tube specs

[trem]

Accepting the fact i am a beginner, and probably stating some generalities that are/may not be 100% accurate.......

I realize that tubes themselves Have Some capacitance, and that they participate in The Miller Effect, but that page from Amp Parts makes tubes look like small speakers.
What is "the truth" regarding tubes and tone.? Even capacitance is linear is it not.? The cap provides a charge within its freq range, but does not vary within that range does it.?
That is to say, if a cap cuts out at 2 kHz, does (or can) it work harder at 800 Hz than it does at 1.2 kHz.?
My same question applies to tubes. Can they really vary as their test says they do.?
Thank You

https://www.amplifiedparts.com/tech-...ent-made-tubes

[J M Fahey]

Short answer and conscious Iīm opening a can of worms.

They contradict themselves, they show HUGE differences in their red grey yellow coloured bar lows mids highs yet they trace the preamp, where difference should be even more marked, since in every case both triodes in a bottle are of the same brand by definition and graphs clearly show all are parallel, meaning the camel hump frequency response you see comes from the circuit parts and values, NOT from tubes, which only show some gain difference , which by the way is expected and acknowledged.

So those poetic descriptions,such as: crisp/bright/thick/harmonic_texture/shimmer/punchy/articulate/presence/lean/sparkle/etc. are just that, poetic words.

The main real difference is gain/transconductance , not frequency response.

Capacitance difference is minute and definitely swamped by the feet of wire they used in the test rig.

They donīt explain how the experiment was made, how many people participated, what does each of them say he thinks he heard , how many disagreed with others and so on.

Oh well.

[trem]

Is there a direct (or any) relationship between Gain and Freq Response. Does more gain allow a stronger bass response for example.?

This is how they say they ran their test.
12AX7 Basics
The 12AX7 is a 9-pin miniature, high-mu, twin triode. What does that mean? The 9-pin miniature part tells us what kind of socket the tube plugs into. It has nine thin pins arranged along the circumference of a circle with a diameter of just under half of an inch. There is a large gap between pins 1 and 9 to ensure that the tube can only be plugged in one way. The high-mu part is referring to the electronic specification called "amplification factor," abbreviated by the Greek letter μ (mu - pronounced 'myoo'). This gives us a general category of amplification that the tube was designed for as compared to other preamp tube types like the "medium-mu" 12AU7. The twin triode part tells us that there are two identical and independent triodes contained within the one tube. A triode is the most basic tube structure that allows for signal amplification and it has three electrodes: plate, grid and cathode.
The Gain Test Setup
Each tube sample was tested in the same (Fender black face style) preamp circuit as shown in the schematic below with bass and treble set to their maximum. A frequency generator was used to measure voltage gain and plot the frequency response from 10 Hz to 18,000 Hz. From the frequency plot it could be seen that there are three extrema at about 50 Hz, 400 Hz and 4,000 Hz. These three frequencies were then used to quickly measure the low, mid and high frequency voltage gain on the same number of samples for each tube in our comparison. The average from each tube's samples was then calculated and used for comparison. The smallest average low frequency gain tube was used as the reference for a gain of "1" and the largest average low frequency gain tube was used as the reference for a gain of "10". This gain reference method was then repeated for both the mid and high frequency averages.
12AX7 Gain Test Schematic
12AX7 Frequency Swap
The Noise Test Setup
Each tube sample was again tested in the same preamp circuit and an oscilloscope was connected to the amplifier output to monitor the amplitude of the voltage spike produced by tapping on the tube's glass envelope. The average voltage spike from each tube's samples was then calculated and used for comparison.
The Listening Test setup
A custom 12AX7 switching box was constructed to allow for a quick switch comparison of six tubes at a time while plugged into the V1 socket of a Marshall JCM800 2203 guitar amplifier. A separate filament transformer was used on the switching box to simultaneously heat all six comparison tube filaments without overheating the amplifier's power transformer. A shielded cable was constructed to plug into the amplifier's V1 socket and transfer its pin 1, 2, 3, 6, 7 and 8 connections to one tube socket at a time in the switching box. Listening tests were then conducted while playing guitar and switching between the tubes at various amplifier control settings to come up with tonal descriptions for each tube.

By Kurt Prange (BSEE), Sales Engineer for Antique Electronic Supply - based in Tempe, AZ. Kurt began playing guitar at the age of nine in Kalamazoo, Michigan. He is a guitar DIY'er and tube amplifier designer who enjoys helping other musicians along in the endless pursuit of tone.

