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Old 25th February 2013, 07:15 AM   #1
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Default 12at7 transformed amp

Click the image to open in full size.Hello, my dual el84 guitar amp with two 12ax7 preamp tubes always sounded very high gain and very compressed on the drive channel when cranked up. I read that subbing a 12at7 for the 12ax7 would tame the gain. Looking at the attached schematic, I tried it in tube 1 and it seemed to lose clarity. But in tube 2 position it turned the amp into a beautiful boutique sounding warm blues/rock dream amp. I've read that a 5751 tube would be better since the current output is better matched to a 12ax7 and is better suited for audio. Looking at the schematic it seems like the second stage of tube 2 acts like a driver tube or maybe a phase inverter so would the 12at7 be better for this amp? I was ready to sell the amp but it's like heaven now. Maybe because it was such a high gain before and the the second stage drives the el84's less eliminating the excessive compression? Thanks for any advise on this.
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Last edited by Clemford; 25th February 2013 at 07:22 AM.
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Old 25th February 2013, 08:56 AM   #2
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Default 12at7 transformed amp

Hi. The first half of tube2 is a "buzz generator", the second half is indeed a cathodyne phase splitter.

The circuit was designed for 12AX7, and the 12AT is a quite different tube, but here you can use one if you like, it won't cause any electrical problems.

The 5751 is closer to the 12AX7, and it has a little more gain than the 12AT7. They generally sound better than the 12AX7 too, at least in Hi-Fi amps.
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Old 25th February 2013, 06:04 PM   #3
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Originally Posted by Vincent77 View Post
Hi. The first half of tube2 is a "buzz generator", the second half is indeed a cathodyne phase splitter.

The circuit was designed for 12AX7, and the 12AT is a quite different tube, but here you can use one if you like, it won't cause any electrical problems.

The 5751 is closer to the 12AX7, and it has a little more gain than the 12AT7. They generally sound better than the 12AX7 too, at least in Hi-Fi amps.
thanksVincent, I'll try a 5751 then. If it sounds better than it does now then I'll feel I hit the tone lottery. thanks.
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Old 25th February 2013, 06:17 PM   #4
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Default 12at7 transformed amp

I had this post in the T/V forum by mistake: Hello, my dual el84 guitar amp with two 12ax7 preamp tubes always sounded very high gain and very compressed on the drive channel when cranked up. I read that subbing a 12at7 for the 12ax7 would tame the gain. Looking at the attached schematic, I tried it in tube 1 and it seemed to lose clarity. But in tube 2 position it turned the amp into a beautiful boutique sounding warm blues/rock dream amp. I've read that a 5751 tube would be better since the current output is better matched to a 12ax7 and is better suited for audio. Looking at the schematic it seems like the second stage of tube 2 acts like a driver tube or maybe a phase inverter so would the 12at7 be better for this amp? I was ready to sell the amp but it's like heaven now. Maybe because it was such a high gain before and the the second stage drives the el84's less eliminating the excessive compression? Thanks for any advise on this. Click the image to open in full size.
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Old 26th February 2013, 09:32 AM   #5
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I've somewhat clumsily merged the two threads. Had to keep the original first post because otherwise it would be owned by Vincent77
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Old 26th February 2013, 11:01 AM   #6
Tesla88 is offline Tesla88  Italy
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Do you notice difference (in a positive way) also on clean channel or only on drive channel ?

First half of V2 used as a gain stage in drive channel , so a 12at7 instead of 12ax7 would lead to a reduced gain sound (good for blues or vintage rock)

Second half is a phase splitter , i prefer a 12AT7 in the phase inverter despite they used 56k resistors for load ...
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Old 26th February 2013, 11:05 AM   #7
Tesla88 is offline Tesla88  Italy
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Anyway , what the hell is a "buzz generator"

Also, was the designer on drunk when designed that schematic? First half of V2 would say "i'm falling"
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Old 26th February 2013, 07:11 PM   #8
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Originally Posted by Tesla88 View Post
Do you notice difference (in a positive way) also on clean channel or only on drive channel ?

First half of V2 used as a gain stage in drive channel , so a 12at7 instead of 12ax7 would lead to a reduced gain sound (good for blues or vintage rock)

Second half is a phase splitter , i prefer a 12AT7 in the phase inverter despite they used 56k resistors for load ...
I pretty much know nothing about design as far as load resistors go but I figured the second stage must be a driver or splitter of some sort so that's why I figured I'd try a 12at7. I'm going to also order a 5751 and hopefully that will even be better. The drive channel could use a touch more gain so fingers crossed it will sound good. Since the 12at7 is a pi tube though it may be a toss up between the two. As far as the clean channel goes, yes, it is much more musical albeit the clean channel was very Fendery before and was the icing on this amp. Now it has a god like vintage Princeton Reverb like tone that is super warm and responsive on both channels. Before, the drive channel was too gainy and the amp got too compressed when driven. Now it's just about perfect. A little more gain would help. It would also be nice to get that natural feedback when letting the notes sustain. Theres a little now but a tad more would be nice. I almost sold this amp the other day but the guy thought it was too Marshall nasally and compressed so he passed. That's when I decided to try the swap. I never would've known. Now the amp is like a boutique tube rectified 2K amp or at least close to it. I don't know if the same result could've been accomplished by changing resistors and caps while keeping the 12ax7 but this sure seems a lot easier. The more usable drive channel was a big bonus on this but the way the amp now opens up without the excessive compression is the winning part.
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Old 26th February 2013, 07:21 PM   #9
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Originally Posted by Tesla88 View Post
Anyway , what the hell is a "buzz generator"

Also, was the designer on drunk when designed that schematic? First half of V2 would say "i'm falling"
not sure what you mean by falling as I mentioned above that I know nothing about design(sure wish I had just a little iota of your guys knowledge but it all looks like Chinese to me and I think I'm too old to learn) but this is a German designed Hughes And Kettner 25th edition amp and having worked in Germany back in the late 80's I can say that most of them seem a bit high strung. Perhaps this explains the excessive compression this amp had (like when Adolph got wound up in his speech's) before the swap.
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Old 27th February 2013, 04:50 AM   #10
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Buzz generator - a (typically 12AX7) triode stage running at very low current (cold biased), designed to clip one half of the signal really hard (asymetrical distortion) thus introducing a lot of second and other even harmonic distortion.

You can recognize it from its position in the signal chain (always the last preamp stage of a high'ish gain preamp) and by a really high value cathode bias resistor which is never bypassed with a capacitor. In this case a 33K which is what they use in SLO Amps as well, same as last stage in a Trainwreck but in that case the cathode bias resistor is 10K.

It is an "icon" of higher gain preamps - to make it sound good, you add one of these cold biased stages with and unbypassed cathode resistor of between 4K7 and 33K as the last preamp stage. With 33K it has a gain of about 2 and clips half of the output so really it has an effective gain of 1, only changes the sound and not the loudness
Never actually heard it called a "buzz generator" before but it is appropriate.

Too much "buzz"? try 10K in place of that 33K.

Cheers,
Ian

PS You may be getting a sonic change from the 2nd half of the 12AT7 as well. This is used for the concertina phase splitter, before overdrive sets in the output impedance of a concertina (at both cathode and anode) is approximately 1/gm. An AT7 has 4 times the gm of an AX7.

Last edited by gingertube; 27th February 2013 at 05:01 AM.
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