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Old 6th November 2012, 01:12 AM   #1
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Default 2A3 with 12AT7

Ok so I am new to hifi and only researching this for a few weeks I wanted to post a first iteration of a topology. The power tube is a 2A3 and the gain/CF are a single 12AT7 tube. Here is my reasoning to save from writing an essay on why I chose this tube. The 12AT7 provides a gain of ~60 but can also deliver plenty of current where the 2A3 requires about 3mA minimum due to its "slew rate".

The 12AX7 was too low current, same with 5751, where the 12AY7 and 12AU7 were plenty of current but perhaps not enough gain. So I settled on the 12AT7 to make a go of it and DC couple it all with enough voltage on the supply to make each stage work.

I can send in 130Vpp into the 2A3 with 7mA current in this design. In terms of the PS, I am considering diodes with a strange setup on the caps/inductors. After reading quite a bit of information about hifi power supplies, there seemed to be two issues: speed and ripple.

Using DuncanAmps PSU Designer for a couple weeks testing stuff, modeling many different setups, I actually found the first cap to be "optimal" for PS speed if made small, around 33nF small. Going smaller didn't seem to help more. Also, the inductors seemed to make a big difference going lower with the henries, but this in addition to cap size obviously increase the ripple. Modeling the PS with real components I think I have a decent PS but want to work on it more though the basic layout is there. To combat the ripple I figured i would split the input/driver off on a separate leg for smoothing. The 2A3 would then get an injection of PS ripple off the B+ to the cathode as I saw some other schematics do. Here is what I have so far...only showing the "main" components. Please tell me what you think. Also, I would appreciate it if someone could tell me if in PSUD for Capacitor resistance to ground if using the ESR value is correct.

EDIT: forgot to mention teh MAIN reason for the 12AT7 and 2A3, both are current production tubes! Doh...I don't want to build an amp on NOS tubes...
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File Type: jpg 2A3-12AT7.jpg (116.8 KB, 337 views)

Last edited by gokite; 6th November 2012 at 01:16 AM.
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Old 6th November 2012, 02:43 AM   #2
GaryB is offline GaryB  United States
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There is a proven 12AT7 - 2A3 design called the Baby Ongaku that was originally published in Sound Practices in the 90s. You can find a copy of the article online at http://www.wavelengthaudio.com/SP-BabyO.PDF

It's been built by quite a few people and sounds good even without the expensive output transformers. I've got a version that I like quite a bit.
---Gary
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Old 6th November 2012, 11:29 AM   #3
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Gary thanks for the schematic, I hadn't seen that before. This is good for research but I won't learn anything copying an existing design - and I'd like to learn more about hifi design. As you can see I'm trying to get rid of the coupling caps as well since it seems they are (1) very expensive, and (2) alter the sound.

I calculated the slew rate of the JJ 2A3 and 5x that resulted in 2.7mA which this delivers. This was for 20kHz. I want to make sure i did that correctly. I attached my math.

The input stage is bootstrapped through the use of the cap to the CF. I am not sure if I calculated the "triode factor" correctly. I attached my math on that as well. I'm not sure I did that right, nor am I sure what I am shooting for. I will probably increase the 10k resistor(s) to 22k or more to kick that value up into the 20-30 range.

Also, I am using a diode rectifier b/c I can use a higher resistance PT (cheaper) and get a similar result as a tube rectifier in PSUD, yet still get a bit more voltage after the fitering. Is there an advantage to the tube rectifier vs diodes for hifi?
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File Type: png TriodeAmpFactor.PNG (26.7 KB, 267 views)
File Type: png SlewRate.PNG (12.6 KB, 248 views)
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Old 6th November 2012, 04:42 PM   #4
TheGimp is offline TheGimp  United States
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One safety point, you do not want your IEC connector safety ground connecting to your star ground.

It should have a dedicated attachment point to your chassis.

