Hi !
Interesting to see that you achieved a good sound at this operating point. I didn't get good results when operating in the "knee" of the loadline.
Here is the loadline of my 6J5 µ-follower stage
Sound is superb, but µ ist still a bit high (because of near infinite load, the mu is near to the µ of the tube)
But there are very few tubes with less µ available (ECC82 has lots of distortion, no way with that)
So I will try with feedback.
Using a 6SN7/7N7/6CG7/6FQ7/6J5 as cathode follower is a bit supoptimal because it doesn't have a high gm. And output impedance of a CF is 1/gm.
the upper Tube of my µ-follower is a EL802 with 40mA/V gm. Gives 25Ohms output resistance. At the right you see the 6J5 (A rather rare german one. I have no USA made ones 🙁 )
Regards, Simon
Interesting to see that you achieved a good sound at this operating point. I didn't get good results when operating in the "knee" of the loadline.
Here is the loadline of my 6J5 µ-follower stage
An externally hosted image should be here but it was not working when we last tested it.
Sound is superb, but µ ist still a bit high (because of near infinite load, the mu is near to the µ of the tube)
But there are very few tubes with less µ available (ECC82 has lots of distortion, no way with that)
So I will try with feedback.
Using a 6SN7/7N7/6CG7/6FQ7/6J5 as cathode follower is a bit supoptimal because it doesn't have a high gm. And output impedance of a CF is 1/gm.
the upper Tube of my µ-follower is a EL802 with 40mA/V gm. Gives 25Ohms output resistance. At the right you see the 6J5 (A rather rare german one. I have no USA made ones 🙁 )
An externally hosted image should be here but it was not working when we last tested it.
Regards, Simon
As small signal amplification tubes go, you almost could not have picked two more different ones.
6SN7 uses high voltages and moderate currents, and is very linear under those conditions.
6DJ8 is a low voltage high GM tube which also happens to have a signifficantly higher mu than the 6SN7, and is great for followers. Without proper RF(like) precautions it is VERY likely to oscilate producing all sorts of audible artifacts. 6DJ8 on 'flying leads' prototypes is unlikely to show it's potential. If you omit the gate damping resistor, it is practically certain to sound bad.
For a line preamp, 6SN7 is already on the top end with it's mu, the 6DJ8 really requires some sort of feedback to get gain down to useful levels in a line preamp - so even there the philosophy of the design would be very different.
6SN7 uses high voltages and moderate currents, and is very linear under those conditions.
6DJ8 is a low voltage high GM tube which also happens to have a signifficantly higher mu than the 6SN7, and is great for followers. Without proper RF(like) precautions it is VERY likely to oscilate producing all sorts of audible artifacts. 6DJ8 on 'flying leads' prototypes is unlikely to show it's potential. If you omit the gate damping resistor, it is practically certain to sound bad.
For a line preamp, 6SN7 is already on the top end with it's mu, the 6DJ8 really requires some sort of feedback to get gain down to useful levels in a line preamp - so even there the philosophy of the design would be very different.
the_manta said:
It's only suboptimal when driving finals into Class AB2. Then, if I were doing that, I'd use a MOSFET anyway. The 6SN7 & Co. works just fine as a cathode follower driver to allow for DC coupling to the grids with fixed bias, and to supply enough charging current for the input capacitance, as most audio pentodes really aren't such a difficult load anyway.
Sometimes you need to ignore sweeping generalizations such as "tubes like 6922 are not good for sound". My (very humble) $0.02 is that from my meager experience, both a 6SN7 and 6922... as well as more maligned triodes such as the 12ax7 and the 12at7... can sound fantastic (to my ear) under the right conditions, but the 6SN7 seems to be more forgiving of boneheads like me, in that is sounds good under a wider range of conditions, wheras the other three are rather picky about how you run them and vary more in charactaristics from tube-to-tube, even though can sound just as good if you find a sweet spot to nestle them into.
aletheian said:
I always disregard audiophool folk "wisdom"; there's all too much of of that out there. Like right now, my current project is using the very much maligned Tee-Vee t00bz
that I can pick up for about $3.00 a pop, and it's sounding mighty fine. Certainly as good as anything that'd use more co$tly audiophool VTs. Put forth a little effort, and you can usually find a good loadline, and/or topology that'll make a bigger selection of VTs sound really good than you might guess. So far, I don't have a need for 12AX7s or 12AT7s, but I wouldn't hesitate to try 'em if I figured they'd get the job done.
Hi Miles,
I have some older preamplifiers that use 12AX7's and 12AT7's. They sound better than many newer 6DJ8 type units. What can I say??
I guess the older (real) engineers actually knew what they were doing. I have a world of faith in Morgan Jones also. I will most probably wear his book out before I'm done with it.
-Chris
I have some older preamplifiers that use 12AX7's and 12AT7's. They sound better than many newer 6DJ8 type units. What can I say??
I guess the older (real) engineers actually knew what they were doing. I have a world of faith in Morgan Jones also. I will most probably wear his book out before I'm done with it.
-Chris
It seems that a lot of people think that there is one simple solution to all problems, and then report disappointment when this fails to produce the goodies.
I have recently learn't that with certain tubes flying leads of any kind will ruin the sound, this knowledge cleared up a lot of confusion for me. Inattention to detail probably accounts for a lot of those reports of ECC88, and such like, sounding tizzy and harsh. This is a hard lesson that only experience and a good scope can teach.
Shoog
I have recently learn't that with certain tubes flying leads of any kind will ruin the sound, this knowledge cleared up a lot of confusion for me. Inattention to detail probably accounts for a lot of those reports of ECC88, and such like, sounding tizzy and harsh. This is a hard lesson that only experience and a good scope can teach.
Shoog
my big reason for not using 6SN7s or 6922s is the price! The good NOS stuff has cranked up in value over the years.
Having said that, I've used 6922s for a White Cathode Follower with good results.
Having said that, I've used 6922s for a White Cathode Follower with good results.
6sn7 vs E88cc
I agree, many newer components using 6dj8s are poor sounding. ^sn7s are generally 'safer', as described below as it is hard to do anything wrong. But then I would also say it is hard to make it sound the best in the world as well.
Anyway, here is some info for any marketeers and scam artists out there propagating false information.
We have seen the hype, whether DIY or manufacturer, that a design is inferior if the 6sn7 is not used? Has X out there, who teaches such done his homework, or is it to corner the market?
I know one gent who was pushing a particular power tube for years; now he is the "Executive Director - Technology" for a hybrid solid state amplifier company.
Continued, what caps and resistors were used? What were the value of the parts and what design was used?
It is true that the higher the frequency response (figure of merit) a tube can reproduce, the more experience and knowledge is required in designing a good circuit, but the results are better.
Let's take a look at the 6sn7. It is not a bad tube, but neither is it world class tube. It can hardly keep its hf response in a basic circuit. Add even the lowest capacitance Ic and the highs deteriorate (actual listening tests). It is a 'safe' and easy tube to design around, hard to do things wrong.
However, as just about everyone knows by now, the 6sn7 needs some sort of buffer design to support its hf response. So feedback is present, even though localized. But I thought feedback was bad according to another forum?
If one uses two or more stages, with a common power supply, we have a frequency dependent feedback problem from one stage to the others through the power supply. So the sound has been degraded even more.
(Don't let them state they solved it, or it isn't a problem. That is just typical marketing hype. They have a problem, especially with integrated amps.)
As one can see, 6sn7 gainstages have some real problems. Poor frequency response, frequency dependent feedback through the power supply (if two or more actual stages), and negative feedback in the buffer stage.
None of those problems exist when using low Rp, high frequency tubes, including the E88cc.
By the way, be careful of capacitor tests in circuits as well. One used a 0.1 value coupling cap, which cannot possibly be accurate.
I agree, many newer components using 6dj8s are poor sounding. ^sn7s are generally 'safer', as described below as it is hard to do anything wrong. But then I would also say it is hard to make it sound the best in the world as well.
Anyway, here is some info for any marketeers and scam artists out there propagating false information.
We have seen the hype, whether DIY or manufacturer, that a design is inferior if the 6sn7 is not used? Has X out there, who teaches such done his homework, or is it to corner the market?
I know one gent who was pushing a particular power tube for years; now he is the "Executive Director - Technology" for a hybrid solid state amplifier company.
Continued, what caps and resistors were used? What were the value of the parts and what design was used?
It is true that the higher the frequency response (figure of merit) a tube can reproduce, the more experience and knowledge is required in designing a good circuit, but the results are better.
Let's take a look at the 6sn7. It is not a bad tube, but neither is it world class tube. It can hardly keep its hf response in a basic circuit. Add even the lowest capacitance Ic and the highs deteriorate (actual listening tests). It is a 'safe' and easy tube to design around, hard to do things wrong.
