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Post subject: 12AX7 vs. 5751
Posted: Fri Apr 05, 2013 1:16 pm
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Can a 5751 replace the 12AX7 for lower gain? Or does the 12AX7 serve another function where that change isn't wise? TIA.
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Post subject: Re: 12AX7 vs. 5751
Posted: Wed Apr 10, 2013 3:16 am
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YES

An easy research with Google ;

http://tubedepot.com/5751.html


One of Stevie Ray Vaughn’s favorite tubes was the 5751 preamp tube. With 30% less amplification than a 12AX7, but 100% interchangeable, an amp using the 5751 preamp tube creates a creamier and smoother tone than with a 12AX7. If your amp has too much gain, the 5751 is the perfect choice to "calm" the amp down and get more break-up without all the volume. We prefer the U.S. made JAN 5751 over any other 5751 available.


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Post subject: Re: 12AX7 vs. 5751
Posted: Thu Apr 11, 2013 1:33 pm
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Thanks for the info. Although I did know most of it, having used the GE 5*****5751 and the JAN 5751, I just wasn't sure about putting one into the X2. Now I understand that the 12AX7 in the X2 is like V1, on a say, Princeton Reverb. Good news. I've got 2 NOS 5751s, a JAN and a GE, so time to do some swapping and have some fun. I also put a pair of JAN6V6GTs in and biased them to 40mV, plus a Merc Mag Axiom output tranny for a PR. Next up is a Weber 10F150, 50W 8 ohm speaker. Once I get the speaker installed, coming next week, I'll really get down to tweaking! Thanks again, bro. I'll be back with an update...for those interested....

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Post subject: Re: 12AX7 vs. 5751
Posted: Sun Apr 28, 2013 11:55 am
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Hi

I would like to check the biasing on my X2, since you have successfully done this could you give me any tips on how this is done (i am an electrician so i am aware of the dangerous voltages) Does the chassis slide out the front of the amp knobs and all or does it slide out backwards, also I realise you ground one multimeter lead to the chassis, where do you put the multi meter probe to measure the mV when you are adjusting the bias pot, and am i right in thinking 40mV is the optimum voltage?

There are a few questions here so thanks and your help would be much appreciated.


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Post subject: Re: 12AX7 vs. 5751
Posted: Sun Apr 28, 2013 4:10 pm
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Remember if it breaks at some point, & you change the tubes, (and the authorized repair guys know about that), you void the Fender warantee. So be extra careful with that idea. I am going to change out the tubes on my X2 to JJ's, and BIAS it myself with JJ's Bias probes (Or if I can do that with a multimeter that will also do.) I may also want to use a hi gain preamp tube to add a little punch to the clean channel as it is too clean for me at higher volumes I need a little break up at that time, however I will have to wait to do that to see if the amp stays stable as shipped, because once I do that I void the warantee which is 5 years long from time of purchase. So I want a few months to see if it holds up and truthfully it sounds great with several of my guitars as shipped (With Fuze tweaks).

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Post subject: Re: 12AX7 vs. 5751
Posted: Sun Apr 28, 2013 4:11 pm
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I am also interested in the technique for removing the chassis.

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Post subject: Re: 12AX7 vs. 5751
Posted: Sun Apr 28, 2013 4:25 pm
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Good info on BIASING the amp on the link below.

http://www.archive.agileguitarforum.com ... ?tid=11815

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Post subject: Re: 12AX7 vs. 5751
Posted: Sun Apr 28, 2013 4:27 pm
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Also this

http://www.strat-talk.com/forum/amp-inp ... -scxd.html

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Post subject: Re: 12AX7 vs. 5751
Posted: Mon Apr 29, 2013 12:57 pm
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Many thanks, that strat forum has excellent instructions on how to bias my amp.


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Post subject: Re: 12AX7 vs. 5751
Posted: Mon Apr 29, 2013 10:17 pm
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Thanks DBM. Very helpful.

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Post subject: Re: 12AX7 vs. 5751
Posted: Tue Apr 30, 2013 2:02 pm
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modwiz wrote:
Thanks DBM. Very helpful.


Google is my friend :lol:

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Post subject: Re: 12AX7 vs. 5751
Posted: Wed May 01, 2013 12:57 pm
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Hi

Just an update, based on the excellent instructions you pointed me towards I dismantled the chassis of my X2 and checked the bias voltage, to my surprise my Valves are biased at 42mV straight out of the factory! :D I left it as it was and put her back together! Although i now wonder if i should have turned it down a tad to 38-40 mV??


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Post subject: Re: 12AX7 vs. 5751
Posted: Wed May 01, 2013 1:42 pm
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Cool.....Or should I put it correctly, hot....... I would leave it as is, the BIAS if it is too hot and you burn it every day with lots of volume, it will wear out the power tubes quicker, but 2 tubes are relatively cheap these days, so tone to me is more important than a few extra months of tube life. If the tone is to your liking keep it like it is....

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Post subject: Re: 12AX7 vs. 5751
Posted: Sun May 05, 2013 3:16 pm
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Yep, real easy to bias the SCX2. The proper way is to check the plate voltage first, then go to WeberVST and use the voltage/bias calculator. It's in the forum section. My voltage was running at 395V, which calls for a 48mV setting on the bias pot, with consideration to measuring both tubes at once from R20. That seemed a bit hot to me so I'm running her at 42mV until she's broken in a bit more. With the JAN6V6GTs and a 5751 preamp tube, a new WEBER 10F150/50W/8ohm and the new Merc Mag OP tranny for a Princeton, I'm now in the tone zone I want to hear. The 5751 is a rugged tube with low noise and microphonics. The reduction from a 12AX7, gain factor of 100, to the 5751 gain factor of 70, plus low noise and microphonics, is a win-win for me. Check it out.


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