The reasoning behind a dedicated attachment point to the chassis is that if you use a common connection, it is possible to accidentally disconnect the safety ground if one is working on the electrical ground.
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Old 6th November 2012, 04:42 PM   #5
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Just as a start, I can't help but notice that you have the 2A3 dissipating 26W where the maximum rating is 15W. Perhaps you are contemplating using the JJ 2A3-40, but I just don't see the need to wring over 80mA out of the tube. I would start at the recommended operating point of 250V @ 60mA.

The 12AT7 is a popular tube but is generally used as a differential amp where the gross 2nd harmonic distortion is cancelled. Notice that the baby ongaku uses the 12AT7 in an SRRP to cancel the distortion. I notice that you are using negative feedback as an attempt to mitigate the distortion produced by the voltage amp stage. Some have found that adding this NFB reduces the perceived dynamics of the amplifier and adds artifacts that some feel render a SE amp less transparent.

So this is a good start and I think your goal of a not using cap coupling is a good one. In the end though, I think you would be better off using a more liner tube with higher transconductance and eliminating the NFB and cathode follower.
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Old 6th November 2012, 09:30 PM   #6
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"One safety point, you do not want your IEC connector safety ground connecting to your star ground."

It will be. Thanks.

"Perhaps you are contemplating using the JJ 2A3-40"

Yes, I plan to use these tubes if I build a 2A3 amp though I am contemplating the design to allow use of different 2A3 tubes. (or maybe a 300B instead). Higher current usually means better tone in the guitar amp world hence ~80mA which is not even close to the 100mA limit of self bias. Why 250v@60mA, aside from being "the book" values?

"I notice that you are using negative feedback as an attempt to mitigate the distortion produced by the voltage amp stage"

I haven't determined if I was going to actually put these components in there. Not being a hifi guy, is there a rule of thumb for a value of distortion that is too high?

"I think you would be better off using a more liner tube with higher transconductance and eliminating the NFB and cathode follower."

What I wanted to do was stick with current production tubes - and I'm not familiar with a ton of "linear" plots. What tubes are common for hifi that are linear? In guitar amps I stuck with ax7, at7, au7, 6sn7, etc...would appreciate a suggestion here as I already have tubes and would like to use them...
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Old 6th November 2012, 11:35 PM   #7
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I biased my JJ 2A3-40 at -45V and 300V @65mA based upon the load lines for my output transformer primary impedance and B+ of the power supply and then confirmed by listening tests. You have to understand that after running true 2A3s for over a decade at 60mA @250 volts the idea of an 80mA 2A3 seems ludicrous, BUT the JJ 2A3-40 is supposed to be a different beast, so I would encourage you to draw up some load lines at 80mA and see if it can work in your application. Then give it a listen and see if it sounds good at this bias point.

If you want to stick with tubes you already own, then you can't get any more linear than a 6SN7. If you are willing to branch out a little then check out the 6N23P. When they come up, I buy my 6N23P-EV by the dozen off eBay for about $10 a tube, although you can get the standard 6N23P for a lot less.

Since you are determined to direct couple (which is a GOOD thing), you should consider a PS that uses a tube rectifier for the B+ and a pair of silicone diodes for the neg supply. Ground the CT of the power transformer as one normally would. With a negative supply you don't have to stack your tubes on huge cathode resistors.
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Old 7th November 2012, 01:10 AM   #8
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I have only two 6sn7s currently and both are questionable in quality. I use them as a backup for the ones I have in the guitar amps that use them. I was hoping to use some novals instead. I did draw the load line for 80mA, in fact many load lines for the 2A3, see the attachment.

I did some searching regarding distortion and the 12at7 is all over the map on recommendations. here are some guys speaking quite favorably about it though:

What is the best use of 12AT7?

Parallel 12AT7 for 2A3 driver?

negative voltage is a good idea. thanks. I'm going to work on this 12AT7 some more before switching to someting else just yet...
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File Type: jpg 2A3-40W-JJ-LL.jpg (179.5 KB, 180 views)

Last edited by gokite; 7th November 2012 at 01:25 AM.
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