Do all power tube types sound the same through a given Reactive Load? (bonus EVH amps info)

GreatGreen

Power User
Considering Cliff's article about the differences between tube types, specifically about how each type of tube doesn't really do anything to the sound other than change a poweramp's damping factor (EL34's have higher damping than 6L6's, which is why 6L6 tubes can sound more mid-scooped than EL34's, etc), does this mean that tube type doesn't matter when connected to a reactive load?

If a Reactive Load presents a static, pre-defined impedance curve to a tube amp, then it isn't really "reactive" is it. If the impedance curve is entirely predefined, then there is nothing for the tube to "react" with and so it shouldn't matter what kind of tube the reactive load sees. Whatever type of power tube it is, it can't impart any impedance or "damping factor" of its own to that load which means any tube poweramp of a given wattage should sound the same through it, right? If this is the case, then a "reactive load" could be more accurately described as "a resistive load shaped in a way that mimics a real speaker's impedance curve," right?


I'm asking this because I have recently compared an EVH 5150 III 100w Stealth (which uses 6L6 tubes) and EVH 5150 III 100w EL34 amp in my studio by reamping a single guitar line through each amp into a Suhr Reactive Load and IR's. Just by eyeballing the controls to match as close as I could, the amps sounded almost exactly the same.

To put this in more objective terms, reversing the phase on one of the amp tracks and playing both tracks together resulted in a mix about 16 db quieter than each track solo'd. That's very close. Close enough that it takes real concentration to tell the differences and it would most certainly be impossible to tell in a mix. To me, this suggests that either the amps are truly identical in every way or (more likely) that the differences in the impedance curves of the tube power sections (6L6 vs EL34) are being negated by the Reactive Load.

By the way, this is in no way a complaint against the Suhr Reactive Load. I think it sounds phenomenal and certainly better than any mic'd cab solution I've ever come up with for sure, and it has pretty much single-handedly made tube amps viable for me to use at all where I live.


Bonus info: This test has shown me that the EVH 5150 III 100w Stealth and 100w EL34 amps have identical preamps. Don't let anybody tell you that one is "fatter" and the other is "more agile" or that one amp has more gain than the other. I have tested them and can confidently tell you it's just not true. The preamps of the 100w Stealth and 100w EL34 are identical... and through a given reactive load, both amps are basically identical as well.

The 50w amps are a different story, btw. Each of the three 50w models (6L6, EL34, and Stealth) actually do feature different preamp sections.
 
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Another thought... this would also explain why changing power tube types in the Axe-Fx's Amp block doesn't really do anything. Power tube differences show themselves through changes in the impedance curves generated by their interaction with speaker cabs... but that impedance curve has already been set by the parameters in the Speaker tab.

Wouldn't this mean that all tube amp models in the Axe-Fx are all being routed into a virtual "reactive load" whose parameters are defined by the Speaker tab?
 
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Not sure about other reactive loads, but the one I built in the '90s to slave a small tube amp in my rack rig used crossover chokes and caps in LC tank circuits along with parallel limiting resistors to set the max reactance of the tanks at their resonant frequency. LC tank circuits tend to 'ring' a little, mimicking the actual physical properties of the speaker at resonance and how the mechanical resonance reflects energy back into the output of the amp.

Likely any decent reactive load does something similar. The impedance curve shows AC impedence throughout the audio band, not DC resistance, BTW. The reflected energy at various frequencies where the speakers physically resonate dynamically affects the way the output section breaks up when driven hard. This is an area where tube type and the circuit design can really show their differences - i.e., how each falls apart when unable to maintain the more-or-less linear response of a clean output section....
 
Not sure about other reactive loads, but the one I built in the '90s to slave a small tube amp in my rack rig used crossover chokes and caps in LC tank circuits along with parallel limiting resistors to set the max reactance of the tanks at their resonant frequency. LC tank circuits tend to 'ring' a little, mimicking the actual physical properties of the speaker at resonance and how the mechanical resonance reflects energy back into the output of the amp.

Likely any decent reactive load does something similar. The impedance curve shows AC impedence throughout the audio band, not DC resistance, BTW. The reflected energy at various frequencies where the speakers physically resonate dynamically affects the way the output section breaks up when driven hard. This is an area where tube type and the circuit design can really show their differences - i.e., how each falls apart when unable to maintain the more-or-less linear response of a clean output section....
What he said. He explains it a lot better than I was going too. I was going to say "No."
 
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