However, as just about everyone knows by now, the 6sn7 needs some sort of buffer design to support its hf response. So feedback is present, even though localized. But I thought feedback was bad according to another forum?
If one uses two or more stages, with a common power supply, we have a frequency dependent feedback problem from one stage to the others through the power supply. So the sound has been degraded even more.
(Don't let them state they solved it, or it isn't a problem. That is just typical marketing hype. They have a problem, especially with integrated amps.)
As one can see, 6sn7 gainstages have some real problems. Poor frequency response, frequency dependent feedback through the power supply (if two or more actual stages), and negative feedback in the buffer stage.
None of those problems exist when using low Rp, high frequency tubes, including the E88cc.
By the way, be careful of capacitor tests in circuits as well. One used a 0.1 value coupling cap, which cannot possibly be accurate.
Re: 6sn7 vs E88cc
Hi,
Tell me about it 🙄
Most likely because they don't throttle them hot enough and end up wasting all that gm.
Though you can get away with running them cool if cathode driven by a pretty good gm tube in cascode.
Cheers!
Hi,
Positron said:
Tell me about it 🙄
Most likely because they don't throttle them hot enough and end up wasting all that gm.
Though you can get away with running them cool if cathode driven by a pretty good gm tube in cascode.
Cheers!
Steve, it's a matter of application. If I had to swing 70V, the 6SN7 would be one of the first tubes I'd reach for (large signal linearity is quite good) and the ECC88 one of the last. If I want a cathode follower for a preamp where the swing is 2-4V, the 6SN7 would not be optimum, but an ECC88 would be close.
These are just very different tubes with very different optimum uses.
These are just very different tubes with very different optimum uses.
No thanks
"Steve, it's a matter of application. If I had to swing 70V, the 6SN7 would be one of the first tubes I'd reach for (large signal linearity is quite good) and the ECC88 one of the last. If I want a cathode follower for a preamp where the swing is 2-4V, the 6SN7 would not be optimum, but an ECC88 would be close.
These are just very different tubes with very different optimum uses."
The problem with that scenario is that the same problems exist using a 6sn7 tube at 70 volts p-p to drive output tubes as it does driving the IC capacitance in a preamp. The Rp is just too high. The input capacitance of the ouput tubes (includes miller capacitance plus stray capacitance) weighs the hf down. Plus the distortion is too high. I don't believe in adding artificial colorations.
There are other tubes such as the 6H30, 5842 etc, for large signals, that will keep the HF response up. If one likes a diffy circuit with these tubes, one can obtain the same gain, get vanishing distortion levels, keep the hf response up much better than a typical 6sn7 stage. Adding another stage just deteriorates the sound even more.
By the way, I have been using a diffy circuit with E88cc tubes in my experimental amp. I get 100 vp-p with less than 0.05% distortion. And the tube has lasted for years. No typical 6sn7 gainstage will come close to that, and going diffy with a 6sn7 lowers the gain to very low levels.
As far as a preamp gain tube, I just would not use a 6sn7. Been there, done that. In fact, I still have some nos from a kid. It 'colors' the sound because of its poor high frequency response, high distortion etc. And adding a buffer stage just makes the sound worse.
I will say thanks but no thanks.
ps. I love your signature. "Communal reinforcement". That is exactly what has happened to the 6sn7 in the audio community, whether true or not.
"Steve, it's a matter of application. If I had to swing 70V, the 6SN7 would be one of the first tubes I'd reach for (large signal linearity is quite good) and the ECC88 one of the last. If I want a cathode follower for a preamp where the swing is 2-4V, the 6SN7 would not be optimum, but an ECC88 would be close.
These are just very different tubes with very different optimum uses."
The problem with that scenario is that the same problems exist using a 6sn7 tube at 70 volts p-p to drive output tubes as it does driving the IC capacitance in a preamp. The Rp is just too high. The input capacitance of the ouput tubes (includes miller capacitance plus stray capacitance) weighs the hf down. Plus the distortion is too high. I don't believe in adding artificial colorations.
There are other tubes such as the 6H30, 5842 etc, for large signals, that will keep the HF response up. If one likes a diffy circuit with these tubes, one can obtain the same gain, get vanishing distortion levels, keep the hf response up much better than a typical 6sn7 stage. Adding another stage just deteriorates the sound even more.
By the way, I have been using a diffy circuit with E88cc tubes in my experimental amp. I get 100 vp-p with less than 0.05% distortion. And the tube has lasted for years. No typical 6sn7 gainstage will come close to that, and going diffy with a 6sn7 lowers the gain to very low levels.
