tone Control Mod
Thanks for the interesting info. I guess the pentode provides a nice degree of isolation. There is still the "problem" that a motorized pot would have to be installed. I'm trying to stay with triodes as much as I can in this project. You could have a remote variable DC "power supply" to control the dual control tubes. Or a pot that only handles DC and no audio signal. |
#19
Posted to rec.audio.tubes
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Eico Tone Control Mod
This is how I would be designing that cathode follower:
1. Divide available supply voltage (165V) into three parts, allow 1/3 to drop across the cathode load, and 2/3 -- across the tube itself. It is not a strict rule, but a guideline. If you leave too little across the tube, you will not be able to run a decent current through it and consequently will lose transconductance, hence output impedance on "tape" will be high. If you leave too little on the cathode resistor, it will shunt the output, consequently, gain will drop (from say 0.9 to 0.7, t.h.d. will rise). So let us choose 55V across the cathode rsistor, 110V across the tube. 2. Refer to the tube anode plots or anode-grid plots and find out what plate current could have been with no grid bias for the given plate voltage. In your case, for 6CG7 @Ua=110V and Ug=0 in theory the tube will have Ia=13mA plate current. Divide this value (13mA) by half and let it be your quiescent current. Thus the working point will be in the middle providing maximum dynamic range. So, in your case, let Ia=6.5mA. 3. Calculate the cathode resistor: Rk= 55V / 6.5mA = 8.5K ~ 8.2K. (Note: in your original design you thought it would be 39K -- too much!). 4. Work out grid bias for your tube to run at Ia=6.5mA @ Ua=110V. From the plots for 6CG7: Ug = -2V (approximately). 5. Calculate cathode self bias resistor: Rb= 2V / 6.5mA = 307R ~ 330R (Note: in your design you assumed it to be 1.5K -- too much.) 6. Just for the circuit verification you can assess the output impedance of your cathode follower. According to the plots, transconductance S=2.5mA/V @ 6.5mA. So your output impedance will be around 1/S = 400 ohm. Not bad! Can handle almost any (valve or transistorised) equipment connected to "tape" output! 7. Now -- soldering iron in hand! (Do not forget about the anti-parasitic resistor in series with the grid!) Tube plots can be found on http://frank.pocnet.net/sheets/093/6/6CG7.pdf Regards, Alex "Jon Yaeger" wrote in message ... Great advice! Thank you. When I get some free time I'll incorporate many of these suggestions and revise the design. I'll then post the circuit. Jon in article , Alex at wrote on 11/27/07 6:48 AM: Hello, John. 1. Current through the valve 6CG7 will be lower than 4mA, since a lot will drop across the cathode resistor 39K. Have you considered to reduce the cathode resistor to, say, 10...15K to get higher transconductance oand hence lower output impedance on "tape" output? 2. Beware of the cathode-to-heater voltage. It shall be less than 100V. For that reason also why not reduce the cathode resistor. 3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu of this tube is low (about 20) and it will not give good power supply noise rejection ratio. 4. If your taperecorder connected to "tape" output happens to have relatively low input impedance (say, 10K) you will lose bass, because your interstage capacitor 0.47uF will not be sufficient. Why not then increase it to 2.2uF? (Otherwise make sure to connect only high impedance loads to your "tape" output. 5. [Warning!] Connect a 100R...1K resistor in series with the grid close to the socket. It will guarantee against parasitic oscillation (in Kolpitz mode) on the Ls and Cs of the circuit, particularly when your input "Balance" pot happens to go to ground. 6. Good thing is that your choice tube has a very common pinout. You can plug almost any twin triode in there and compare results. I believe, ECC85 is a better option. And if you decide to play with high transconductance ones, like 6ES8 (to get very low output impedance on "tape"), you will indeed appreciate advice #5. Regards, Alex "Jon Yaeger" wrote in message ... My latest project is to complete some modifications to an Eico 2080 integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Mikkel Simonsen, formerly of R.A.T. 2. Remove the tone control circuitry; 3. Design a new faceplate for the reduced number of controls. In order to achieve goal #1, I used one of the tone control tube positions as a cathode follower, to drive the 50K motorized volume control. Mikkel's circuitry included a driver for a mute relay. I am housing his circuit inside of a metal box installed under the chassis, to reduce interference as much as possible. A schematic of one channel the proposed design is posted on: http://www.yaegeraudio.com/eico_mod.pdf I welcome any comments / criticisms / suggestions on the proposed design. Thank you. Jon |
#20
Posted to rec.audio.tubes
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Working on Classic amps: butchery or necessary updating?
