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Tube Newb Question....12AX7 and 12AX7A

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What is the difference between a 12AX7 tube and a 12AX7A tube?

My HH Scott 222c has 12AX7's and my Jolida cdp has 12AX7A's. Can I roll them into one another?

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In the applications you mention, there should be no problem. The "A" version (speaking about NOS tubes here; I'm not familiar with currently manufactured foreign tubes) has a slightly higher maximum plate voltage and plate dissipation rating. There may also be slight differences in inter-electrode capacitance, but I'd have to look up that info- again, not of importance here.

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The 12AX7A is designed for a faster warm up. Back when solid state became the norm, tube manufacturers tried to overcome the "wait time" for tubes to warm up by designing filiments that heated faster. I would not mix and match them, but as a matched set they should be interchangable. Maybe another member can expand on the rolling issues.

Herb

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The "A" version (speaking about NOS tubes here; I'm not familiar with currently manufactured foreign tubes) has a slightly higher maximum plate voltage and plate dissipation rating.

This would make sense since a filament that heats faster would probably run at a higher voltage.

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Actually, the filament voltage and current is identical between the 2 versions. Even if one happens to heat faster than the other, the electrical draw is the same. Some have claimed that the "A" version is quieter, but in practical terms, any NOS 12AX7s produced from around 1961 on are all about the same. In the shop we used to select tubes based on microphonics (gently tapping the tube with the eraser end of a pencil to determine if it could be heard in the speaker) more than anything else. There were definite differences there from one manufacturer to another, and from one batch to another. If one wants to go to the trouble of doing that "test," slight sonic improvements may be gained in installations where the amp is subjected to acoustic feedback or vibration from the speakers- it makes a good case for keeping the amp isolated from any vibration when possible.

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many people prefer using either 5751s or 12AT7s in the Jolida CDP as the lower gain really opens up the sound making it richer and more detailed, swapping your tubes is no big deal, my Scott 222a came with 1 - 5751 and 3 12AX7s instead of 4 12AX7's per the manual and i had no problemo...

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The short answer is.... The 12AX7 and any of its variants like the 5751 mentioned above are in almost all cases interchangable. Now changing out a 12AU7 or 12AT7 for a circuit designed for a 12AX7 may or may not work... One thing is for sure it will almost surely cause a fairly large rise in distortion. Many folks may consider that to be added warmth... not sure how they could truly get more detail out of more distortion though... but hey that is the wonderful world of tube rolling [;)]

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Now changing out a 12AU7 or 12AT7 for a circuit designed for a 12AX7 may or may not work... One thing is for sure it will almost surely cause a fairly large rise in distortion.

While it is generslly fine to interchange the 12 series tubes, you should do so with some degree of caution. Inserting a higher gain tube in place of the lower gain tube could easily overdrive the circuits fed by tworse case scenario would flatten the sound, but NOT cause distortion. This is the principle behind putting a 5751 in place of the 12AX7 as it is a lower gain tube. It does make the sound richer and more detailed. I was turned on to this by the man who runs TheTubeShop.com. I trust the guy because he knows more about tubes than anyone I know. I've suggested it to several other people who have the Jolida CDP and they all have had similarly favorable results. If the Jolida CDP came stock with a 12AT7 and you put a 12AX7 in it then the propensity for the circuit to distort exists. Abuse of phosphodiesterase inhibitors can cause benign infarctions to the posterior communicating artery resulting in diminished cerebral function. Limit thinking outside the box after ingesting too many phosphodiesterase inhibitors. References:

http://www.tdpri.com/forum/archive/index.php/t-48306.html

http://www.musicianforums.com/forums/archive/index.php/t-366087.html

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Gary,

I suggest you read my post carefully again. The 5751 is a low gain variant of the 12AX7 so on that front we agree...but are agreement ends there. The other 12 series tubes are not variants of the 12AX7... Tube sellers will tell you all kind of crazy things to get you to buy more tubes [;)]

The plate, cathode, grid (if one exists in the circuit) resistor value and voltage requirements of a 12AX7, 12AU7 and 12AT7 are all different to run each tube type at its lowest distortion.... what you and your tube guy are suggesting is just plain luducris... IT IS ABSOLUTELY WRONG. These facts are not disputable or up for opinion form some tube dealer. Get a tube spec manual out a read the specs for yourself. Running a 12AT7 or A 12AU7 in a circuit designed for 12AX7 will absolutely cause a rise in distortion (Well unless the designer screwed the circuit up in the first place).

