Hello people!
I just finished prototyping one of my first tube project.
It's a simple line preamp, made of a cathode follower for each channel, ECC81 by Siemens is the tube. 10k cathode resistor, 220ohm grid stoppers and the usual things. Output cap is a 100uF electrolytic bypassed by a 100nF polyester.
The more interesting part is the PSU. I found 4 220/110V transformers for free, thinking they were autotransformers, useful for powering USA equipment or the like.
Instead they are fully functional transformers, as I measured open connection from the 220V primary to the 110V secondary. So let's use them for an high voltage PSU!
I made a SS voltage doubler with 1N4006 diodes, 2x470uF 200V caps (from old pc smps psu) + 150ohm resistor + 100uF 385V cap bypassed with 10nF cap. This ended out about 322V pretty stable and cool. Of course filaments have a separate 6-0-6 @ 2A tranny.
The question part is: I wanted to add a choke in the psu, because hum was too much, even considering that I build that little thing on a wooden board, no shielding etc etc. So I found a fluorescent lamp reactor that I thought could act like a choke, and I wanted to put it in place of the 150ohm resistor, right?
Wrong. I made a wiring error and I put it after the last filter cap, directly on the plate pins of the ECC81. I thought that before correcting the error, I could see if this worked, and how.
I plugged it in, and it worked, the hum is reduced by a significant amount.
My question is: what's going on in there? What's the role of a choke as a sort of "plate load" in a cathode follower? I mean, no resistor between the choke and the plates...
Or I will gain better hum reduction if I put another cap after the choke?
I just finished prototyping one of my first tube project.
It's a simple line preamp, made of a cathode follower for each channel, ECC81 by Siemens is the tube. 10k cathode resistor, 220ohm grid stoppers and the usual things. Output cap is a 100uF electrolytic bypassed by a 100nF polyester.
The more interesting part is the PSU. I found 4 220/110V transformers for free, thinking they were autotransformers, useful for powering USA equipment or the like.
Instead they are fully functional transformers, as I measured open connection from the 220V primary to the 110V secondary. So let's use them for an high voltage PSU!
I made a SS voltage doubler with 1N4006 diodes, 2x470uF 200V caps (from old pc smps psu) + 150ohm resistor + 100uF 385V cap bypassed with 10nF cap. This ended out about 322V pretty stable and cool. Of course filaments have a separate 6-0-6 @ 2A tranny.
The question part is: I wanted to add a choke in the psu, because hum was too much, even considering that I build that little thing on a wooden board, no shielding etc etc. So I found a fluorescent lamp reactor that I thought could act like a choke, and I wanted to put it in place of the 150ohm resistor, right?
Wrong. I made a wiring error and I put it after the last filter cap, directly on the plate pins of the ECC81. I thought that before correcting the error, I could see if this worked, and how.
I plugged it in, and it worked, the hum is reduced by a significant amount.
My question is: what's going on in there? What's the role of a choke as a sort of "plate load" in a cathode follower? I mean, no resistor between the choke and the plates...
Or I will gain better hum reduction if I put another cap after the choke?
Just wanted to share the improvements...
I put a 160uF 330V (camera flash cap) after the choke, and an elevated reference for the heaters (about 38V, made with a 1Mohm resistor from the B+, 33uF cap and 120K resistor to ground).
Those measures killed the remaining hum: with full volume I have only a faint hiss.
I put a 160uF 330V (camera flash cap) after the choke, and an elevated reference for the heaters (about 38V, made with a 1Mohm resistor from the B+, 33uF cap and 120K resistor to ground).
Those measures killed the remaining hum: with full volume I have only a faint hiss.
Now I've got a real problem.
I get random cracking sound on the left channel: what kind of component faliures may do this? Resistors? Capacitors? in the psu or in the audio path? The tube?
I substituted the tube but it's still there... and 2 times yet the preamp has started a strong humming sound throught the speakers, that cured by simply turning off and back on the preamp.
The only components I'm suspect of are:
- the 2 100uF 25V caps in the audio path, on the cathode of the cathode follower. They have only 7V on them, but...
- a 100K 2W resistor as a bleeder on the B+, after the 2nd filter cap and before the choke. It gets quite hot, but still touchable.