Andre Jute wrote: Robert Casey wrote about Yaeger's Eico wiresnips: At least there'd be no serious criticism that he is hacking up a rare and great piece of equipment... If in fact that it is "el cheapo", and that if he can improve it, then more power to him. I agree. He could learn something. Up to fifteen years ago I thought nothing of rebuilding a QUAD II in my own image but eventually I decided that Peter Walker & Co knew more than I do and restored all mine to original, and over a period of years called in those that had left here modded and did the same restoration to original on them. In my defense I point out that Patrick still blithely alters Quad Ii quite extensively. So now I think how much work you can justifiably do on a "classic" is relevant to the time period in which you do it, and to the original quality of the "classic". It would be a crime to hack up a Quad II or a Radford, but Dynaco and Eico and so on are pretty common and therefore fair game. The Quad II case is confused by the fact that there is a couple of modern factory reissues that include many of the updates I did, and that Patrick now does, so in fact throwing the sanction of official approval over our humble endeavours. Andre Jute Visit Jute on Amps at http://members.lycos.co.uk/fiultra/ "wonderfully well written and reasoned information for the tube audio constructor" John Broskie TubeCAD & GlassWare "an unbelievably comprehensive web site containing vital gems of wisdom" Stuart Perry Hi-Fi News & Record Review I had an enthusiast from the UK send me pics of his efforts to upgrade Quad-II, and all he'd done was replace R&C and fit IEC and better wiring. This does not address the tragic failings in the Walker design. The guy said Quad-II fetch 700 BP which was a lot of money he thought. I said I didn't care if they fetched 7,000 BP, they are still just old junk. If you had a Morris Oxford in mint condition, its still a lousy motor car. I have no idea what the schematic differences are between any Quad-II re-issues, or what the schematic is for Quad 40. There are various ways to improve Quad-II listed at my website pages. I recall about 100,000 Quad-II amps might have been made. If 3 remain in 3 different museums around the world, I am happy for those visiting to see them. Nothing much stops anyone from seriously modding the other 99,997 amps that could be lurking around. Patrick Turner. |
#21
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Eico Tone Control Mod
Alex,
I'm very much obliged for your time and excellent advice! Thank you. Jon in article , Alex at wrote on 11/28/07 6:31 AM: This is how I would be designing that cathode follower: 1. Divide available supply voltage (165V) into three parts, allow 1/3 to drop across the cathode load, and 2/3 -- across the tube itself. It is not a strict rule, but a guideline. If you leave too little across the tube, you will not be able to run a decent current through it and consequently will lose transconductance, hence output impedance on "tape" will be high. If you leave too little on the cathode resistor, it will shunt the output, consequently, gain will drop (from say 0.9 to 0.7, t.h.d. will rise). So let us choose 55V across the cathode rsistor, 110V across the tube. 2. Refer to the tube anode plots or anode-grid plots and find out what plate current could have been with no grid bias for the given plate voltage. In your case, for 6CG7 @Ua=110V and Ug=0 in theory the tube will have Ia=13mA plate current. Divide this value (13mA) by half and let it be your quiescent current. Thus the working point will be in the middle providing maximum dynamic range. So, in your case, let Ia=6.5mA. 3. Calculate the cathode resistor: Rk= 55V / 6.5mA = 8.5K ~ 8.2K. (Note: in your original design you thought it would be 39K -- too much!). 4. Work out grid bias for your tube to run at Ia=6.5mA @ Ua=110V. From the plots for 6CG7: Ug = -2V (approximately). 5. Calculate cathode self bias resistor: Rb= 2V / 6.5mA = 307R ~ 330R (Note: in your design you assumed it to be 1.5K -- too much.) 6. Just for the circuit verification you can assess the output impedance of your cathode follower. According to the plots, transconductance S=2.5mA/V @ 6.5mA. So your output impedance will be around 1/S = 400 ohm. Not bad! Can handle almost any (valve or transistorised) equipment connected to "tape" output! 7. Now -- soldering iron in hand! (Do not forget about the anti-parasitic resistor in series with the grid!) Tube plots can be found on http://frank.pocnet.net/sheets/093/6/6CG7.pdf Regards, Alex "Jon Yaeger" wrote in message ... Great advice! Thank you. When I get some free time I'll incorporate many of these suggestions and revise the design. I'll then post the circuit. Jon in article , Alex at wrote on 11/27/07 6:48 AM: Hello, John. 1. Current through the valve 6CG7 will be lower than 4mA, since a lot will drop across the cathode resistor 39K. Have you considered to reduce the cathode resistor to, say, 10...15K to get higher transconductance oand hence lower output impedance on "tape" output? 2. Beware of the cathode-to-heater voltage. It shall be less than 100V. For that reason also why not reduce the cathode resistor. 