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Post subject: Super Champ XD phase inverter replacement tube.
Posted: Tue Sep 07, 2010 5:55 pm
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The phase inverter tube is one of the hardest working tubes a class AB amp. Mesa has a 12ax7 tube that is labeled "SPax7." supposedly a hard worker. Does anyone have experience with this tube ? What is the best phase inverter valve out there? Thank You. PDyckman


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Posted: Wed Sep 08, 2010 11:32 am
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What is important in a phase inverter is that the tube is balanced on both sides.

The SpAX7 is a tube identified by Mesa as sounding good in preamp stages, not necessarily for phase inverter sections.

The best thing is to request a balanced 12ax7 for the inverter, from whatever manufacter you choose.

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Posted: Wed Sep 08, 2010 11:34 am
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firstrat wrote:
The best thing is to request a balanced 12ax7 for the inverter, from whatever manufacter you choose.


+1

Arjay

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Posted: Wed Sep 08, 2010 12:02 pm
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I have and use SPAX7, it is (according to MESA) a "high grade substitute for 12ax7 / ECC83"; It is a good preamp valve, I tested it on my BDRI (the drive channel was really sounding compare to GTs) and also on my Vox (today I uses Siemens ECC83). As MESA didn't produce valves, it's only a sorting for Russian and/or Chinese makers. But it's difficult to know because there is a kind of plastic around to avoid microphonics.


At last, + 1 for previous mails, if the valve is not mentionned matched or balanced triodes, it is not the best candidate for the "long tailed stage".

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Posted: Wed Sep 15, 2010 9:45 am
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The SCXD PI is of the concertina style, using only half the tube for PI duties so a matched tube won't help you. The other half of the tube is a typical Fender gain stage.


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Posted: Wed Sep 15, 2010 10:10 am
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djd100 wrote:
The SCXD PI is of the concertina style, using only half the tube for PI duties so a matched tube won't help you. The other half of the tube is a typical Fender gain stage.


Indeed, you are correct. Linking the power tubes cathodes with a solid-state diode is a pretty innovative design.

Good catch!

Arjay

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Posted: Wed Sep 15, 2010 12:18 pm
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++1 djd100 ! It s not the usual inverter based long tailed stage, but the variable "resistor". In fact the second triode is necessary to have gain due to the fact that the concertina is equivalent to a follower. Consequenlty, there is no need of triode matching.

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Posted: Wed Sep 15, 2010 8:37 pm
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Isn't that circuit off the cathodes a noise suppression or avoidance method ? It elevates the cathode a little above the ground potential, right ? Art

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Posted: Thu Sep 16, 2010 8:08 pm
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Nope ! That circuit is just for checking the power tube bias in DCmv. Duh ! My bad. Art

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