Just ordered one of these - Ampeg SVT Vacuum Tube Direct Injection

[Super Turbo Deluxe Custom]

Confronted with a potential bass amp purchase that doesn't come equipped with a balanced XLR DI built in, I pulled the trigger on this.

They are out of production now, so I found a used one with a warranty.  I am hardpressed to find anything negative about them.  I don't know if anyone has ever looked for a tube DI, but they are anywhere between $600 and $1,600, so this one was a steal at $279.  Plus, for just being a DI, so many of them have features I just don't want, like knobs.  Hey folks, I'm just trying to get a signal in a balanced XLR form to the board for FOH sound.  Unity gain setting (with optional db pad) and ground lift, don't need a whole lot of bells and whistles.  You start adding knobs, it's a preamp.

And...I got to use one in the studio before, so I'm not going into it blind.

 

[Kadmium]

You start adding knobs, it's a preamp.

I would suggest that already happened when they added a tube. What does the tube actually do in this DI? Is it a 12AX7?

 

[Super Turbo Deluxe Custom]

I wanna say it's a 12au7.  I've downloaded the manual, and it doesn't say, only the what the usual preamp tubes are.  12ax7, 12ay7, 12au7.  Since it's used, I'll have no way of knowing if what was in there is original.  I called Ampeg yesterday just to ask which one they recommend, but hung up after 10 minutes of holding. 

The one I played on previously, it adds a little tube girth.  That subtle spongey compression, and a bit non muddy bass.  Perhaps you're right about it being borderline preamp territory, but you couldn't go instrument<this thing<power amp and expect to have any reasonable EQ or tone shaping.

It's a DI with a tube in it, where some of the other tube DIs with knobs that boost and cut different frequencies start really pushing into preamp territory.  This has four features, 2 of which are common to most passive DIs, a preset db pad and a ground lift.  The other 2 options are a tube bypass for the 1/4" thru and balanced XLR portion.

 

[Super Turbo Deluxe Custom]

I would suggest that already happened when they added a tube. What does the tube actually do in this DI? Is it a 12AX7?

Did some digging, it's a 12AX7.

Rather than think of it as a stand alone preamp, think of it as a DI with an effect.

 

[Super Turbo Deluxe Custom]

It arrived.  It has a 12AU7 in it. Did more digging, that's what's supposed to be in it, at least on this day, from this source.

Toyed around with it, just plugged my (passive) bass into this, and this right into a power amp and into a cab with no horn so I could A/B the thing on an uncolored signal.  The difference is noticeable, albeit subtle.  I haven't used the XLR portion into a PA yet, which is why I got it.  Plugged into the front of an SS pre, the effect is noticeable as I A/B the tube.  For comparison, my Carvin hybrid amp has no difference when A/B ing the tube in it.  Why is this one different?

The pad on it is a nice unexpected feature.  At 6 db, as opposed to 10 and 15 db, I'm used to seeing with DI boxes and bass amps, it seems like a good amount to cut.  Of course, I just turn the amp up.