Swap 12AT7 for 5751

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northernguitarguy
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Swap 12AT7 for 5751

Post by northernguitarguy » Tue May 21, 2019 10:03 am

Hello all....new poster here. Loving my new Rocker 15 Terror. I have plenty of backup tubes, except for the 12AT7. A friend tells me that a JAN-GE 5751 tube I have will make a great substitute. Just checking before I give it a shot.

Any thoughts? Thanks in advance.

PBA
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Re: Swap 12AT7 for 5751

Post by PBA » Thu May 23, 2019 12:27 pm

You'd need to try it to see if you like it but it should work ok and won't damage your amp.

Rlw59
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Re: Swap 12AT7 for 5751

Post by Rlw59 » Thu May 23, 2019 4:58 pm

12AT7 is the fx loop driver/recovery tube. AT7s are used because they can comfortably drive and recover a wide range of impedances. (Most pedals have high impedance inputs and low impedance outputs, but some have low inputs and high outputs.)

You can certainly use a different type as a temporary emergency substitute.

And you can sub in others to see if you prefer their tone. If you like the results, it's safe to use the one you like best.

But, other types may wear out faster than normal. Still years, just not as many years.
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The signal always runs through the loop tube, but it's meant to be a clean unity-gain circuit. The driver circuit drops the preamp signal back down to instrument level and provides a low output impedance, then the recovery circuit provides a high input impedance and boosts the signal back up to preamp level.

I've only had one amp with a tube loop, and I never got around to tube rolling that slot. (Most of my amps haven't had loops; none of my current amps have them.) So I don't know what changes (if any) you'd hear.

5751 and 12AT7 have similar gain (70%/60%), but IIRC the current/impedance characteristics are different. Possibly might interact differently with some pedals, but I wouldn't expect any noticeable change when not using the loop.
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But now I'm wishing I had subbed in a 12AU7 and a 12AX7. I'm guessing that would lower or raise the signal going to the PI?

northernguitarguy
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Re: Swap 12AT7 for 5751

Post by northernguitarguy » Sat May 25, 2019 1:00 am

PBA wrote:
Thu May 23, 2019 12:27 pm
You'd need to try it to see if you like it but it should work ok and won't damage your amp.
Thanks! I though it would be safe, but wasn't sure.

northernguitarguy
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Location: Ontario, Canada

Re: Swap 12AT7 for 5751

Post by northernguitarguy » Sat May 25, 2019 1:01 am

Rlw59 wrote:
Thu May 23, 2019 4:58 pm
12AT7 is the fx loop driver/recovery tube. AT7s are used because they can comfortably drive and recover a wide range of impedances. (Most pedals have high impedance inputs and low impedance outputs, but some have low inputs and high outputs.)

You can certainly use a different type as a temporary emergency substitute.

And you can sub in others to see if you prefer their tone. If you like the results, it's safe to use the one you like best.

But, other types may wear out faster than normal. Still years, just not as many years.
------------------------
The signal always runs through the loop tube, but it's meant to be a clean unity-gain circuit. The driver circuit drops the preamp signal back down to instrument level and provides a low output impedance, then the recovery circuit provides a high input impedance and boosts the signal back up to preamp level.

I've only had one amp with a tube loop, and I never got around to tube rolling that slot. (Most of my amps haven't had loops; none of my current amps have them.) So I don't know what changes (if any) you'd hear.

5751 and 12AT7 have similar gain (70%/60%), but IIRC the current/impedance characteristics are different. Possibly might interact differently with some pedals, but I wouldn't expect any noticeable change when not using the loop.
---------------------
But now I'm wishing I had subbed in a 12AU7 and a 12AX7. I'm guessing that would lower or raise the signal going to the PI?
Thanks for the detailed reply! :D

JJJ
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Re: Swap 12AT7 for 5751

Post by JJJ » Sat Jun 22, 2019 10:57 am

yeah not really. 5751s might swap with a 12ax7, 12ay7, etc but the 12at7s are specifically made for driver positions so if you need a spare, I'd buy some more 12at7s. If you're looking for JAN tubes there are general electric 12at7s around cheap, but IME the current Russian stuff is just fine

All the 12a types have different bias requirements, different current draw at those points and different internal resistance. I have managed to swap 5751s, 12ay7s and 12ax7s just fine but I would not sub a 12at7 for any of those, or the other way around

northernguitarguy
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Re: Swap 12AT7 for 5751

Post by northernguitarguy » Sat Jun 22, 2019 12:51 pm

JJJ wrote:
Sat Jun 22, 2019 10:57 am
yeah not really. 5751s might swap with a 12ax7, 12ay7, etc but the 12at7s are specifically made for driver positions so if you need a spare, I'd buy some more 12at7s. If you're looking for JAN tubes there are general electric 12at7s around cheap, but IME the current Russian stuff is just fine

All the 12a types have different bias requirements, different current draw at those points and different internal resistance. I have managed to swap 5751s, 12ay7s and 12ax7s just fine but I would not sub a 12at7 for any of those, or the other way around
Thanks for the reply, JJJ. Cool to learn all this, I appreciate it. In the end, same friend tossed me a NOS Sylvania 12AT7 and it works great! I'll keep the Shuguang as a backup. I like having a backup tube for every spot.

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