3. [Obvious]. Ripple on the 165V supply rail shall be low, since the mu of this tube is low (about 20) and it will not give good power supply noise rejection ratio. 4. If your taperecorder connected to "tape" output happens to have relatively low input impedance (say, 10K) you will lose bass, because your interstage capacitor 0.47uF will not be sufficient. Why not then increase it to 2.2uF? (Otherwise make sure to connect only high impedance loads to your "tape" output. 5. [Warning!] Connect a 100R...1K resistor in series with the grid close to the socket. It will guarantee against parasitic oscillation (in Kolpitz mode) on the Ls and Cs of the circuit, particularly when your input "Balance" pot happens to go to ground. 6. Good thing is that your choice tube has a very common pinout. You can plug almost any twin triode in there and compare results. I believe, ECC85 is a better option. And if you decide to play with high transconductance ones, like 6ES8 (to get very low output impedance on "tape"), you will indeed appreciate advice #5. Regards, Alex "Jon Yaeger" wrote in message ... My latest project is to complete some modifications to an Eico 2080 integrated amplifier, which uses 7591s in PP configuration, and is similar in many respects to the ST-70 circuit. Among my goals we 1. To install a remote volume control, using the very nice kit sold by Mikkel Simonsen, formerly of R.A.T. 2. Remove the tone control circuitry; 3. Design a new faceplate for the reduced number of controls. In order to achieve goal #1, I used one of the tone control tube positions as a cathode follower, to drive the 50K motorized volume control. Mikkel's circuitry included a driver for a mute relay. I am housing his circuit inside of a metal box installed under the chassis, to reduce interference as much as possible. A schematic of one channel the proposed design is posted on: http://www.yaegeraudio.com/eico_mod.pdf I welcome any comments / criticisms / suggestions on the proposed design. Thank you. Jon |
#22
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Eico Tone Control Mod
Thanks, Jon. Sorry for delay, my continuity manager has been
on holiday. Points I would make have been made already. Except, AFAIK (thread's gone from my server), for the issue of bootstrapping. A little discussion on whether to use cathode bias, or fixed, might be interesting. I note that the output impedance for the circuit you have shown increases tenfold if you change the source impedance from a short to an open circuit. This may not matter much, but OTOH you don't appear to need the bootstrapping. What do you think Alex? cheers, Ian |
#23
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Eico Tone Control Mod
"Ian Iveson" wrote in
eyonder.co.uk: X-Newsreader: Microsoft Outlook Express 6.00.2900.2869 (thread's gone from my server) Sure the previous articles will have gone from the server, but why don't you have a threaded saved local version of them? I have messages going back to '02 on this humble PII233 PC, and back further on another PC's newsreader install. They're only text messages;- they don'y take up a lot of room! [Well ... !] But the defaults in many newsreaders (incl Agent which mine is an earlier version of) are set to "purge" after definition which IMO is an insane idea. Well, unnecessary, anyway, if based on threads you are reading or have read. I have no idea how Outlook (Outbreak?) Express handles default purge settings. But I suggest you look into it;- ! |
#24
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Eico Tone Control Mod
Ross wrote:
Sure the previous articles will have gone from the server, but why don't you have a threaded saved local version of them? I have messages going back to '02 on this humble PII233 PC, and back further on another PC's newsreader install. They're only text messages;- they don'y take up a lot of room! [Well ... !] But the defaults in many newsreaders (incl Agent which mine is an earlier version of) are set to "purge" after definition which IMO is an insane idea. Well, unnecessary, anyway, if based on threads you are reading or have read. I have no idea how Outlook (Outbreak?) Express handles default purge settings. But I suggest you look into it;- ! Hi Ross, But... Maybe it was the MI5 posts, or perhaps somewhere there is a buffer of varying capacity, whatever: the thread went away with unexpected speed. And, it will take months, someday, to tie up hundreds of loose ends already in my library of loose ends. Sometimes I think it doesn't matter. Sometimes I'm just lost for words. Other times I'm just too slow to condense too many words into a coherent posting. It would be nice to discuss the cathode follower, and especially this bootstrapping mullarky. By the time I'd checked through my ideas, Alex had dealt with them and the thread seemed to have degenerated into an expert/novice type of exchange. Being neither, I felt estranged. Anyway, what are the ups and downs of cathode or grid bias for a CF, I wonder. A potential divider between HT and ground could be used for the grid. The input impedance could then be 470k, which would be OK with the source of 47k, IIRC. I'm not keen on cathode followers at all, although they are sometimes necessary. I don't see any advantage in unnecessarily complicating their operation. To address your point, ahem, I could buffer a heap of r.a.t locally but then I'd just get slower. It's a motivation thing. I can't help thinking, a bit more each day, that there's no-one here to talk to. Ian |