As far as a preamp gain tube, I just would not use a 6sn7. Been there, done that. In fact, I still have some nos from a kid. It 'colors' the sound because of its poor high frequency response, high distortion etc. And adding a buffer stage just makes the sound worse.
I will say thanks but no thanks.
ps. I love your signature. "Communal reinforcement". That is exactly what has happened to the 6sn7 in the audio community, whether true or not.
anatech said:
Hi Chris. I would quibble with that as a universal. The Scott LT-110B currently occupying my evenings shipped with a 12AT7 common cathode output, 68k on the plate and around 600 uA standing bias. The GE curves indicate a plate impedance of ~40 kohms pre-feedback. Scott used ~10 dB direct from plate to grid to ammeliorate some of consequences. No wonder the recommended minimum load is 100k!
At least solid state makes things a whole helluvalot easier. The gain of a BJT is so high that how it behaves is determined by the circuit externals, and not the device itself. So long as the BJT can function at whatever frequencies it is to process, the type selected doesn't make any difference, unless you need some special property, like an unusually low noise figure or something.
It's a case of trendiness in action. 6SN7s, 12AX7s, 12AT7s: bad! 6DJ8: good! However, what's really important is using the right VT for the right job. Yeah, they've been saying that 6SN7s aren't good because the r(p) is too high. Is it really? Do you really need a DC to daylight passband? If you want that, skip triodes altogether, use something like the 12BY7A heavily loaded and with HF compensation coils to push the HF cutoff above 1.0MHz. Unless it's a vid amp, you don't see that in audio circuits. Although now that I mentioned it, I suppose this will become the next great trend? 😀
Or how about putting that EVIIIIIIIIIILLLLLLLLL NFB to good use for what it was intended: to help make performance less dependent on the active devices, and more dependent on circuit externals?
It's a case of trendiness in action. 6SN7s, 12AX7s, 12AT7s: bad! 6DJ8: good! However, what's really important is using the right VT for the right job. Yeah, they've been saying that 6SN7s aren't good because the r(p) is too high. Is it really? Do you really need a DC to daylight passband? If you want that, skip triodes altogether, use something like the 12BY7A heavily loaded and with HF compensation coils to push the HF cutoff above 1.0MHz. Unless it's a vid amp, you don't see that in audio circuits. Although now that I mentioned it, I suppose this will become the next great trend? 😀
Or how about putting that EVIIIIIIIIIILLLLLLLLL NFB to good use for what it was intended: to help make performance less dependent on the active devices, and more dependent on circuit externals?
12a^7, 6sn7
The 12at7 is a very high distortion producer, as well as limited bandwidth. Take a look at the curves from the links below. And actual measurements have been performed.
http://www.tubezone.net/pdf/12at7ecc81.pdf
http://www.triodeel.com/12at7p3.gif
http://www.pentalaboratories.com/pdfs/ECC81.pdf
The grid to plate capacitance is approx 1.6pf. Factor a gain of approx 40 and the miller capacitance is approx. 64pf. This is higher than low capacitance ICs. Any IC will adversely affect the highs as well as any output tube (triode in all cases) capacitance.
A 12ax7
http://www.nj7p.org/Tube4.php?tube=12AX7A
has a grid to plate capacitance of 1.7pf. At 60 gain, the miller is 102pf. It almost drowns in its own capacitance. Don't even think of adding an IC let alone the capacitance of the output triode.
A 12au7
http://www.nj7p.org/Tube4.php?tube=12AU7A
has a grid to plate capacitance of 1.5pf. For 10 gain, the miller is only 15pf. Its transconductance is higher, but the Rp is still around 6k, give or take. Again, any IC or output triode will affect the highs.
A 6sn7
http://www.nj7p.org/Tube4.php?tube=6SN7GTA
has a grid to plate capacitance of 4 pf. For 10 gain, the miller is 40pf. Its transconductance is higher, but the Rp is still around 7k, give or take. Again, any IC or output triode will affect the highs.
The 12at7 is a very high distortion producer, as well as limited bandwidth. Take a look at the curves from the links below. And actual measurements have been performed.
http://www.tubezone.net/pdf/12at7ecc81.pdf
http://www.triodeel.com/12at7p3.gif
http://www.pentalaboratories.com/pdfs/ECC81.pdf
The grid to plate capacitance is approx 1.6pf. Factor a gain of approx 40 and the miller capacitance is approx. 64pf. This is higher than low capacitance ICs. Any IC will adversely affect the highs as well as any output tube (triode in all cases) capacitance.
