6922 vs 6SN7

[SY]

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.

[Positron]

"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.

[rdf]

Originally posted by anatech

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. ....

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!

[Miles Prower]

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?

[Positron]

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.

[Positron]

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.

[the_manta]

Originally posted by Positron
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.

Way back I thought the same about the ECC81. Now I changed my decision. Look THIS thread please.

[rdf]

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. :P)

[anatech]

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

[SY]

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.