[potatofarmer]

Look at the graph more carefully. Each brand's trace is almost identical over the frequency range they looked at. The only difference is a slight vertical offset - this is the difference in gain. Since they used only one example from each tube brand, this graph shows a worst case scenario of their differences... and they're all basically identical. The graph makes the exact opposite point they're saying it does - namely, that brands of preamp tube don't influence "tone," or at least frequency response.

If you're concerned about bass, any given 12AX7 has no problem putting out near maximum gain at fractions of 1 Hz. Just look at any tube tremolo circuit, where they do exactly that.

As for your other questions, you might want to look into how simple (first-order) high-pass filters work. The grid/anode and grid/cathode capacitances are listed on a tube's datasheet and generally considered to be essentially universal for every manufacturer of that tube type. All 12AX7s, for example, will have the same interelctrode capacitances... more or less. It's enough to say that with typical preamp stages, the amount of treble that's rolled off due to Miller capacitance is generally inconsequential... and even when it isn't, changing brands of tube won't change things.

[J M Fahey]

Is there a direct (or any) relationship between Gain and Freq Response. Does more gain allow a stronger bass response for example.?

No, and their experiment proves that.
Iīll cut and paste their *measured* curves so we donīt need to go back and forth to another window:



how to read his graph:

* you have a vertical scale, labelled as Voltage gain (dB) , it shows from a low of 14 dB to a high of (marked) 28dB

* you have a horizontal scale, showing frequencies from 10 Hz to 10000 Hz.

We are interested in frequencies from about 80Hz , lowest frequency produced by a guitar string, to about 3500/4000Hz which is where most guitar speakers drop dead.
Very few reach 5000 Hz but most users find them brittle/icepicky/unbearable (modern Jensen) or very harsh and brittle (JBL) so we stop at 5000 Hz.

What do the graphs mean?
They show how much gain does that circuit have at any frequency. Cool, huh?

So if I want to know what is the voltage gain of the circuit using the yellow tube at, say, 100 Hz, first I find 100Hz on the frequency scale , go straight up until I meet the yellow line, then go left from that exact point and read gain on the gain scale, in this case itīs the first thin horizontl line above 22 dB

And how do I know its value? it is not marked !!!!!!!!!!!!

True, often itīs not practical to mark *all* lines , no space for regular letters and those which would fit are unreadably small, BUT they label some (14/16/ .... /28) and show some intermediate lines where itīs easy to guess athe value.
In this case, from thick to thick horizontl line we have 2 full dB, so 5 intermediate lines must show : 2/5 = 0.4 dB
So the mystery thin line now means 22 + 0.4 = 22.4dB Cool, huh?

A similar trick is used on frequency values, only 4 are written but if you want to find, say, 2000Hz , itīs the first vertical line to the right of 1000 Hz .
Count left to right from 10 Hz and youīll find vertical lines for 20/30/ .... 80/90/100 and so on

Now YOU find it: what is the gain of the violet tube at 3000 Hz?
Find 3000Hz, go straight up to the violet curve and then full left, read the value or calculate how much above or below a marked value is it.

OK, back to business, now that you know how to *read* a frequency response curve or graph, letīs see what does this one tell us.

A perfect flat response must be a perfect horizontal line.

What woud that mean?

That the tube has the same gain at all frequencies.

A tube with more highs than lows would show a tilted or rising curve, rising to the right.

A bassy one would be the opposite: rising towards the left.

A dark/muddy one woud drop towards the right .

A thin one would drop towards the left.

A nasal one would show a hump in middle frequencies.

A scooped one would drop in the middle, and be higher left and right.

So as you see, a proper frequency curve can and does show or at least give you a pretty good idea of how something will or does sound, those are not just random squiggles on a piece of paper but a representation of reality.

So if tubes were bassier/punchier/warmer/whatever that MUST be shown on different curves, but if they are all the same they sound the same.

IF those tubes (and thousands others) were tested in a simple jig: a tube with cathode and plate resistors, a power supply both HV and filaments , were fed audio at the grid and measured at the plate, forgive me for spoiling the mystery movie by telling you how it ends; they will all have the same frequency response, thus same sound

Which is often said as: tubes have flat frequency response from DC to well beyond the Audio spectrum.
Most into the tens or even hundreds of MHz (specially 12AT7), with the notable exception of 12AX7 which has higher than normal capacitance so it usually dulls above the audio range in most normal Audio circuits.

Of course, IF they ran such plain test they could not claim sound differences if all curves were straight horizontal lines (which they would be)

Beware, tubes with different gain would have horizontal lines, only at different height (you now know and checked that different height means different gain).

To put it another way, and pay attention to this detail, because itīs important: all curves, no matter what the colour, would be parallel at different frequencies.

Now this sales page introduces the tone control curve , which depends on the tone controls, not the tubes, and it results confusing to most and obscures the fact that the curve contradicts the subjective labelling.

