that circuit sure went 'round big time -
Marantz 7, Dyna PAS, CJ-whatever- I never see Chinese clones of Stu Hegeman's approach :12ax7 feedback pair driving lumped passive RIAA eq (Citation I & IV) nor Al Hart's Grommes cascode 12ax7 -Trevor Lees/Precision fidelity) - btw its interesting that Hegeman and Hart went "forwards" with IC based phono preamps.
Marantz 7, Dyna PAS, CJ-whatever- I never see Chinese clones of Stu Hegeman's approach :12ax7 feedback pair driving lumped passive RIAA eq (Citation I & IV) nor Al Hart's Grommes cascode 12ax7 -Trevor Lees/Precision fidelity) - btw its interesting that Hegeman and Hart went "forwards" with IC based phono preamps.
Attachments
Power supply output impedance plus RC decouplers I think can be used to influence the way a typical tube phono circuit sounds.
Perhaps tricks such as diode biasing. (IIRC the Van Alstine circuit took on a hard sound). A first stage RC decoupler with small cap was nice on my Precision Fidelity C7 as was a Zener referenced TIP50 vs whatever IC chip Bruce moore picked. (Cartridge was Sonus Blue- speakers - Magnepan MG1-IMP)
I wouldn't worry much about rolling a 6922 cathode follower - unless the present tube was old and worn-out. Also ideally a CF should not see a lower Z load than plate follower - but at small swings seen to drive a power amp - won't matter.
Perhaps tricks such as diode biasing. (IIRC the Van Alstine circuit took on a hard sound). A first stage RC decoupler with small cap was nice on my Precision Fidelity C7 as was a Zener referenced TIP50 vs whatever IC chip Bruce moore picked. (Cartridge was Sonus Blue- speakers - Magnepan MG1-IMP)
I wouldn't worry much about rolling a 6922 cathode follower - unless the present tube was old and worn-out. Also ideally a CF should not see a lower Z load than plate follower - but at small swings seen to drive a power amp - won't matter.
Last edited:
So, because of the negative feedback, a brand new New Sensor 6922 and a 1960 NOS tube blessed by the Pope would sound exactly the same. What about a 6dg8 or other variant? Would there be variations in performance if a non-specified type was used?
In principle gain stages and NFB work that way.
That´s what it was invented for.
Only way it "fails" (not its fault really) is when tube gain is so abismally low (VERY worn tube or improper type selected) that open loop gain is not enough to sustain it.
NFB works by having extra gain on tap, and "wasting" that surplus to correct errors.
But in a properly designed circuit, with competent tubes, there will be little to no tube to tube variations.
Again, that is the purpose of NFB.
That´s what it was invented for.
Only way it "fails" (not its fault really) is when tube gain is so abismally low (VERY worn tube or improper type selected) that open loop gain is not enough to sustain it.
NFB works by having extra gain on tap, and "wasting" that surplus to correct errors.
But in a properly designed circuit, with competent tubes, there will be little to no tube to tube variations.
Again, that is the purpose of NFB.
But in a circuit without negative feedback, there would be measurable variation between tubes, but it would be essentially random? There wouldn't be common variations between tubes made at different factories?
The same electrical behavior in each individual tube stage remains, with or without global feedback.
Rather, the effective input voltage to the entire circuit is modified by the feedback to compensate
for differences between the input and output of the entire circuit.
Each stage still behaves the same, with or without the NFB. The input voltage (and so also the output voltage)
of each stage does change because of the feedback. But the individual stage output/input relationship
(ideally) remains the same, with or without global feedback.
Global NFB tube phono stages have real limitations in their accuracy due to low gain, especially under 100Hz.
Most mass produced parts, like tubes, resistors, etc. are never individually tested by the factory. Too expensive.
Sometimes a relatively small number of sample parts are tested to verify process control.
Rather, the effective input voltage to the entire circuit is modified by the feedback to compensate
for differences between the input and output of the entire circuit.
Each stage still behaves the same, with or without the NFB. The input voltage (and so also the output voltage)
of each stage does change because of the feedback. But the individual stage output/input relationship
(ideally) remains the same, with or without global feedback.
