The 12AY7 isn't supposed to add warmth to the signal. The early Fender amps, and especially the 5E3 Deluxe, benefitted from the clarity and modest amplification of the 12AY7. It's making these amps more controllable and provides more headroom. In other applications it might be perceived as lifeless and sterile.
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SurfGuitar101 Forums » Gear »
12AT7 replacement for Texotica reverb
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Joined: May 04, 2014 Posts: 734 
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Posted on Feb 02 2016 07:20 AM
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Joined: Sep 23, 2014 Posts: 1433 Memphis, TN 
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Posted on Feb 02 2016 10:32 AM
Thanks for the info, I'll give it a shot and see how it sounds. Sorry for being a pest, I just don't know what the heck I'm doing. —MooreLoud.com - A tribute to Dick Dale. New EP Louder Than Life available on bandcamp and website. Taking guitar lessons from Jimmy Dale. |
Joined: Sep 24, 2011 Posts: 1424 Wellington, NZ 
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Posted on Feb 02 2016 04:13 PM
All 12A_7 tubes exhibit the same heater filament current draw (300mA at 6.3V, or 150mA at 12.6V), so substituting any one of them for another won't stress the amp's power transformer. Furthermore, most 12A_7 types are readily substitutable without modifications to the circuit being required. However, some 12A_7 types are specifically designed for higher plate current (like a 12AU7 and a 12AT7), whereas other types (12AX7, 5751) are designed for higher voltage gain. To get the most out of each type of tube (i.e. to do the job it was intended for), you need to set the gain stage up so as to ensure optimum transconductance/voltage gain. This means biasing the triode optimally and using and optimal load resistor (be it plate load, or cathode load, or both). Typically for a circuit optimally designed for a 12AU7, you will have lowish value load resistance (~10k to 20k) and a higher value bias resistor (2k7 to 4k7), than you would for something like a 12AX7 (where you typically have between 47k and 220k for the load and about 820R to 2k2 for the bias). You can happily run a 12AU7 triode in a stage designed for a 12AX7, but you won't get the most out of the 12AU7 because the plate current will be quite low, so it will sound pretty lifeless and gutless. Whereas you can more easily run a 12AT7 in a stage designed for a 12AX7, and while the 12AT7 won't sound as gutsy as a 12AX7, it will not sound as bad as a 12AU7 in that slot. But if you tried to run a 12AX7 in a slot optimally designed for a 12AU7 (i.e. with a really low load), you could damage the 12AX7 (and the circuit), because the latter won't be able to handle the high plate current. On the other hand a 12AU7 makes an excellent substitute for a 12AT7 in a reverb driver slot on a blackface or silverface amp, if you want lower gain that a 12AT7. This is because both types are designed for highish plate current, but a 12AU7 has an amplificaton factor of 19, compared to a 12AT7 amplification factor of 60. For a 12AY7 running in either a 12AT7 or a 12AX7 slot, it won't do any harm. A 12AY7 exhibits a lower amplification factor (45) and lowish-plate current (but higher transconductance) than a 12AX7 (amplification factor = 100). All other things (i.e. plate load and bias) being equal, a 12AY7 will result in less gain but slightly more bandwidth than a 12AX7, but less gain and less bandwidth than a 12AT7. A 12AT7 sounds gutsier than a 12AY7, because it has a higher amplification factor than a 12AY7. —He who dies with the most tubes... wins Last edited: Feb 02, 2016 16:14:15 |
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Joined: Nov 16, 2013 Posts: 4537 Wisconsin
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Posted on Feb 02 2016 04:35 PM
What a great summary, in understandable language no less. Stellar. Wes DISCLAIMER: The above is opinion/suggestion only & should not be used for mission planning/navigation, tweaking of instruments, beverage selection, or wardrobe choices. |
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Joined: Sep 23, 2014 Posts: 1433 Memphis, TN
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Posted on Feb 02 2016 05:21 PM
Great explanation, tubeswell. This needs to be stickied, very well done. MooreLoud.com - A tribute to Dick Dale. New EP Louder Than Life available on bandcamp and website. Taking guitar lessons from Jimmy Dale. |
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Joined: Sep 23, 2014 Posts: 1433 Memphis, TN
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Posted on Feb 06 2016 12:31 PM
I just wanted to let you guys know that I replaced the 12AX7 with a GE 5751, and the 12AT7 with a GE 12AY7, and it definitely reduced that harsh, high pitched sound by about 40%. I am definitely happy with the results, and will leave those tubes in my tank. I will keep the other ones for a backup. I know this thread wasn't meant to discuss Fender tanks, but I felt it was still relevant as it is a discussion about tubes, and someone else had already brought up some questions about their Fender tank. I certainly didn't mean to hijack. Thanks for all the help and advice! MooreLoud.com - A tribute to Dick Dale. New EP Louder Than Life available on bandcamp and website. Taking guitar lessons from Jimmy Dale. |
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