So how can you read something useful from that tone control curve?

Somewhat easy if you remember what I said above: if all curves are parallel, then there is not a tube based difference, hence all sound the same.

OK: please check the curves: they are parallel; they all squiggle up and down but by the same amount (as in: at any frequency, they have the same response).

Example:
* difference between the yellow curve and the violet one at 100Hz: 6 thin horizontal lines or 6*0.4=2.4dB
* at 1000 Hz : 6 lines or 2.4 dB
* at 5000Hz: 6 lines or 2.4 dB

check it all over the place, difference is always (in this case) 2.4 dB which means that violet has 2.4dB more gain than yellow, at *all* frequencies, meaning it has plain 2.4dB more gain but it is in NO way is brighter/smoother/darker/warmer/you-name it , NO WAY.

This is how they say they ran their test.

Nice but irrelevant:

12AX7 Basics
The 12AX7 is a 9-pin miniature, high-mu, twin triode. What does that mean? The 9-pin miniature part tells us what kind of socket the tube plugs into. It has nine thin pins arranged along the circumference of a circle with a diameter of just under half of an inch. There is a large gap between pins 1 and 9 to ensure that the tube can only be plugged in one way. The high-mu part is referring to the electronic specification called "amplification factor," abbreviated by the Greek letter μ (mu - pronounced 'myoo'). This gives us a general category of amplification that the tube was designed for as compared to other preamp tube types like the "medium-mu" 12AU7. The twin triode part tells us that there are two identical and independent triodes contained within the one tube. A triode is the most basic tube structure that allows for signal amplification and it has three electrodes: plate, grid and cathode.

The Gain Test Setup
Each tube sample was tested in the same (Fender black face style) preamp circuit as shown in the schematic below with bass and treble set to their maximum. A frequency generator was used to measure voltage gain and plot the frequency response from 10 Hz to 18,000 Hz. From the frequency plot it could be seen that there are three extrema at about 50 Hz, 400 Hz and 4,000 Hz. These three frequencies were then used to quickly measure the low, mid and high frequency voltage gain on the same number of samples for each tube in our comparison. The average from each tube's samples was then calculated and used for comparison. The smallest average low frequency gain tube was used as the reference for a gain of "1" and the largest average low frequency gain tube was used as the reference for a gain of "10". This gain reference method was then repeated for both the mid and high frequency averages.

OK, the test shows parallel curves, so each tube is flat , both in an absolute way and relative to others.
Measurements do NOT back the subjective/verbal tone differences described, which can safely be called poetic or imaginative.

12AX7 Gain Test Schematic
12AX7 Frequency Swap

The Noise Test Setup
Each tube sample was again tested in the same preamp circuit and an oscilloscope was connected to the amplifier output to monitor the amplitude of the voltage spike produced by tapping on the tube's glass envelope. The average voltage spike from each tube's samples was then calculated and used for comparison.

Now this can show differences, microphony depends a lot on how firm and tight are internal tube elements and thereīs variations even on same brand and model.

The Listening Test setup
A custom 12AX7 switching box was constructed to allow for a quick switch comparison of six tubes at a time while plugged into the V1 socket of a Marshall JCM800 2203 guitar amplifier. A separate filament transformer was used on the switching box to simultaneously heat all six comparison tube filaments without overheating the amplifier's power transformer. A shielded cable was constructed to plug into the amplifier's V1 socket and transfer its pin 1, 2, 3, 6, 7 and 8 connections to one tube socket at a time in the switching box. Listening tests were then conducted while playing guitar and switching between the tubes at various amplifier control settings to come up with tonal descriptions for each tube.

The measurements and curves supplied by themselves do NOT confirm tonal differences.
They do NOT explain or detail how listening tests or who was used as listner; ideal woud have been to have a panel made of, say, 10 or 20 people at least, and tests carrid behind a thin but opaque curtain , with both listeners and testers NOT knowing what tube was being used at any moment (which of course is recorded on computer/tape/punched paper/etc. or switched from a faraway room, where testers donīt see the public and viceversa)

Am I an obsessive fan of mechanical results instead of human ones?

No, quite the contrary, but I trust human even over measurements results IF consistent and repeatable along time, and that is the base of psychocoustics.

Now if self contradicting, or correctly guessing 50% of the time (same as flipping a coin ) or twisted by expectations ... they become unreliable (and thatīs an understatement).

By Kurt Prange (BSEE), Sales Engineer for Antique Electronic Supply - based in Tempe, AZ. Kurt began playing guitar at the age of nine in Kalamazoo, Michigan. He is a guitar DIY'er and tube amplifier designer who enjoys helping other musicians along in the endless pursuit of tone.

mmmmmhhhh, sales huh?

[nickb]

I get the impression the listening tests were done by Kurt still in front the amp with his guitar whilst switching tubes. No mention af any kind of controlled experiment i.e. essentially worthless. But he does do pretty presentations.

And here's the kicker, even if the listening results were reliable "It should not be assumed that every single 12AX7 from a specific brand will match its place on this graph". In other words don't expect to get the same results as they did. With at least a +/-30% tolerance in characteristics you might as well buy several cheap tubes and pick the one you like best.

[Enzo]

And does anyone ever drive them into distortion for these comparisons?

[nickb]

And does anyone ever drive them into distortion for these comparisons?

I had a stab at it. It's far from perfect and if I were to do it again I'd do it differently, but it is what it is. Skip down to >>part 2<< for the overdrive bit

[J M Fahey]

Hey, incredibly good and exhaustive experiments !!!!

[nickb]

Hey, incredibly good and exhaustive experiments !!!!

Aw, shucks - thank you.

[trem]

Aw, shucks - thank you.

It Would Be interesting to see you exhaust yourself some more.....over the long term with repeatable and definitive results.
Your part 2 was very intriguing indeed. Certainly impressive to somebody at my level of understanding.
I realize (like Juan said) this is a can of worms...totally NOT My Intent.
Just that i see all these claims and reviews about tubes, and i wonder what "The Real Story" is.
Thank You Very Much

[Malcolm Irving]

As discussed above, tubes themselves have unlimited bass response and a high frequency response way beyond the audio range. It's the interaction of a tube with the surrounding circuitry which determines the frequency response (e.g. the interaction of Miller capacitance with grid-stopper resistance).

However, all tubes are nonlinear devices. In hi-fi design various ideas are applied to minimise the effect of the non-linearity; while for guitar amplifiers we usually like the sonic effects of the nonlinearity. Even when playing ‘clean’ there is a significant amount of distortion.

As a result of differences in the spacing and geometry of the elements in a tube, different examples of tubes (of the same nominal type) can have subtly different nonlinearity. This will not really affect frequency response, but can give a difference in the sound, and gives some credence to the concept of ‘tube rolling’.

[Justin Thomas]

I always thought that one single PARTICULAR tube could sound different from another single particular tube in a single particular circuit that is not compensated for tube differences in any way, but that what upset the tube scientists among us are these sweeping claims that ALL (for example) EH 12AX7s will make YOUR amp (which could be any one paritcular amp of potentially billions that could use your particular tube) sound THIS way, while any Mullard reissue will make your particular amp sound THAT way, and so the tube vendors are selling based on specious claims that they can predict how every amplifier on Earth will sound based on their limited testing of a very tiny sampling of tubes from each shipment. Maybe if they said "this exact specific tube sounded THIS way in THIS exact amplifier; it probably won't sound the same in YOUR amplifier..." then maybe we could all get along on this issue...

The other side of the argument is that any variation in tone caused by a tube change can be compensated for by altering circuit parameters to equalize for differences in gain. If the circuit under test is modified to compensate for manufacturing differences of electronic properties, then tubes will sound remarkably alike.

The only factors that cannot be compensated for in regards to tubes I thought were microphony, noise, and other such things that are outside the control of the user once the tube rolls off the line... I mean, I can't exactly open up all my SED 6L6s and firm up their micas from rattling, can I? But they still sound awesome.

Using these criteria, I change tubes for different tones because I do not have the scientific equipment to calibrate my circuits so that I can get repeatable performance from any tube; and even if I <DID>, I'm too damn lazy to bother. Pulling one out of a socket and inserting another is SOOOOOooooooo much easier.

Justin

Sorry Malcolm, seems I repeated you...

[Enzo]

Sure. You may find an EH12AX7 and a SOVTEK 12AX7 much closer together than say two Sovetks or two EHs. Every brand of tube has a broad range of performance, and they overlap a lot.

[trem]

Sure. You may find an EH12AX7 and a SOVTEK 12AX7 much closer together than say two Sovetks or two EHs. Every brand of tube has a broad range of performance, and they overlap a lot.

That has certainly been my (limited) experience as well. Not only overlap, but as you guys mentioned already...tolerances within same tube.
Choosing tube-XYZ does not always deliver same results when buying the same tube=XYZ three months later.
What can you say...same old argument. If a guy tries tube ABC...then tries tube XYZ and says..... Holy Cow, that sounds much better.! How can i say he is wrong.?
And that was not really my purpose here. Just wondering about the "science" behind some of these claims by dealers.
Enzo
Fahey
Justin
etc etc
Thank You all once again