Global NFB tube phono stages have real limitations in their accuracy due to low gain, especially under 100Hz.
Most mass produced parts, like tubes, resistors, etc. are never individually tested by the factory. Too expensive.
Sometimes a relatively small number of sample parts are tested to verify process control.
Last edited:
Which works out to mean any minor deviations in tube performance get “flattened out” by the negative feedback loop, right?
No, it's the entire circuit's output that is altered by GNFB. Each tube stage still does exactly the same thing, either way.
It's the input signal that is altered to compensate (and hence the output signal is altered as well), not the performance
of any individual tube stage, which remains the same.
It's sort of like if you have a power amp with a bass boost, and you set your preamp to cut the bass to compensate.
Your ear/hand is the feedback. Except that an electrical feedback system can do that much more precisely than
you can by turning a knob, and also can correct it automatically in real time if the amplifier's bass boost changes.
It's the input signal that is altered to compensate (and hence the output signal is altered as well), not the performance
of any individual tube stage, which remains the same.
It's sort of like if you have a power amp with a bass boost, and you set your preamp to cut the bass to compensate.
Your ear/hand is the feedback. Except that an electrical feedback system can do that much more precisely than
you can by turning a knob, and also can correct it automatically in real time if the amplifier's bass boost changes.
Last edited:
Doesn't that work out to the same thing, that deviations induced by the tubes get canceled out by the feedback?
Individual tube variations are "nice to know" but YOU will hear the full tube preamp, you will not grab little crocodile clips and listen point by point "inside" the circuit.
Definitely not the way to use a preamp, unless OCD hits hard.
Hope not 🙂
Definitely not the way to use a preamp, unless OCD hits hard.
Hope not 🙂
If it is working good and sounding good, leave it alone.
Enjoy.
If it does not sound good, like noisy, distorted, etc. then take it to a good repair house.
If you do not like it because it sounds bright; or bloated bass, or no midrange, etc. then check the rest of your system . . .
All the way from the phono cartridge and tone arm, some Vinyl recordings, and through the preamp and amp, to the loudspeakers, room, and speaker placement in the room, as well as the listening position.
Unless I missed a detail on one of the audio tubes, all of them are self biased, which helps to reduce the differences of quiescent current of different tubes (I did not check the power supply tubes).
Also, this is a very complicated preamp.
It has a lot of switches, a lot of potentiometer wipers, and a lot of tube sockets.
As the amplifier ages you may need to deal with the possibility that it will need to be serviced.
Complex is great; Simplicity is more reliable.
Compare a get-around-town Toyota, to a Lamborghini.
Just my opinions.
Enjoy.
If it does not sound good, like noisy, distorted, etc. then take it to a good repair house.
If you do not like it because it sounds bright; or bloated bass, or no midrange, etc. then check the rest of your system . . .
All the way from the phono cartridge and tone arm, some Vinyl recordings, and through the preamp and amp, to the loudspeakers, room, and speaker placement in the room, as well as the listening position.
Unless I missed a detail on one of the audio tubes, all of them are self biased, which helps to reduce the differences of quiescent current of different tubes (I did not check the power supply tubes).
Also, this is a very complicated preamp.
It has a lot of switches, a lot of potentiometer wipers, and a lot of tube sockets.
As the amplifier ages you may need to deal with the possibility that it will need to be serviced.
Complex is great; Simplicity is more reliable.
Compare a get-around-town Toyota, to a Lamborghini.
Just my opinions.
Last edited:
I expect these might go bad:
Switches, Potentiometer wipers, tube Sockets, Electrolytic caps, Coupling caps.
Switches, Potentiometer wipers, tube Sockets, Electrolytic caps, Coupling caps.
Switches and pots seem straight forward enough. I should be able to tell when they fail.
In what way would a tube socket fail? Not grip the tube pins properly?
Electrolytics make sense. Every one I’ve tested so far is within 10% or so, so seems good so far.
Why do you call out coupling caps specifically?