Randomly the cracking sound disappears, like now. It has cracked for 5 minutes, and now it has stopped.
I've already excluded things like the power amp (I'm driving the power stage of a Marantz 1060), cables, interconnections...
Somebody has ideas?
I get random cracking sound on the left channel: what kind of component faliures may do this? Resistors? Capacitors? in the psu or in the audio path? The tube?
I substituted the tube but it's still there... and 2 times yet the preamp has started a strong humming sound throught the speakers, that cured by simply turning off and back on the preamp.
The only components I'm suspect of are:
- the 2 100uF 25V caps in the audio path, on the cathode of the cathode follower. They have only 7V on them, but...
- a 100K 2W resistor as a bleeder on the B+, after the 2nd filter cap and before the choke. It gets quite hot, but still touchable.
Randomly the cracking sound disappears, like now. It has cracked for 5 minutes, and now it has stopped.
I've already excluded things like the power amp (I'm driving the power stage of a Marantz 1060), cables, interconnections...
Somebody has ideas?
I once tried the plate choke thing with the fluorescent ballast and the sound was good but there were cracking noises, they were on like 5 minutes then dissapeared, then back again; really strange. I took the "choke" out and problem resolved, no more noises.
So the choke could be the cause? I will try removing it... maybe I will replace it with a 100ohm resistor.
Btw what's the technical cause for this crackling sound? Is the choke core saturating for DC current? I only have a few mA of load...
Btw what's the technical cause for this crackling sound? Is the choke core saturating for DC current? I only have a few mA of load...
I
I have no idea...i never tried a real choke in its place so i don't know if it was the choke's fault or just the fact that the 6AS7 didn't like that load for unknown reasons. It was a 6AS7 cathode follower line stage. Sound was great much better than with a resistor but in the end i couldn't fix the crackling noise , not until i removed the choke. But i thought to myself one day i shall buy a good choke (i needed more Henries i believe).
I have no idea...i never tried a real choke in its place so i don't know if it was the choke's fault or just the fact that the 6AS7 didn't like that load for unknown reasons. It was a 6AS7 cathode follower line stage. Sound was great much better than with a resistor but in the end i couldn't fix the crackling noise , not until i removed the choke. But i thought to myself one day i shall buy a good choke (i needed more Henries i believe).
Hello people, I replaced the chocke with a 5W 120ohm resistor, and the cracking is still there, just with some more hum.
All the solder joints appear to be well made...
I will recheck anyway.
All the solder joints appear to be well made...
I will recheck anyway.
Hi Giaime,
Odd fault. Try running it in the dark. When it starts cracking, look under and over the chassis for small arcs. Just a thought.
-Chris
Edit: one day I'll learn to spell
Odd fault. Try running it in the dark. When it starts cracking, look under and over the chassis for small arcs. Just a thought.
-Chris
Edit: one day I'll learn to spell
Can you scope the voltage at both ends of the plate resistor <reactor's old location>? Crackling can often be capacitors arcing internally.
Also use a DRY wooden stick and beat/tap on your components... you might find a poor connection INSIDE a compnent... not your soldering.
Also use a DRY wooden stick and beat/tap on your components... you might find a poor connection INSIDE a compnent... not your soldering.
Hello friends, and hello Chris, now His Moderating Majesty 😉
It seems that the crackling sound has been solved. Things I've done:
- substitute the fluorescent lamp choke with a 120ohm resistor, before the last filter cap which goes to the plates. Still cracking, big time hum.
- substituted the 100uF/100nF combo on the cathodes (the output caps) with a parallel of 2 0.47uF polyester caps (big ones rated for AC mains), still cracking but... hey! They sound better 😀
- at this point, I re-put in the choke. Now the psu is: 2 1N4006 as doublers, 2 470uF as voltage doubler caps, 150ohm resistor, 100uF cap (bypassed by 0.47uF polyester, I'm thinking to move this bypass to the LAST filter stage just before the plates), fluorescent lamp choke, 160uF. Still cracking sound but zero hum this time.
- err... I feel so stupid. The cracking sound was made by the old rusty potentiometer I'm using. After some brute force turns it cleaned, and now it's quiet. Also a tube change helped (it's still a Siemens ECC81... I've got a carboard full).
So, another problem solved!
Now I plan to make that bypass cap upgrade on the last filter cap (practically direct on the plates)... and maybe thinking of put the thing in a box (now it's on a wooden board
), or maybe add a phono preamp to it.
Sorry poobah, no scope here. I just found an old one, but it's still in repair. In the meantime I found some beautiful scope test probes... for free! 😀
Now I'd like to hear the opinions. Is a so-simple cathode follower line stage useful? Yes, it beats hands down the pre stage of my Marantz, but... should I add a gain stage in front of it, maybe another ECC81, like the commercial preamps, or I will just degradate the signal?
Or should put my effort in rebuild it in a decent enclosure with quality components?
Or should I try a phono preamp with it? I've seen many designs that run a cathode follower after the RIAA network: can I use the existent cathode follower of my design, and simply put a selector to decide if the input has to go to the cathode follower (for a line level input) or has to go throught the gain stage and the RIAA? Is this doable?
Thanks so much for the help.
Those Siemens flash so brightly at startup... I've read it's normal when they're cold. Btw they're on AC filaments referenced to +30V derived from the B+.
It seems that the crackling sound has been solved. Things I've done:
- substitute the fluorescent lamp choke with a 120ohm resistor, before the last filter cap which goes to the plates. Still cracking, big time hum.
- substituted the 100uF/100nF combo on the cathodes (the output caps) with a parallel of 2 0.47uF polyester caps (big ones rated for AC mains), still cracking but... hey! They sound better 😀
- at this point, I re-put in the choke. Now the psu is: 2 1N4006 as doublers, 2 470uF as voltage doubler caps, 150ohm resistor, 100uF cap (bypassed by 0.47uF polyester, I'm thinking to move this bypass to the LAST filter stage just before the plates), fluorescent lamp choke, 160uF. Still cracking sound but zero hum this time.
- err... I feel so stupid. The cracking sound was made by the old rusty potentiometer I'm using. After some brute force turns it cleaned, and now it's quiet. Also a tube change helped (it's still a Siemens ECC81... I've got a carboard full).
So, another problem solved!
Now I plan to make that bypass cap upgrade on the last filter cap (practically direct on the plates)... and maybe thinking of put the thing in a box (now it's on a wooden board
), or maybe add a phono preamp to it.Sorry poobah, no scope here. I just found an old one, but it's still in repair. In the meantime I found some beautiful scope test probes... for free! 😀
Now I'd like to hear the opinions. Is a so-simple cathode follower line stage useful? Yes, it beats hands down the pre stage of my Marantz, but... should I add a gain stage in front of it, maybe another ECC81, like the commercial preamps, or I will just degradate the signal?
Or should put my effort in rebuild it in a decent enclosure with quality components?
Or should I try a phono preamp with it? I've seen many designs that run a cathode follower after the RIAA network: can I use the existent cathode follower of my design, and simply put a selector to decide if the input has to go to the cathode follower (for a line level input) or has to go throught the gain stage and the RIAA? Is this doable?
Thanks so much for the help.
Those Siemens flash so brightly at startup... I've read it's normal when they're cold. Btw they're on AC filaments referenced to +30V derived from the B+.
Hi Giaime,
Still just plain old me until you see a "cop".
I would use a follower to drive the lower impedance input, helps with cables too. Many older preamps included a follower stage. Follower stage is always in service.
A phono preamp is a good idea, it just drives your line stage through a selector switch. Build it in good with room to add things as you experiment.
Some of the simplist things can drive you crazy trying to find them.
-Chris
Still just plain old me until you see a "cop".
I would use a follower to drive the lower impedance input, helps with cables too. Many older preamps included a follower stage. Follower stage is always in service.
A phono preamp is a good idea, it just drives your line stage through a selector switch. Build it in good with room to add things as you experiment.
Some of the simplist things can drive you crazy trying to find them.
-Chris
Thank you for your suggestions, Chris. You're so helpful, as usual 🙂
So I'm planning to add a phono section to my little preamp. Now, I've got some fixed points:
- I've got 320V B+. I have many "pieces of sand" to regulate it if someone show me a simple schematic of how to do it.
- I've got 2A heater transformer. Not more. So another tube isn't a problem.
