Quick test: Presonus Bluetube v2 adjustable hybrid preamp

[pkane]

Presonus Bluetube v2 is a 2 channel, pro preamp with a mic and instrument inputs, 48v phantom power, and balanced and single-ended inputs and outputs. The curious part is the inclusion of a 12AX7 vacuum-tube stage with adjustable control allowing all that tube "warmth" to be mixed in variable proportions with the solid-state amplification. While I can run more measurements on this device using Apogee Element24 DAC/ADC and REW, I wanted to see primarily the effect of the Tube Drive on the harmonic content at the output. All connections between Element24 and Bluetube were balanced. Output with the Tube Drive turned off was at 3.6v. With Tube drive at maximum setting the output fell to 3.1v

1. Tube Drive turned off (all solid state). Not awful for a moderately priced preamp:

2. Tube drive turned on, minimum setting. Wow!

3. Tube drive at 10 o'clock:

4. Tube drive at 11 o'clock:

4. Tube drive at 12 o'clock:

5. Tube drive at maximum setting. That warmth should really be easy to feel by now

 

[pkane]

Adding multi-tone test:

1. Tube Drive turned off:

2. Tube Drive at the minimum setting:

3. Tube drive at the maximum setting:

 

[pkane]

Frequency response is fairly flat above 100Hz, but dips by about 2.5dB between 100Hz-20Hz. It doesn't seem to be affected by the inclusion of the tube:

 

[SIY]

But that really has zero to do with it being a tube and everything to do with being a distortion generator. A 12AX7 in a competent circuit has remarkably low distortion before feedback is applied. You could make this same box with the same effect without using a tube.

/pedant

 

[pkane]

But that really has zero to do with it being a tube and everything to do with being a distortion generator. A 12AX7 in a competent circuit has remarkably low distortion before feedback is applied. You could make this same box with the same effect without using a tube.

/pedant

Ok,ok. But that’s what tubes are good at, so this is using them in a way fit for purpose.

 

[Dogen]

Thanks - this is really interesting! How would you describe the difference in sound between no tube and max tube? The expected second harmonic “richness”? That might be obvious to a guitar player...

 

[watchnerd]

But that really has zero to do with it being a tube and everything to do with being a distortion generator. A 12AX7 in a competent circuit has remarkably low distortion before feedback is applied. You could make this same box with the same effect without using a tube.

/pedant

But sales of a tube effects box without a tube wouldn't be as good as one with a tube.

 

[pkane]

Thanks - this is really interesting! How would you describe the difference in sound between no tube and max tube? The expected second harmonic “richness”? That might be obvious to a guitar player...

The difference I found when playing acoustic music or vocals through it made the sound just a bit thinner, less coherent, like what I’d imagine would be the result of adding a some noise modulation or dither. It’s not a huge effect, even at the maximum tube setting. I think if the source sound is overly bright or compressed this can help make it sound just a bit more natural, but overall I didn’t like the effect. It’s almost like it dilutes the audio. How’s that for a subjective audiophile description?

 

[SIY]

But sales of a tube effects box without a tube wouldn't be as good as one with a tube.

Likely true. But an actual tube is optional.

 

[watchnerd]

Likely true. But an actual tube is optional.

So here is the follow-up question, given the tube is optional:

Why a 12AX7, specifically?

My thought is that it could simply be because it's common in guitar amps.

 

[SIY]

Likely. Unfamiliar things require more effort to sell.

 

[pkane]

So here is the follow-up question, given the tube is optional:

Why a 12AX7, specifically?

My thought is that it could simply be because it's common in guitar amps.

I should have another tube I purchased with the amp that’s supposed to be an improvement according to tube rollers. I’ll have to dig it up to see if it makes any difference. Last time I built anything with tubes was more than 40 years ago. I gave up on them then after being shocked a few times by the business side of a 500v transformer

 

[watchnerd]

I should have another tube I purchased with the amp that’s supposed to be an improvement according to tube rollers. I’ll have to dig it up to see if it makes any difference. Last time I built anything with tubes was more than 40 years ago. I gave up on them then after being shocked a few times by the business side of a 500v transformer

Cool, curious to see what you find and what the measurements say.

But why question was why not something like a 6DJ8?

 

[SIY]

Cool, curious to see what you find and what the measurements say.

But why question was why not something like a 6DJ8?

Not as recognizable a name to the guitar crew.

My question is, why not a FET or opamp, but same answer- marketing.

 

[watchnerd]

Not as recognizable a name to the guitar crew.

My question is, why not a FET or opamp, but same answer- marketing.

I had the same thought.

It seems reductionist, but that given the product type and market segment, probably also true.

 

[pozz]

Nice work.

I hope we see more pro gear tested in the future.

 

[restorer-john]

So here is the follow-up question, given the tube is optional:

Why a 12AX7, specifically?

My thought is that it could simply be because it's common in guitar amps.

Because it's fashionable to stick it in anything. What's the world coming to?

Here's a Samsung Networked BluRay player I picked up for $3 with a pair inside (and 6 channels of Class D amplification). It's got a nice smoked glass top, but is destined to be gutted in the name of science. Looks like 4 of the 6 channels go through the pair of 12AX7s.

 

[LightninBoy]

But that really has zero to do with it being a tube and everything to do with being a distortion generator. A 12AX7 in a competent circuit has remarkably low distortion before feedback is applied. You could make this same box with the same effect without using a tube.

It is a distortion generator and that is the point. This unit is about artistic expression, not transparent playback. It is popular to record bass with this and similar units.

That said, the pure solid state performance is better than I expected.

 

[watchnerd]

Because it's fashionable to stick it in anything. What's the world coming to?

Here's a Samsung Networked BluRay player I picked up for $3 with a pair inside (and 6 channels of Class D amplification). It's got a nice smoked glass top, but is destined to be gutted in the name of science. Looks like 4 of the 6 channels go through the pair of 12AX7s.

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Okay....this is shocking.

What's the Venn diagram intersection of people into Blu Ray and people who want tubes in their Blu Ray players???

 

[restorer-john]

Okay....this is shocking.

What's the Venn diagram intersection of people into Blu Ray and people who want tubes in their Blu Ray players???

It's beyond me. That's why I had to pick it up just to have look inside. First of all, I thought, I bet it is just for looks, but no. Initial inspection (brief) showed 4 channels heading to the tubes...

I'll gut it later in the week and post some pics in another thread.