6EU7 vs 12AX7

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6EU7 vs 12AX7

Postby DoneB4 on Fri Jul 05, 2013 12:49 am

Just thought I would share my experiences with these two tubes in a split cathode Marshall Super Lead circuit. This amp started life as a Bogen PA amp and I converted it myself. As the amp came with a Mullard 6EU7 as the first preamp tube, I went ahead and used it as V1. I knew this tube is considered to be pretty much a 12AX7 with a different pinout, so why not? Well...the amp fired up fine the first try and sounded great except for an intermittent runaway squeal and a VERY microphonic chassis. Pulling the 6EU7 eliminated these issues, so figuring I had a bad tube, I bought another Mullard on ebay. Popped it in, same issues. Finally decided to rewire the socket for a 12AX7. I used all the same components/wire as before, just moved the leads to different pins. Problem solved! I tried a few different tubes, including an original Mullard longplate, and all were fine.

I know this is not exactly conclusive proof, but I suspect the 6EU7 and the 12AX7 are not quite as similar as the specs would suggest? Both 6EU7s had identical microphonic characteristics. If I tapped the chassis, or even moved the guitar cable over the carpet(!), you could hear it LOUDLY through the speaker. Every 12AX7 was MUCH quieter. YMMV, but this is what I found.
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Re: 6EU7 vs 12AX7

Postby OldSchoolDave on Fri Jul 05, 2013 3:32 am

I have a Heathkit 14 watt amp modified for guitar that uses a 6EU7 in V1. Didn't notice extreme microphonics, but I did want to clean it up just a tad. Did some googling and bought 3 different 6EV7's. Dunno where I found the info, but I was under the impression they were lower gain versions of the same tube. Unfortunately, they didn't work at all in that chassis :( .

I'm tempted to rewire the socket for 12A#7 series, as you did.

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Re: 6EU7 vs 12AX7

Postby DoneB4 on Fri Jul 05, 2013 4:44 am

Dave, if you do switch tube types, I'd be very interested to know if the "gain" was lowered with the 12AX7. Seems to me the 6EU7 just had higher amplification factor in my amp compared to the 12AX7, even though they are supposed to be the same :?: I say this because, with the 6EU7, I had to move the amp FAR away from the cab just to prevent acoustic feedback (squealing). Granted, this was at full amp volume 8) If I turned volume, treble or presence down, the feedback died down slowly, mirroring the behavior of a microphone in a PA that's up too loud. With the 12AX7, I can place the amp right on top of the cab with no problem.
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Re: 6EU7 vs 12AX7

Postby OldSchoolDave on Fri Jul 05, 2013 2:56 pm

DoneB4 wrote:Dave, if you do switch tube types, I'd be very interested to know if the "gain" was lowered with the 12AX7.


With yet another house move ahead of us, it'll be awhile before I tackle that amp :( .

It could be that the 6EU7's are just more likely to be microphonic, similar to 12AY7's (tweed players have to hunt for non-microphonic ones).

Dunno :?: .

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Re: 6EU7 vs 12AX7

Postby DoneB4 on Sat Jul 06, 2013 1:37 am

OldSchoolDave wrote: It could be that the 6EU7's are just more likely to be microphonic, similar to 12AY7's (tweed players have to hunt for non-microphonic ones).

Dunno :?: .

Dave



That's entirely possible, Dave. The interesting thing, though, is that apparently the 6EU7 was developed specifically as a low noise/microphonic substitute for the 12AX7. That's what I've read everywhere on the internet anyway, so it must be true :D
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Re: 6EU7 vs 12AX7

Postby PeterS on Sun Jul 21, 2013 2:54 am

I have a couple of amps with 6eu7's and haven't had any microphonic problems with them, I suspect you just happen to have some bad tubes. I have dozens of old 12ax7's and most of them are microphonic to varying degrees. At least rewiring for a 12ax7 makes everything a bit more 'standard'. 6eu7 is a superior tube for low hum and noise but (a) this isn't hi-fi, and (b) a bad 6eu7 isn't superior to anything.

On the other side of the coin: I have an Ampeg that needs a 7199. I have no 7199, but I have lots of things like 6u8 and 6BR8 that are similar except the pinout. So I spent many happy hours fooling around making a socket adaptor. Should have rewired the socket but didn't want to mod a vintage amp. Now I find that none of the substitute tubes give quite the same performance and I spent quite a while sorting out parasitics etc (not that you'll have those issues with 6eu7 vs 12ax7). All good fun but if you're just looking for a result: it would have made more sense to pay the $30 or $40 for a decent 7199.
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Re: 6EU7 vs 12AX7

Postby DoneB4 on Sun Jul 21, 2013 5:28 am

PeterS wrote:I have a couple of amps with 6eu7's and haven't had any microphonic problems with them, I suspect you just happen to have some bad tubes. I have dozens of old 12ax7's and most of them are microphonic to varying degrees. At least rewiring for a 12ax7 makes everything a bit more 'standard'. 6eu7 is a superior tube for low hum and noise but (a) this isn't hi-fi, and (b) a bad 6eu7 isn't superior to anything.

On the other side of the coin: I have an Ampeg that needs a 7199. I have no 7199, but I have lots of things like 6u8 and 6BR8 that are similar except the pinout. So I spent many happy hours fooling around making a socket adaptor. Should have rewired the socket but didn't want to mod a vintage amp. Now I find that none of the substitute tubes give quite the same performance and I spent quite a while sorting out parasitics etc (not that you'll have those issues with 6eu7 vs 12ax7). All good fun but if you're just looking for a result: it would have made more sense to pay the $30 or $40 for a decent 7199.


I really do think the 6EU7 is a little higher gain than a 12AX7. No scientific proof, just my opinion after having two different 6EU7s behave the same as each other, but very different from every 12AX7 I've tried after the rewire. The 12AX7 are all different types, but they all behave pretty much the same as each other.
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