agarner
66 posts
Dec 12, 2019
5:01 PM
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So I have a small 5w tube amp, and like many others I had feedback issues. I will preface this by saying I have no technical knowledge about anything related to electronics, so I did my homework and tossed a 12AU7 preamp tube in and.... no more feedback. Lost some volume, but the squeal stopped.
I was at the music store the other day getting my keyboard worked on and asked the amp tech if there were other options other than my 12au7 swap. He obviously laughed and rolled his eyes at me, and let me know that what I was doing wasn't ideal. He then showed me a catalog of options, and informed me that not all 12AX7 tubes have equal gain levels. Bingo! I grabbed a "Chinese" brand tube with a 4 out of 10 factor and the amp is back to being playable, loud with tone, with reduced feedback
Now some of you probably already know what I'm going to post, but I bet this may be of help to some of you. The link shows possibilities. Its also cool to see how certain tubes accentuate the lows, mids, and highs.
Enjoy all.
https://www.tubesandmore.com/tech-corner/12ax7-comparison-current-made-tubes
Last Edited by agarner on Dec 13, 2019 9:05 AM
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Tblues1
101 posts
Dec 13, 2019
5:45 AM
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There likely is some very minimal difference in gain between 12AX7's, but not likely to be significant.
12AU7 can be a bit "Bland/Dull".
You might want to try a 12AY7 in place of the 12AU7 for better results.
Another option would be a 5751 in place of the 12AX7, if loosing too much gain is a concern.
I run a 12AY7 and a 5751 in my tube amp and am happy with the results.
It really comes down to what you find acceptable.
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barbequebob
3625 posts
Dec 13, 2019
8:24 AM
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There are some tonbal differences in the different brands and the way the 12ax7 tubes are manufactured. the ones with the long plates as a general rule will have less treble and more bottom end and mids and most of the shorter plate 12ax7's are often the exact opposite (however an exception to this in newer production 12ax7's are the Tung Sol 12ax7 reissues). I've found this to be true with NOS tubes as well.
Bear in mind the design of the circuitry of the amp does play a role in how well a tube works in certain amps.
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Sincerely,
Barbeque Bob Maglinte
Boston, MA
http://www.barbequebob.com
CD available at http://www.cdbaby.com/cd/bbmaglinte
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snowman
524 posts
Dec 13, 2019
8:56 AM
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thanks for posting this----yr webpage had a ton of info thanks
also
the ones with the long plates as a general rule will have less treble and more bottom end and mids and most of the shorter plate 12ax7's are often the exact opposite
thanks good info
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agarner
67 posts
Dec 13, 2019
9:04 AM
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Before you all responded, I had no idea what the "long" and "short" plates actually did. This is good to know.
I'm always learning. Thanks for the information.
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barbequebob
3626 posts
Dec 14, 2019
8:43 AM
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The best longplate 12ax7 I've ever used was the NOS GE's made from the mid 50's to the mid 60's and probably the closest in sound to it coming from new production tubes are the long plate Mullard reissues. I use a real '59 Bassman and an old friend of mine who used to deal with vintage amps gave me an NOS long plated GE12ax7 to test out and I got totally floored by it and I don't impress very easily. Hands down, THE best 12ax7 I've ever used.
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Sincerely,
Barbeque Bob Maglinte
Boston, MA
http://www.barbequebob.com
CD available at http://www.cdbaby.com/cd/bbmaglinte
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eetechTom
58 posts
Dec 14, 2019
8:18 PM
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Good stuff, always learning!
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