In what way would a tube socket fail? Not grip the tube pins properly?
Electrolytics make sense. Every one I’ve tested so far is within 10% or so, so seems good so far.
Why do you call out coupling caps specifically?
Any electronic circuit should be designed to cope with individual component deviations, no matter what.
It's about reliability actually. Save the DIY-folks, the rest of the world population press the switch and expects the miracle happen.
In our time now, quality continuation seems the challenge.
It's about reliability actually. Save the DIY-folks, the rest of the world population press the switch and expects the miracle happen.
In our time now, quality continuation seems the challenge.
SoaDMTGuy,
I am not a statistician.
But I read a lot on Tubes / Valves, and guess what are the problems . . .
Switches, pots, tube sockets, electrolytics, coupling caps, oh . . . and tubes.
Some other problems are poorly designed amps that never worked properly in the first place.
I am not a statistician.
But I read a lot on Tubes / Valves, and guess what are the problems . . .
Switches, pots, tube sockets, electrolytics, coupling caps, oh . . . and tubes.
Some other problems are poorly designed amps that never worked properly in the first place.
Oh that's not what I think of as tube rolling. Different brands of the same tube are just the same tube with negligible differences. You'll be listening for the number of angels dancing on a pin. Go ahead.
You won't be able to source "closely matched" pairs. For some reason all tube dealers (at least the ones I have seen offer tubes, maybe that was just the minority) are proudly using outdated emission testers like a Hickock TV7. They match at one single operating point. If that is the same operating point the tubes run at in your appliance you are good. If not the tubes might not be that matched anymore.
So if you feel a "close match" is necessary, then buy ten tubes and an eTracer or measure them in circuit to match them.
Also be aware, that brand new tubes need a few hours (about 2-4) for emission of the cathodes to stabilise. Which means current draw goes up. When that state is reached you can attempt to do a "close match".
SoaDMTGguy,
Why you are changing the tubes, comparing essentially the same types?
Are you listening for the differences of sound?
Are you testing your hearing to check how much resolution it has?
Most of the circuits in your preamp use local negative or 2 stage negative feedback.
That, plus the self biasing tends to make the sound pretty much the same.
What is your ultimate goal for that preamp?
Are you satisfied with it?
This is Post # 39.
The thread has been wandering all over the map.
As the original poster, you can help us hone in on what is desired.
We need to work toward a goal, and toward a resolution.
Just my opinions
Why you are changing the tubes, comparing essentially the same types?
Are you listening for the differences of sound?
Are you testing your hearing to check how much resolution it has?
Most of the circuits in your preamp use local negative or 2 stage negative feedback.
That, plus the self biasing tends to make the sound pretty much the same.
What is your ultimate goal for that preamp?
Are you satisfied with it?
This is Post # 39.
The thread has been wandering all over the map.
As the original poster, you can help us hone in on what is desired.
We need to work toward a goal, and toward a resolution.
Just my opinions
My goals are to explore sonic changes between compatible tubes.
Factor 1: Will replacing the original, used tubes of unknown lifespan with new (either new production or NOS) have an audible difference?
Factor 2: Will changing tubes between different manufacturers or vintages have an audible difference?
There is much “lore” about tubes, brands, and tonality, which I wanted to explore. I have been reading the descriptions for various 6dj8-family tubes here: http://www.audiotubes.com/6dj8.htm
I have the sense that much of this “tube lore” is nonsense, but I want to gain a better overall understanding, either through reading everyone’s comments here, or actually swapping tubes around, or both.
Factor 1: Will replacing the original, used tubes of unknown lifespan with new (either new production or NOS) have an audible difference?
Factor 2: Will changing tubes between different manufacturers or vintages have an audible difference?
There is much “lore” about tubes, brands, and tonality, which I wanted to explore. I have been reading the descriptions for various 6dj8-family tubes here: http://www.audiotubes.com/6dj8.htm
I have the sense that much of this “tube lore” is nonsense, but I want to gain a better overall understanding, either through reading everyone’s comments here, or actually swapping tubes around, or both.