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12A-7 Tubes

 
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GreenCustom24
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Joined: 16 Jan 2015
Posts: 47
Location: Wilmington, North Carolina

PostPosted: Thu Jan 26, 2017 2:30 am    Post subject: 12A-7 Tubes Reply with quote

Hey. So I'm doing research on preamp tubes. Reducing gain with 12AT7, 12AU7, etc. I'm finding all sorts of conflicting info online about swapping them out. One would think they'd be direct drop in replacements for a 12AX7, but apparently they arent. a 5157 would be. Anyway... is it or is it not safe for the amp/use/whatever to replace a 12AX7 with one of those? I'm looking to go super clean, so I think a 12AU7 (least amount of gain I can find) would be best. Half the guys say it's "not ideal" or even bad for the amp and the other half do it all the time...
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George
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Joined: 03 Jul 2009
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Location: UK

PostPosted: Thu Jan 26, 2017 7:35 am    Post subject: Reply with quote

As I understand it, yes it is safe to change a preamp tube to a lower gain tube, BUT it would just make your amp lower in volume overall and your gain pot wouldnt have as much range (it would stop at the previous 12AX7s 2 o'clock, for example) . Unfortunately it won't add any extra clean headroom that isn't already available. To get more headroom, a more efficient speaker could help, or sometimes variants of power tubes are said to squeeze out a bit more.

If you never go above a certain level on the gain pot and there's a little pita sweet spot you want to elongate then maybe a lower gain preamp tube would be useful, though.
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NickS
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Joined: 14 Jan 2008
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Location: Nr. Basinggrad, UK

PostPosted: Thu Jan 26, 2017 9:43 am    Post subject: Reply with quote

Whether it gives you more clean headroom depends on where you're looking for it and where your gain controls are. As George says, it's not going to help your power amp stage but for example, if you have both halves of the first preamp valve before your gain control, a high-gain first valve could lead to clipping with a hot input; I've seen people say that lower gain valves can tame some amps that are over-hot, e.g. the Peavey Valveking. Actually, I've seen mods to completely bypass one or two of the preamp valves on a Valveking.
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Doog
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Joined: 24 Apr 2006
Posts: 19525
Location: London

PostPosted: Thu Jan 26, 2017 10:31 am    Post subject: Reply with quote

Obviously it depends on your amp, but I had success with JJ E34L (not a typo, rather than the standard EL34), which have a higher grid voltage (apparently), which helped with clean headroom on my Marshall DSL50.

Maybe a combo of a similar approach (lower gain preamp tubes, higher voltage power tubes) will sort you out? Don't overlook what a bias can do to your sound also..
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George
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Joined: 03 Jul 2009
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PostPosted: Thu Jan 26, 2017 11:17 am    Post subject: Reply with quote

^ I've heard of the same tactic for Vox amps (there's a "military" EL84M or something), but my problem isn't the overall headroom, it's that I drive the preamp just a little too hard so that when boosting, the reverb gets a bit messy. Starting to wish I'd got the AC30 now which has an FX loop. Oh well, ho hum etc.
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GreenKnee
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Joined: 29 Oct 2009
Posts: 573
Location: Sheffield, England

PostPosted: Thu Jan 26, 2017 1:23 pm    Post subject: Reply with quote

I successfully achieved a clean Fender DeVille by swapping the preamp tubes to 12AY7s, but I think what made the biggest difference was biasing the tubes to a lower voltage. I don't think this is possible on all amps, but is pretty simple on amps that you can. On the DeVille I measured the voltage of the power tubes from the 'Bias Teat Point' and then adjusted the 'Bias Pot' until it read 60mV.

Be careful and good luck
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GreenCustom24
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Joined: 16 Jan 2015
Posts: 47
Location: Wilmington, North Carolina

PostPosted: Fri Jan 27, 2017 2:17 am    Post subject: Reply with quote

Nice. Thanks guys. I'll definitely look more into swapping them out. I'm looking into either a Jet City amp or a used Peavey Windsor. If I get the Windsor I wanna put KT88s in the power section and 12AU7s in the preamp. Once i found out you could swap the power tubes in that thing to pretty much whatever you wanted I was kinda sold.
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