- I'd like to use just ONE tube for the phono gainstage, a section for each channel. I've got several ECC81 (Siemens), an ECC82 (Siemens, beautiful shiny silver plates, never seen this 😱), and some ECC83 (Siemens, Sylvania).
Some suitable schematics? Remember, I want to use just one more tube. If it's completely out of question... well... I can add another more tube, but if I can do everything with 2 tubes it would be great 😀
Thanks to all!
So I'm planning to add a phono section to my little preamp. Now, I've got some fixed points:
- I've got 320V B+. I have many "pieces of sand" to regulate it if someone show me a simple schematic of how to do it.
- I've got 2A heater transformer. Not more. So another tube isn't a problem.
- I'd like to use just ONE tube for the phono gainstage, a section for each channel. I've got several ECC81 (Siemens), an ECC82 (Siemens, beautiful shiny silver plates, never seen this 😱), and some ECC83 (Siemens, Sylvania).
Some suitable schematics? Remember, I want to use just one more tube. If it's completely out of question... well... I can add another more tube, but if I can do everything with 2 tubes it would be great 😀
Thanks to all!
To get a phono signal (on the order of a millivolt) up to line level (on the order of a volt) will take 60dB of gain. That's going to be tough with one tube. Three is more likely.
Giaime said:
look for RCA phono stage
one ECC83 p/ch.
http://www.google.com/search?as_q=&...as_dt=i&as_sitesearch=&as_rights=&safe=images
Something like this is out of question?
Or this:
Maybe this:
This?
And this? Is someone against NFB?
Also note that I've got a couple of GE EF86: I haven't been able to find a EF86 based design, but I know they exist...
Or this:
Maybe this:
This?
And this? Is someone against NFB?
Also note that I've got a couple of GE EF86: I haven't been able to find a EF86 based design, but I know they exist...
Hi Giaime,
Build them to see what you like. The single section will be too variable depending on tube characteristics.
-Chris
Build them to see what you like. The single section will be too variable depending on tube characteristics.
-Chris
Yeah, I thought so.
I like more this one, for it's simplicity:
http://www.diyaudio.com/forums/attachment.php?s=&postid=335624&stamp=1077837811
but this one has the same B+ rail voltage as my preamp, so I don't have to recalculate things:
http://www.diyaudio.com/forums/attachment.php?s=&postid=613068&stamp=1112741651
Someone care to express?
I like more this one, for it's simplicity:
http://www.diyaudio.com/forums/attachment.php?s=&postid=335624&stamp=1077837811
but this one has the same B+ rail voltage as my preamp, so I don't have to recalculate things:
http://www.diyaudio.com/forums/attachment.php?s=&postid=613068&stamp=1112741651
Someone care to express?
As any owner of a Leak preamp will attest, the EF86 is far too noisy for hifi performance in a phono stage.
The two tube RCA circuit (and, for that matter, the two tube Dynaco circuit) were not very good as regards distortion, conformance to RIAA, stability of RIAA curve, source impedance, slewing, overload recovery, and drive capability. They were designed with one critereon in mind: cheap. The circuit was improved somewhat by Marantz and Audio Research by adding a buffer, bringing the tube count to three.
The two tube RCA circuit (and, for that matter, the two tube Dynaco circuit) were not very good as regards distortion, conformance to RIAA, stability of RIAA curve, source impedance, slewing, overload recovery, and drive capability. They were designed with one critereon in mind: cheap. The circuit was improved somewhat by Marantz and Audio Research by adding a buffer, bringing the tube count to three.
Thank you for pointing out those considerations, SY. I appreciate them very much.
So, let's exclude the EF86. From the two options in my last post, what would you choose? Or are they too similar to the RCA circuit that is to be avoided?
Btw, I still get some random cracking/frying on my left channel. I'm planning to replace the potentiometer, but I'm not sure it's actually the pot...
Thanks to everyone for the help!
So, let's exclude the EF86. From the two options in my last post, what would you choose? Or are they too similar to the RCA circuit that is to be avoided?
Btw, I still get some random cracking/frying on my left channel. I'm planning to replace the potentiometer, but I'm not sure it's actually the pot...
Thanks to everyone for the help!
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