A 12ax7
http://www.nj7p.org/Tube4.php?tube=12AX7A
has a grid to plate capacitance of 1.7pf. At 60 gain, the miller is 102pf. It almost drowns in its own capacitance. Don't even think of adding an IC let alone the capacitance of the output triode.
A 12au7
http://www.nj7p.org/Tube4.php?tube=12AU7A
has a grid to plate capacitance of 1.5pf. For 10 gain, the miller is only 15pf. Its transconductance is higher, but the Rp is still around 6k, give or take. Again, any IC or output triode will affect the highs.
A 6sn7
http://www.nj7p.org/Tube4.php?tube=6SN7GTA
has a grid to plate capacitance of 4 pf. For 10 gain, the miller is 40pf. Its transconductance is higher, but the Rp is still around 7k, give or take. Again, any IC or output triode will affect the highs.
hf response
Hi Miles,
"Is it really? Do you really need a DC to daylight passband?"
Actually one needs to go much further than 20khz. Even the radiotron designers handbook deals with problems of response of amplifiers well above 20khz.
Any impulse reponse needs to be at least 10 times higher than the fundamental. Cymbals are a particular case. over 40% of its energy is above 20khz. 60% is below 20khz of course. To preserve what we hear, the response cannot barely be 20khz.
Try messing around with the electronics at 50khz or higher and hear what happens.
Hi Miles,
"Is it really? Do you really need a DC to daylight passband?"
Actually one needs to go much further than 20khz. Even the radiotron designers handbook deals with problems of response of amplifiers well above 20khz.
Any impulse reponse needs to be at least 10 times higher than the fundamental. Cymbals are a particular case. over 40% of its energy is above 20khz. 60% is below 20khz of course. To preserve what we hear, the response cannot barely be 20khz.
Try messing around with the electronics at 50khz or higher and hear what happens.
I'ld rather use a tube capable of sprinting than rely on prosthetics. Feedback's just another tool, in the Scott it was a crutch. The 12AT7 was replaced by a choke-loaded (for convenience, CCS on the way), 2 green LED-biased 5687 running around 6ma per side. Into the 10k input impedance of my sound card the distortion is comfortably under 0.1% pure second with anything higher well below the limits of 24-bit audio at 4 volts p-p. If Spice is believable that should drop at least another 10 dB into the more common 47k load. The Raytheon curves suggest below 4kohm plate impedance at the running plate voltage. For me this is the right tube for the job.
I agree with the principle above in general and have taken the Tubelab approach of poring over curves when checking out sales and picking tubes for their curves, not their reps. Some amazing unknown performers are still waiting for the right circuit. (Best example so far was a $2 triode-diode-diode that swung 160 p-p at <0.5% pure second coupled to a cathode follower, no feedback. 😛)
I agree with the principle above in general and have taken the Tubelab approach of poring over curves when checking out sales and picking tubes for their curves, not their reps. Some amazing unknown performers are still waiting for the right circuit. (Best example so far was a $2 triode-diode-diode that swung 160 p-p at <0.5% pure second coupled to a cathode follower, no feedback. 😛)
Hi rdf,
I didn't say all. I do think that there are many designs where a 12AX7 has been used to it's best advantage, same goes for other tubes in the 12A(TUX)7 group. There have certainly been some dogs around too.
Hey Steve,
You keep dragging your comparison back to a line stage output. You are arguing a point while focusing in one direction only. Open your eyes man!
Each tube type has it's strengths. I personally do not care that much for the cheap little 6DJ8. It was designed as a UHF tuner tube in cascode operation. It was never conceived as an audio tube. So it started life as a cheap TV tube. Yes, it can perform well and I will not take that away from it. It does well when designed with properly, but then again, so do the other tubes. Mr 6SN7 kicks the 6DJ8 when you need large voltage swings. BTW, the 6DJ8 was designed with high plate current and low plate voltage in mind. It's not happy when used at the higher plate voltages and lower currents that were typical of audio designs. Hmmmm, high voltage and low current. Sounds like a high impedance circuit to me. That would be Miller time I guess. 😀
Most older audio equipment used a cathode follower, transformer or a circuit from the plate and feedback. Rarely did the signal come off a plate to the outside world (with no feedback) in the better designs. Also, the standard input impedance of the day was 100 K or higher. So you can't fairly compare the performance of an old design to new standards of impedance either.
I understand that you have your viewpoints as they relate to marketing your equipment. Cool, but your audience here is somewhat better informed than the average public buyer.
-Chris
I didn't say all. I do think that there are many designs where a 12AX7 has been used to it's best advantage, same goes for other tubes in the 12A(TUX)7 group. There have certainly been some dogs around too.
Hey Steve,
You keep dragging your comparison back to a line stage output. You are arguing a point while focusing in one direction only. Open your eyes man!
Each tube type has it's strengths. I personally do not care that much for the cheap little 6DJ8. It was designed as a UHF tuner tube in cascode operation. It was never conceived as an audio tube. So it started life as a cheap TV tube. Yes, it can perform well and I will not take that away from it. It does well when designed with properly, but then again, so do the other tubes. Mr 6SN7 kicks the 6DJ8 when you need large voltage swings. BTW, the 6DJ8 was designed with high plate current and low plate voltage in mind. It's not happy when used at the higher plate voltages and lower currents that were typical of audio designs. Hmmmm, high voltage and low current. Sounds like a high impedance circuit to me. That would be Miller time I guess. 😀
Most older audio equipment used a cathode follower, transformer or a circuit from the plate and feedback. Rarely did the signal come off a plate to the outside world (with no feedback) in the better designs. Also, the standard input impedance of the day was 100 K or higher. So you can't fairly compare the performance of an old design to new standards of impedance either.
I understand that you have your viewpoints as they relate to marketing your equipment. Cool, but your audience here is somewhat better informed than the average public buyer.
-Chris
Steve, regarding bandwidth, I don't follow your point. Let's look at the worst-case input capacitance, 40pF. Further assume that the tube is used at an amp input and is being driven by 5k of source impedance (a pretty high number). The f3 is 850kHz. I suspect that's far enough above the human hearing range to not matter.
I've measured the distortion spectra of a LOT of ECC88 diff amp circuits. Let's see, you've got 100V pp; I assume that's a differential number? So each half is running 50Vp-p, or about 17VRMS. That's still fairly high for that tube, high enough that I suspect your distortion number is after feedback- the "irreducible" 3rd order distortion for the best ECC88 type I've measured (CCa) at that level is about twice that. At that signal level, a diff pair of 6SN7 will have about 0.02% open loop third harmonic. Morgan Jones reported that a diff pair of 6SN7 was able to swing 177VRMS (250VRMS) at 0.11%.
Now, let's look at the argument about output stage drive. For universality, we consider the Williamson amp as our example. The driver stage has a source impedance of about 8k7, the output stage has an input capacitance of 55pF, so we're facing an f3 over 300kHz. That might be too constrictive for you, but around here, output transformers have cutoffs 5-10 times lower.
I'm not trying to sell this tube or make audio lore, I use both in my system, they are just very different tubes with very different strengths.
And I caveat all this by assuming GOOD examples of both tubes, not things like relabeled 6ES8 or 7A4.
edit: I hadn't noticed until Chris pointed it out that you're in the business of making and selling amps. I do not intend to step on your commercial toes, but I won't avoid them, either. If you're using ECC88 in your preamps, I can't object to that since I do the same thing.😀
I've measured the distortion spectra of a LOT of ECC88 diff amp circuits. Let's see, you've got 100V pp; I assume that's a differential number? So each half is running 50Vp-p, or about 17VRMS. That's still fairly high for that tube, high enough that I suspect your distortion number is after feedback- the "irreducible" 3rd order distortion for the best ECC88 type I've measured (CCa) at that level is about twice that. At that signal level, a diff pair of 6SN7 will have about 0.02% open loop third harmonic. Morgan Jones reported that a diff pair of 6SN7 was able to swing 177VRMS (250VRMS) at 0.11%.
Now, let's look at the argument about output stage drive. For universality, we consider the Williamson amp as our example. The driver stage has a source impedance of about 8k7, the output stage has an input capacitance of 55pF, so we're facing an f3 over 300kHz. That might be too constrictive for you, but around here, output transformers have cutoffs 5-10 times lower.
I'm not trying to sell this tube or make audio lore, I use both in my system, they are just very different tubes with very different strengths.
And I caveat all this by assuming GOOD examples of both tubes, not things like relabeled 6ES8 or 7A4.
edit: I hadn't noticed until Chris pointed it out that you're in the business of making and selling amps. I do not intend to step on your commercial toes, but I won't avoid them, either. If you're using ECC88 in your preamps, I can't object to that since I do the same thing.😀
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