CE Distribution / Antique Electronic Supply / Amplified Parts just put out this PDF where they plotted the frequency response of a single tube of each of the 6L6 family tubes they sell.
Okay, so this looks like a really useful document initially. They put a bunch of tubes on a tube tester, picked the average one, plugged it into an amp, plotted its frequency response, and let a guitarist subjectively describe the tone.
That last bit is a little like asking a mechanic what oil you should put in your car and then asking a guy who likes to drive how the different oils taste on bread, so I'm going to try to avoid commenting on "sizzling leads" versus "balanced and tight." If all you want is lurid prose about how many orgasms each tube brought the guitarist, there's no need to bring a tube tester or frequency plotter into this.
So ignoring the poet, we have the manufacturer's rated specs on the left. These are pretty much crap, as modern manufacturers largely just copy whatever was on the original data sheets. The maximum plate voltage is just a suggestion, let's be honest. Dissipation kills tubes, not voltage. And do we expect guitarists to know what their screen voltage is? Considering how common screen grid failure is, it would've been nice to include maximum screen dissipation buuut it doesn't matter anyway because the manufacturer's numbers are junk.
Back In The Day(tm), for a tube to be sold as a "6L6GC" it had to meet certain specs for plate dissipation, transconductance, envelope size, etc. No one cares about this any more, so it's understandable that there's some variation in rating and size from one manufacturer to another. Not to mention some of these tubes aren't even 6L6GCs, but Russian military tubes which were designed to be clones of 6L6GCs. That's another whole bag of worms, and it's fairly irrelevant, but it is interesting that some New Sensor 6L6GCs can take up to 40W on the plate comfortably.
What would really be interesting would be a comparison of transconductances, but considering a lot of these tubes are "close enough" to a 6L6GC I suspect there wouldn't be too many surprises. Specs on those Russian tubes (6P3S-E) are hard to track down, so it'd be nice to finally get some solid data. Anyway.
There are handy bar graphs of "lows," "mids," and "highs." This was kind of a mistake, or rather, how they define "low" and "high" is problematic. The open low E is 82 Hz, but this chart defines "lows" as 50 Hz. Even a baritone B string is 62 Hz, so the measurement of "lows" on these bar graphs is useless.
"Mids" are defined as 700 Hz, which is pretty reasonable.
"Highs" are defined as 6 kHz, which is not pretty reasonable. Most guitar speakers have already started rolling off around 5 kHz. 3kHz is around the "icepick" region and probably would've been a better choice.
To generate the frequency response graphs (and the guitarist gibberish,) they built a little single-ended guitar amp to put the tubes in. I take issue with a couple things here. First off, they didn't make a neutral Hi-Fi amp, they made a guitar amp. They describe the preamp as being a Blackface Fender design, and that's good for frequency plotting because there's certainly no limit to the low end, but unfortunately there's also a freaking tone stack! Yes, you can get a Blackface tonestack to be almost linear by turning up the mids to 10 and turning the bass and treble to zero, but did they do this for the frequency response plotting? Did they instead take the tone stack out of the circuit? These details aren't provided.
Anyway, these are graphs of just individual tubes so buying tube X and actually getting frequency response X is probably a crapshoot. Those little bumps are the result of manufacturing variances; if they averaged 10 of the same tube the responses would look much more similar brand-to-brand. Plotting dB on a linear scale instead of a logarithmic one is a little disingenuous too; even speaker manufacturers don't try that. Anyway, the biggest difference they show is roughly 2dB which is just under the threshold of what the human ear can discern as a difference in volume.
There is also the sizeable mid-hump which all the tested tubes exhibit. In the guitar community, 6L6s are widely considered to be somewhat "mid scooped" and the swap to EL34s will return these missing mids. At the very least, we can now clearly show everyone that the mid scoop comes from the circuit around the tubes, not the power tubes themselves. I suspect this bump in the frequencies is more related to the circuit, than a characteristic response of a 6L6.
In this case, the circuit these power tubes are plugged in is quite simple. Like I mentioned above, we have to assume they took the tone stack out of the circuit before measuring the frequency response of each tube. The circuit itself is single-ended, with no negative feedback. This is potentially a huge oversight - most guitar amps use a global NFB loop around the power tubes and phase inverter. I honestly can't think of a single 6L6-based amp that doesn't. I'm sure they're out there, of course.
The point being that NFB reduces distortion - the output is fed back to the input out of phase, so if the tube generates a boost at the output, that gets fed back to an earlier stage as a notch, and the result is a flat response. So, it makes sense that you would remove NFB if you wanted to plot a theoretical response of each tube to see if there really are significant differences from brand to brand.
But what happens when you put these tubes in your amp?
Even without NFB, a push-pull amp will cancel a noticeable amount of (even ordered) distortion. As the fluctuations in frequency response are largely sporadic, there would be significant cancellation of some deviations from linearity. Some would reinforce each other, so I suspect the overall response would look less choppy, though approximately the same.
With NFB though, all of that choppiness is going to cancel, so the only difference you're going to see is the overall y-axis offset - also known as the difference in emission & transconductance from tube to tube.
So to wrap this whole thing up:
- All these tubes are basically identical.
- There are some apparent deviations in frequency response at around +/- 2dB.
- These differences are inaudible.
- Even if they were audible, most amps have NFB, so these deviations would cancel out.
- Only one tube of each brand was tested, so we have no idea if there are any trends among brands, or if CE Distribution just happened to select tubes that were basically identical.
- This document is just a piece of marketing material, and shouldn't be viewed as anything but an advertisement.
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