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Post by zpilot on May 10, 2012 14:13:19 GMT -7
I have a bunch of NOS 12AT7's in my stash so I thought I'd try one in my JAZ's PI position and I think I'm going to like it. As always I will need to do a show with it before I can really tell. The one I used is a CV4024 and labeled KQDD/K so I believe it is manufactured by Mullard. It has a little less gain than the stock 12AX7LPS and is cleaner so I have bumped the volume and tone controls up a little to compensate. I usually have my MV maxed so no change there. Anyone else tried this?
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Post by zpilot on Jun 17, 2012 17:46:47 GMT -7
I've played a couple of shows and an audition now with the different PI and it's absolutely a permanent change to this amp for me. You JAZ guys gotta try this. It lets me get the tone controls up more in the middle of their range where I think they sound better and also allows more headroom in the power amp. I don't mean to be re-engineering Doc's design but I think this really changes the character of this amp for the better.
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Post by bluzman on Jun 17, 2012 22:47:02 GMT -7
Yes... The LPS can make it a little brighter and I like it less bright. I have tried different 12AX7s to tame it but will try the 12AT7. thanks for the tip.
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Post by twangstang on Jun 27, 2012 19:27:08 GMT -7
I'm thinking about trying this. Which NOS would you recommend in the P.I?
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Post by zpilot on Jun 27, 2012 21:55:37 GMT -7
The only one I have tried is a Mullard but I would expect similar results with any NOS 12AT7 tube, especially a JAN. The important thing is it needs to be balanced to work properly for this function. Mike at KCA has a good stock at resonable prices.
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Post by twangstang on Jun 28, 2012 6:31:08 GMT -7
Thanks for the reply. I don't believe that the P.I needs to be balanced in the Jaz. I don't believe that Doc designed it that way but I could be wrong. The only one I have tried is a Mullard but I would expect similar results with any NOS 12AT7 tube, especially a JAN. The important thing is it needs to be balanced to work properly for this function. Mike at KCA has a good stock at resonable prices.
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Post by jesslm02 on Jun 28, 2012 8:22:49 GMT -7
It's my understanding that all phase inverters should be balance/matched as close as possible. The phase inverter is a very important part to the function of a push-pull designed amp. Will it work if they are not balanced? Yes, but it will not provide the optimal signal, tone and function to the power amp stage.
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Post by twangstang on Jun 28, 2012 11:34:16 GMT -7
The Jaz isn't a push pull circuit, it's cathode bias so my understanding is that you don't need a balanced P.I. Maybe the Doc will chime in and clarify.
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Post by zpilot on Jun 28, 2012 22:45:08 GMT -7
The JAZ power amp is a push-pull circuit and the tubes are cathode biased. The PI should be balanced in order to drive each phase symmetrically. Although it's not absolutely necessary that this tube be balanced it doesn't make much sense to buy a high end amp like a "Z" and skimp on this. After all, the up-charge for this is no more than five bucks.
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Post by jesslm02 on Jun 29, 2012 5:13:05 GMT -7
Zpilot is right. And FWIW, most tube amps that have two or more power tubes, are push-pull circuits. There are some other rare designs but generally this is the case. But just because they are push-pull doesn't mean they have to be cathode biased, they could be fixed bias as well. The majority of the Doc's stuff is cathode bias because it makes it easier to swap tubes. In a fixed bias amp you would have to adjust the bias for your tubes if you were to change them out. In the cathode bias design, just grab a matched set of tubes and plug them in. It pretty much does the biasing for you, and you can't adjust it.
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Post by twangstang on Jun 29, 2012 8:10:11 GMT -7
I talked to Dr. Z and you don't need a balanced P.I in the Jaz although it doesn't hurt anything to have it balanced. I have two LPS tubes, one balanced and the other is not. I don't hear any difference between the two.
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Post by jesslm02 on Jun 29, 2012 10:33:26 GMT -7
I haven't experimented with Jaz, but I've swapped phase inverters in other Z amps to a newer balanced one and was able to detect a pretty significant change. Sometimes the different sides of the tube will wear at different rates and become unmatched. I've noticed the biggest differences in matched versus unmatched when you get into natural power tube overdrive. If you mostly play it clean, then you won't have to worry too much about it as long as it's close. They are rarely perfectly matched anyways.
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Post by zpilot on Jul 6, 2012 23:44:31 GMT -7
As I posted before I have played several shows with the PI changed to a 12AT7 and was liking it. These shows required a really clean tone with a lot of headroom and that tube really worked better than the stock tube. However, last week I rehearsed with our worship team. I needed a more contemporary slightly overdriven sound and the JAZ just wasn't cutting it. I changed back to the stock 12AX7LPS and the magic was back. I could easily use this amp for any gig I play with just tube or speaker changes.
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Post by twangstang on Jul 10, 2012 10:08:30 GMT -7
I tried putting a NOS Jan 12AT7WC in the P.I. It made the amp sound a little thinner. It was a cleaner tone but I'm not too sure I like it. I'll give it a little more time.
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