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 Post subject: phase inverter
PostPosted: Mon Jan 04, 2010 11:22 pm 
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Bottle Rocket

Joined: Mon Jan 04, 2010 11:15 pm
Posts: 2
I have a 1980 60W Mesa Boogie Mark II(B?) I believe. I am replacing the tubes in it and I know it is fairly important to put a balanced tube in the phase inverter slot. Does anyone know which preamp tube is the phase iverter?


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 Post subject: Re: phase inverter
PostPosted: Tue Jan 05, 2010 3:04 am 
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Dual Recto

Joined: Mon Jul 16, 2007 1:01 am
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Location: SoCal
I can't find the exact tube layout for a IIB, but generally speaking, the phase inverter tube is closest to the power tubes and/or furthest away from the input jack.
Not to burst any bubbles, but the recommendation of a balanced 12AX7 is a leftover from audiophile tube hi-fi days. A guitar amp has many thousands of times more distortion inherent in it's design, so a balanced PI is like using non-fat mayo on a bacon double-cheeseburger in order to make it health food. Of course, YMMV. Please let us know if you hear any difference (in a blind "taste test" :D )


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 Post subject: Re: phase inverter
PostPosted: Tue Jan 05, 2010 5:29 am 
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Donating Member

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MrMarkIII wrote:
I can't find the exact tube layout for a IIB, but generally speaking, the phase inverter tube is closest to the power tubes and/or furthest away from the input jack.

+1. On IICs it's the one 12ax7 that stands with the power tubes.

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 Post subject: Re: phase inverter
PostPosted: Tue Jan 05, 2010 12:18 pm 
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Bottle Rocket

Joined: Mon Jan 04, 2010 11:15 pm
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Ok that helps. I'll switch them around. I was thinking that the one 12ax7 that stands with the power tubes is the reverb tube. I'm just trying to get the best tone possible.


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 Post subject: Re: phase inverter
PostPosted: Tue Jan 05, 2010 8:38 pm 
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Mark IV
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I have found that the Sovtek 12AX7LPS works really well as the PI in most amps...

ty

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 Post subject: Re: phase inverter
PostPosted: Tue Jan 05, 2010 9:55 pm 
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Mark III
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Joined: Sat Aug 16, 2008 7:39 pm
Posts: 300
Location: New York
I totally agree with MrMark.
/cheers

If the resistors ect. connected to the damn tube have a tolerance of + or - 20% how in the hell
is a balanced P.I going to correct that. IMHO, sometimes your unbalanced tube, with the help of a
roll of dice might just balance that P.I section.. :shock:


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 Post subject: Re: phase inverter
PostPosted: Tue Dec 27, 2011 8:34 am 
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Mark I

Joined: Tue Oct 16, 2007 2:41 am
Posts: 29
just found this about PI

http://www.guitaramplifierblueprinting. ... verter.pdf


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 Post subject: Re: phase inverter
PostPosted: Tue Dec 27, 2011 10:32 am 
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Donating Member

Joined: Sun Jul 06, 2008 9:48 am
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MrMark's analogy was funny and true!

I've spoken to techs at Mesa Boogie, and they will tell you that you do NOT need a balanced phase inverter. In contrast, some of the old school amp techs I've visited will insist that the phase inverter must be balanced. I've tried balanced and standard 12AX7s and can tell no difference.

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 Post subject: Re: phase inverter
PostPosted: Tue Dec 27, 2011 10:35 am 
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Mark III
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musicman700 wrote:


Thank you, musicman700, for this. Interesting.

Dennis

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 Post subject: Re: phase inverter
PostPosted: Tue Dec 27, 2011 11:25 am 
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Mark I

Joined: Sat Oct 03, 2009 12:59 am
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Location: Chicagoland
+1 on Myles Rose and all the information he shares on his Guitar Amplifier Blueprinting site.


Juan


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 Post subject: Re: phase inverter
PostPosted: Tue Dec 27, 2011 10:45 pm 
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Mark IV
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it *was* interesting, although somewhat a bit disjointed and at times self contradictory. First time I read it I thought, "huh?" Had to read it again. Jury still out on final verdict.

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 Post subject: Re: phase inverter
PostPosted: Thu Oct 25, 2012 9:37 pm 
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Mark I

Joined: Mon Oct 08, 2012 12:58 pm
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musicman700 wrote:


I think I'll have to look into changing the PI soon.

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 Post subject: Re: phase inverter
PostPosted: Fri Mar 24, 2017 2:17 pm 
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Mark III
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Joined: Wed Feb 27, 2013 12:45 pm
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Location: Southwest Michigan
I was researching the PI tube as I am considering using a 12AT7 in the PI slot of my TC-50 (there already is an AT7 in the reverb driver slot). The Myles Rose document on the PI tube was somewhat confusing, or maybe it's just my older brain - or lack thereof...
When he says, "Going from a 12AX7 to a 12AT7 in the PI (phase inverter) will yield a change in output tube
distortion, touch, and output dynamics in most amps." He doesn't really indicate which direction the sound will change, meaning, more power tube distortion? more sensitivity? Or less of either 2? Anyone made the switch to an AT7 in their PI? If so, what changes did you notice, if any?

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