12AX7 Tube Substitutions

Hpilotman

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V1 and/or V2 positions.

Quote:
"5751 tube is a close equivalent of 12AX7 with slightly reduced gain. This makes 5751 a better choice for many high linearity low distortion audio circuit. 5751 tube also works great in guitar amps in place of 12AX7 to tame the gain."

Quote:
"Description:
Low-noise Tube for Best Clean Tone!

The TAD Selected 5751 is our first choice for noticeably improved clarity, smoothness, and a true vintage tone character for most guitar amps! There will still be plenty of gain - so if you own a high-gain preamp that does not have these classic crunch characteristics, the 5751 tube will work wonders. In hi-fi preamps and CD players, the TAD selected 5751 will lower the noise floor and sound smoother and clearer than many 12AX7 tubes.

Especially this low-noise tube is recommended for best clean tone with big bottom, warm midrange and silky top end. Performs great for HiFi too! Replaces any 12AX7A/WA, ECC83, 7025 etc.

NOTE: SRV used a 5751 to improve his clean tone with vintage Fender amps producing that thick and three-dimensional bold tone."



https://tubedepot.com/t/tubes/preamp-tubes/5751
 
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slider313

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FYI: the 7058 is not a drop in replacement for a 12ax7 as it needs a 12.6v series heater line, not the 6.3v parallel heater lines you find in most guitar amps.
 

Torren61

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FYI: the 7058 is not a drop in replacement for a 12ax7 as it needs a 12.6v series heater line, not the 6.3v parallel heater lines you find in most guitar amps.

Thanks for the heads up. I was relaying the information I got from this site.

http://www.audiotubes.com/

They sell NOS and ANOS and hard to find tubes.

No mention of the issue with the 7058 tube.

"7058: Like the 6681 above, this is a specially selected 12AX7. The JEDEC registration says it was originally made for mobile operation from 6 cell storage batteries, and has additional lot tests for filament cycle times, Low frequency Vibration test and a 500 hour test at 15 volts on the filament and Maximum Plate dissipation. Already burned in, this tube has very stable output and makes an incredible audio or guitar amp tube. Rare, so if you find them, give them a try!"
 

slider313

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Thanks for the heads up. I was relaying the information I got from this site.

http://www.audiotubes.com/

They sell NOS and ANOS and hard to find tubes.

No mention of the issue with the 7058 tube.

"7058: Like the 6681 above, this is a specially selected 12AX7. The JEDEC registration says it was originally made for mobile operation from 6 cell storage batteries, and has additional lot tests for filament cycle times, Low frequency Vibration test and a 500 hour test at 15 volts on the filament and Maximum Plate dissipation. Already burned in, this tube has very stable output and makes an incredible audio or guitar amp tube. Rare, so if you find them, give them a try!"

The "key" phrase there is 15 volts on the filament. The 6681 is a "mobile" rated tube with a beefed up filament and a direct replacement for a 12ax7. They were used in police, fire department and coast guard communication radios to withstand many on/off cycles.
 

Torren61

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The "key" phrase there is 15 volts on the filament. The 6681 is a "mobile" rated tube with a beefed up filament and a direct replacement for a 12ax7. They were used in police, fire department and coast guard communication radios to withstand many on/off cycles.
You should email him and tell him that. He won't believe me because I don't know what I'm talking about most of the time, lol. I have my degree from Dunning-Kruger. :D
 

Telekarster

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Here are some tubes that you can sub for 12AX7 tubes. Some of these can be found at a much cheaper NOS price than 12AX7s and are better. So don't get suicidal about the world running out of tubes just yet.

12AX7A
ECC803S, E83CC, E803CC, 6L13
B759
12DF7
7025
12AD7
12DT7
12BZ7
7729
6681
7058

Thanks man! Gosh.... I wish there was some sort of cross ref chart (maybe there is?) for all of our most popular tubes that would show proper alternatives. EL84's, 6L6, etc. etc. My understanding thus far is that, while some tubes are "close" in the properties to XYZ, they are not the same... but I'm always unclear of exactly what that means, and whether or not a substitute tube could cause harm to the amp, or burn out faster, etc.
 