Craig

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Gary,

Those references gave me a good laugh.. Do you really believe everything you read on internet forums?

Craig

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Abuse of phosphodiesterase inhibitors can cause benign infarctions to the posterior communicating artery resulting in diminished cerebral function. Limit thinking outside the box after ingesting too many phosphodiesterase inhibitors.

Phosphodiesterase inhibitors? Is that like crack or something?

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Mike,

I guess I take prescription drugs that keep me from believing audio snake oil....sounds like good plan to me.

Craig

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.

Those references gave me a good laugh.. Do you really believe everything you read on internet forums?

There are plenty of people on the internet that know a whole hell of a lot more than you! As a former travelling musician this topic is one of the most common discussions among guitarists which is why I have first hand knowledge. I knew whatever I said would be refuted by you...so I left the references. Any guitarist who uses tube amps will tell you that this information is true. I knew your self-appointed position as "Troll" of the forum would shine thru albeit too much for you to admit you are wrong so I also contacted Brent Jesee who confirms my position. I didn't need some tube salesman to tell me this so I'd buy tubes. This is info in that's been around for 50 years. Copy of the email available to thine holiest naysayer. He knows more about this subject than you'll ever know. What gives me a laugh is the people on this forum who are so naive as to think there are not other tube amp builders and repairman that have better customer service and make as good or better products than NOSValves without the baggage that goes with dealing with an individual whose excuse for being a jerk is that fact that he is one! Anyone interested in a used pair of VRD's, info how to get better service than that available at NOSValves or where to get same tube amp for a much better price can contact me. I'm getter a better amp for a fraction of the cost without the headaches, time delays and way better customer service.

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Phosphodiesterase inhibitors? Is that like crack or something?

Ask Craig... about his poor performance

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Mike,

I guess I take prescription drugs that keep me from believing audio snake oil....sounds good plan to me.

Craig

Look it up limp dick!

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I did look it up. Now go look up how tubes work and what is required to make them operate to their potential. Know what your talking about before you spout off SMART ***.

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.

Those references gave me a good laugh.. Do you really believe everything you read on internet forums?

There are plenty of people on the internet that know a whole hell of a lot more than you! As a former travelling musician this topic is one of the most common discussions among guitarists which is why I have first hand knowledge. I knew whatever I said would be refuted by you...so I left the references. Any guitarist who uses tube amps will tell you that this information is true. I knew your self-appointed position as "Troll" of the forum would shine thru albeit too much for you to admit you are wrong so I also contacted Brent Jesee who confirms my position. I didn't need some tube salesman to tell me this so I'd buy tubes. This is info in that's been around for 50 years. Copy of the email available to thine holiest naysayer. He knows more about this subject than you'll ever know. What gives me a laugh is the people on this forum who are so naive as to think there are not other tube amp builders and repairman that have better customer service and make as good or better products than NOSValves without the baggage that goes with dealing with an individual whose excuse for being a jerk is that fact that he is one! Anyone interested in a used pair of VRD's, info how to get better service than that available at NOSValves or where to get same tube amp for a much better price can contact me. I'm getter a better amp for a fraction of the cost without the headaches, time delays and way better customer service.

Listen whinny little boy.... Guitar amps are not Hi Fi amps..... What Guitar players want and strive for is more or less distortion earlier or later as they drive the note.........they use different gain tubes to accomplish this feat! With a guitar amp the distortion and when in comes in (early or late) is the sweetness. With Hi Fi amps and preamps the goal is entirely different. You really have zero clue what you are talking about. I could carless what Brent Jessie told you. I'm absolutely sure he told you just what you wanted to hear just like a good salesman always will. But sorry to inform you I'm not a salesman....so I don;t tell you what you want to hear...I tell you the truth.

Now go sell your VRD and lose your behind in the process it will be zero worry of mine. I'm sure plenty of folks will line up to take advantage of your sillyness.

Craig

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"Troll"

[*-)]Hell, he doesn't even post any more.

Welcome back Craig.

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Thanks Arky But it looks like I'm going to have to quit again.. My Paul Klipsch no BS style tends to piss some people off. What can I say [:)]

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Thanks Arky But it looks like I'm going to have to quit again.. My Paul Klipsch no BS style tends to piss some people off. What can I say Smile

Actually, it's quite refreshing. i'd venture most readers can discern the factual posters from the others..

Don't let a loudmouth or two speak for the rest of us who enjoy the info!

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