#25
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Eico Tone Control Mod
"Ian Iveson" wrote
Ross wrote: Sure the previous articles will have gone from the server, but why don't you have a threaded saved local version of them? I have messages going back to '02 on this humble PII233 PC, and back further on another PC's newsreader install. They're only text messages;- they don'y take up a lot of room! [Well ... !] But the defaults in many newsreaders (incl Agent which mine is an earlier version of) are set to "purge" after definition which IMO is an insane idea. Well, unnecessary, anyway, if based on threads you are reading or have read. I have no idea how Outlook (Outbreak?) Express handles default purge settings. But I suggest you look into it;- ! Hi Ross, But... Maybe it was the MI5 posts, or perhaps somewhere there is a buffer of varying capacity, whatever: the thread went away with unexpected speed. Yes;- recently my own isp appears to have much less retention. And, it will take months, someday, to tie up hundreds of loose ends already in my library of loose ends. Well put! I fear I am in a very similiar position ... maybe worse! Sometimes I think it doesn't matter. Sometimes I'm just lost for words. Other times I'm just too slow to condense too many words into a coherent posting. Can relate to that ... It would be nice to discuss the cathode follower, and especially this bootstrapping mullarky. Well, you could start a new thread? By the time I'd checked through my ideas, Alex had dealt with them and the thread seemed to have degenerated into an expert/novice type of exchange. Being neither, I felt estranged. Feelings (and thoughts!) are temporal;- Anyway, what are the ups and downs of cathode or grid bias for a CF, I wonder. A potential divider between HT and ground could be used for the grid. The input impedance could then be 470k, which would be OK with the source of 47k, IIRC. I can't comment, I'm sorry. At this moment! I'm not keen on cathode followers at all, although they are sometimes necessary. I don't see any advantage in unnecessarily complicating their operation. Of course not unnecessarily, but what of any refinement possible? To address your point, ahem, I could buffer a heap of r.a.t locally but then I'd just get slower. It's a motivation thing. I'm already slowed right up. Even so, I mark threads that are or look interesting & download bodies, mark read the non-interesting etc. Sometimes I can change my mind about the latter. Thus at least I have some reading for later when I might have time. This across several groups. I hardly ever have time to post ... (Just as well;- anyway, I am still merely hoarding, pre-constructional;-) I can't help thinking, a bit more each day, that there's no-one here to talk to. Sounds like depression too. Well, maybe that's just me projecting! OTOH perhaps it's not quite intended to be utterly utile for talk as such! You mean 'talk with', of course, anyway ... -- Ian It's a tough life being a bachelor at times, yes;-? Ross -- "Truckin - like the doodah man once told me you got to play your hand sometime - the cards ain't worth a dime if you don't lay em down Sometimes the light's all shining on me Other times I can barely see Lately it occurs to me What a long strange trip it's been" The Grateful Dead, American Beauty, "Truckin". |
#26
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Eico Tone Control Mod
I agree, Ian, a fixed bias is better.
Use (low noise resistors) 1M from grid to ground and 2.2M from grid to supply voltage (165V) and you are in business. One disadvantage is some cold emission field stress while the tube is warming up with the positive grid bias. Another disadvantage -- the circuit will not work if you plug in a low current (high-mu) triode. It might not be able to pass 6.5mA at all, will go into positive grid voltage "saturation". The cathode resistor needs to be recalculated and changed for every tube type. In a bootstrap configuration literally any tube will work (better or worse) without the circuit change. Regards, Alex "Ian Iveson" wrote in message . uk... Thanks, Jon. Sorry for delay, my continuity manager has been on holiday. Points I would make have been made already. Except, AFAIK (thread's gone from my server), for the issue of bootstrapping. A little discussion on whether to use cathode bias, or fixed, might be interesting. I note that the output impedance for the circuit you have shown increases tenfold if you change the source impedance from a short to an open circuit. This may not matter much, but OTOH you don't appear to need the bootstrapping. What do you think Alex? cheers, Ian |
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