Wally

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Thanks man! Gosh.... I wish there was some sort of cross ref chart (maybe there is?) for all of our most popular tubes that would show proper alternatives. EL84's, 6L6, etc. etc. My understanding thus far is that, while some tubes are "close" in the properties to XYZ, they are not the same... but I'm always unclear of exactly what that means, and whether or not a substitute tube could cause harm to the amp, or burn out faster, etc.

Telekarster, check out that link. Tube Data Sheet Locator has all of the tube information one might need.
 

Wally

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Fwiw, there are others that can sit in these Noval sockets and function. Here is a computer tube….
https://tdsl.duncanamps.com/show.php?des=5963
Another computer tube…
https://tdsl.duncanamps.com/show.php?des=5965

I don’t necessarily advise going out and looking for these, but they are used by some people. I have some of these from some odd stash of tubes I ran across. That is how I came to be aware of them.

Tube Depot for one lists the 5963 as a 12AU7 sub.
 

Telekarster

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Telekarster, check out that link. Tube Data Sheet Locator has all of the tube information one might need.

:eek::eek::eek::eek: What???? Wow!!! Thanks Wally! This is a big help to me! Gosh.... I suppose I should've asked that question before now. Reminds me of that film I Robot - "That, Mr. XYZ, is the right question..." ;)
 

mkdaws32

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5751, 12ay7 sound better in some amps
I like the 5751 in the PI position in my amp to get more clean head room - hits the power tubes less hard. Sometimes I want squeaky clean and sometimes I don't so it's been back and forth between a 5751 and an EC83 a few times now :) But the 5751 and 12ay7 are lower gain - I wouldn't call them 12ax7 substitutes, exactly. I tried a 5751 in the first preamp stage of that same amp and it completely neutered it - sounded like an old Gorilla solid state amp, lol!
 

jimgchord

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I like the 5751 in the PI position in my amp to get more clean head room - hits the power tubes less hard. Sometimes I want squeaky clean and sometimes I don't so it's been back and forth between a 5751 and an EC83 a few times now :) But the 5751 and 12ay7 are lower gain - I wouldn't call them 12ax7 substitutes, exactly. I tried a 5751 in the first preamp stage of that same amp and it completely neutered it - sounded like an old Gorilla solid state amp, lol!
Depends on the amp. The 5751 is a very close sub other than gain factor. Srv used them as preamp tubes in his fenders. Those were the opposite of neutered.
 

mkdaws32

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Depends on the amp. The 5751 is a very close sub other than gain factor. Srv used them as preamp tubes in his fenders. Those were the opposite of neutered.
Yep - completely different pre-amp design - totally agree. I was just posting the experience in my amp to illustrate that it is not an exact sub in all cases.
 

Wally

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Yep - completely different pre-amp design - totally agree. I was just posting the experience in my amp to illustrate that it is not an exact sub in all cases.

What amp are you dealing with there? Enquiring minds and all, right?
 

mkdaws32

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What amp are you dealing with there? Enquiring minds and all, right?

Hughes and Kettner TubeMeister 18. I use the clean channel only, which has a gain control for overdriving the pre-amp. The clean channel is very Marshall 18-watt-ish sounding (not sure of it's design) and with the volume and gain turned up you get a really great classic breakup. The Lead channel is quite gainy and voiced very differently - some say like a Soldano SLO-100. It shares the EQ stack making it impossible to use both channels without greatly changing the tone controls. I don't like the Lead channel at all with any settings, anyway, so I just use the clean channel as a single channel amp. That clean channel is well worth it. If I boost it with a clean boost or a light OD, I can get all of those classic high gain tones at low volume. It has a really good built-in power soak on hand that is post power tubes and does not flush all of the tone down the drain ;)
 




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