12AT7 phase inverter tone

[SonicProvocateur]

So yesterday, I fired up the trusty Mark III after a week of vacation with my fiance, and I got some volume/channel dropouts. Of course I immediately freak out thinking it's a power cap or the grid resistors again or something, but I calm down and diagnosed it as the PI tube. But instead of chucking in one of the AX's I had laying around I decided to throw in this really nice Mesa labeled Sylvania 12AT7 (blackplate) with maybe a couple hours on it I had around and wow, kudos to all the people who suggested that in other threads!

The tone is more lively and spacious for sure! Being a blue stripe, I didn't really notice the drop in gain (it felt more like a 5% loss, which is fine!) but the added articulation and subtle midrange voicing it adds is really cool. I need to switch over the 5881 power tubes back to my Sylvania's after the power tube shootout from this thread: http://forum.grailtone.com/viewtopic.php?f=16&t=56777 and see how she really sounds!

With all the talk of "reduced gain" over the years I thought "meh, this tube cant be for me" but surely, it may just stay in the amp for awhile! Very awesome indeed!

 

[sduck]

Somewhere around here I have a pdf written by one of the big name tube engineers - perhaps the lead guy at groove tubes, I don't remember. It's all about PI tubes, and how to choose them. While a lot of it is fairly technical, his bottom line is that using a 12at7 is the way to go - they handle the stresses better than the 12ax7's do, and sound better also.

 

[Whoopysnorp]

I don't see why gain would drop when using a 12AT7 as the phase inverter instead of a 12AX7. Maybe if some of your gain was coming from the PI hitting the power amp so hard with the preamp's output signal that it was distorting. I haven't yet tried a 12AT7 in my Mark III, but it seemed to increase the clarity on my Randall RM100.

 

[dodger916]

I've been running a Mullard 12AT7WA/CV4024 in the PI of my C+ for years. Real improvement IMO.

 

[EOengineer]

Which position is a PI in a III? Is it the tube closest to the power tubes?

 

[jpdennis]

... instead of chucking in one of the AX's I had laying around I decided to throw in this really nice Mesa labeled Sylvania 12AT7 (blackplate) with maybe a couple hours on it I had around and wow, ...

Glad I found this thread. I am just now toying with this very idea in my no stripe Mark III & my .50 Cal (the EL84 version). I put 12AY7s in V2 & V3 in my old Fender 75 and the girl just seemed much more articulate, clear, AND quieter - this was at the full power level and it has not blown up

Point is, I am assuming this will not do anything adversely to my amps :?:

 

[dodger916]

Which position is a PI in a III? Is it the tube closest to the power tubes?

Should be; it is in other Marks.

 

[SonicProvocateur]

I put my Sylvania 6L6 set #2 in and it sounded like a blanket over the cab. Switched cabs to my good studio cab and sounded the same. Switched to my favorite Sylvania 6L6's and the tone was still withdrawn and "blanketed". Took out the 12AT7 and replaced with a AX and it returned to normal (badass) - wondering if it sounded so keen with the 5881's? Will report back soon!

Might be like Whoopysnorp was saying and the preamp signal hits the poweramp a little harder with the AX versus the AT, or just the AT tone is that much different...hmmm

 

[dodger916]

I put my Sylvania 6L6 set #2 in and it sounded like a blanket over the cab. Switched cabs to my good studio cab and sounded the same. Switched to my favorite Sylvania 6L6's and the tone was still withdrawn and "blanketed". Took out the 12AT7 and replaced with a AX and it returned to normal (badass) - wondering if it sounded so keen with the 5881's? Will report back soon!

Might be like Whoopysnorp was saying and the preamp signal hits the poweramp a little harder with the AX versus the AT, or just the AT tone is that much different...hmmm

Do you have another AT7 you could try? I did not experience this with the Mullard AT7 I insatlled in my C+.

 

[Monsta-Tone]

Did you notice any perceptible loss in volume with the AT?

A lot of Fender guys will run an AX7 as a PI in their Black & Silver face amps to get more overdrive out of them.
I never thought of trying it in reverse.