Tube EQ Curves

"Makers, takers and fakers. There are no other kinds!"

ENDITOL said:

I wonder if anyone has ever tested and charted the EQ curves of different tubes? I'm sure it would vary between amps but there must be a baseline. For instance how are the mids different between EL-34 and 6550? Or even different brands of preamp tubes? :confused-smiley-010

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Actually, to my knowledge it was never done! Mainly because there isn't any difference!

A tube is a linear amplifier, when biased correctly. It has a bandwidth of at least a few MEGAcycles, if not hundreds of megacycles. This means it amplifies ALL the frequencies the same from its lowest to its highest limit! There is no measurable difference over the audio range.

Different types and brands have differences in gain, which will result in some differences in tone. These differences in tone are pretty subtle. Frankly, there's a lot of absolute hype and crapola out there that is used to sell one brand of tube over another. There are crappy brands, of course. Some brands of 12AX7s make unwanted noises like microphonics and some have a little bit more gain. Some have more hum. NONE will have peaks and dips over the audio frequency range!

When hifi amplifiers were being pioneered no one chose one type of tube over another for a specific tone. They chose a tube for how much gain or power it would deliver versus how much it cost. A pair of 6550s can deliver as much power as a quad of 6L6's. They also mean half the tube socket cost and savings in real estate on the chassis. EL34s need half the drive of 6L6s. This might save you a stage of preamp, or maybe you could use a cheaper 12AU7 than a higher gain 12AX7. You also had to factor that prices could vary on 6550s or EL34s, depending on availability. If they got too expensive a quad of 6L6s might work out cheaper.

In a guitar amp circuit you can get tone variances because you are driving the tubes into distortion, which brings out new differences to your ear. These differences are almost impossible to predict because they mainly depend on the circuit parameters far more than the specific tube. Trying to plot a curve for tone response during distortion would be pointless, because the curve would never be the same twice.

If someone tries to tell you otherwise, first check to see if he's reaching for your wallet. Then blindfold him for a test. Better yet, make him pony up some serious coin as a side bet! Should be good for some free beer!None

"All generalizations are dangerous, including this one!"

ENDITOL said:

Interesting, thanks for the thoughtful reply. So the circuit defines what changes there are? A lot of tube dealers and amp makers seem to make it sound like the tone changes - e.g. EL-34 will have a more slender low end and pronounced mids compared to 6L6 etc. Or that certain preamp tubes sound 'warmer' or more defined etc. I suppose it's all just subjective language. :food-smiley-004:

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Exactly! Although the tube will amplify a low frequency at the same level as a high treble one there can still be subtle differences. If a tube has a better distortion figure and generates lower hum/noise even an untrained ear will notice how the "clarity" or "definition" of a note sounds better. I suspect that this is what really generates all the arguments. Most decent brands of preamp tubes are good enough to sound "clear" in themselves. EHs and JJs and some of the Chinese and Sovteks are "good 'nuff" in quality. The new Tungsol 12AX7 is the quietest and most humfree of the lot. Your original question had to do with frequency response and despite all the extra stuff I've typed the simple answer remains that they're all the same in that regard.

The circuit certainly does behave differently with different kinds of tubes, but for specific technical reasons. Consider the original JTM 45 Marshall. It was a dead-on copy of one of the Fender Bassman circuits that also used power tubes from the 6L6 family (5881s). The only difference was a 12AX7 instead of a 12AT7 as the phase inverter driver tube to the outputs. This tube had a bit more gain so it drove the 5881s easier into distortion. They also used Celestion speakers. Typically Fender had used Jensens, Oxfords and whatnot. The difference is that Celestions are a "mid to high" fast response speaker and the others were "mid to low" with a bit slower response.

Very early on Marshall had a problem finding enough 6L6 types at a decent price and tried the EL34. As I said, like any tube it had a flat gain at all audio frequencies but it was about twice as sensitive for the same amount of drive signal as a tube from the 6L6 family! This meant that when you cranked the amp up the output tubes went into power distortion far earlier than in a Fender. With all these differences the Marshall sound was born!

It's all these circuit differences that make it so much fun to be a tech! 15 minutes ago a customer brought me another old Trayor head to "Marshall-ize". By changing the circuitry I can make it sound like pretty well any vintage amp. A modern "crunch monster" is out of the question because it all hinges on how many stages of preamp tube gain are available.

When it's finished I'll likely "forget" to call him for a few days. I want some fun for myself! :rockon2: