Question about 12AT7/E180CC tubes!

[c_corie]

My amp can take 12ax7 input tubes and I've rolled - GE short/long plates, RCA short/long, Mullard I63 Amperex label, Tung Sol D getter and halo, and I've heard Telefunken ribbed plates. For new tubes I tried some basic Chinese preferred stuff from thetubestore, and I have JJ 803S gold pin on order.

I'm currently using the Tung Sol O getter long plates- but I've been swapping a lot.

The amp is made by Earle Weston- 2-12ax7 4-el34 triode push pull, 5ar4 or 274b. Power tubes are preferred series blue base el-34 and I have a preferred series 5ar4 on order.

The amp can also take 12at7 type. I only have Daystrom "made in Germany" with seams in the top and a number 21 stamped on the bottom- no idea what they really are....

In the past with other amps I liked Amperex and BEL 6DJ8/6922- are there any great Amperex made 12AT7 or any other weird long plate E180CC types? I want a warm sound with detail and don't want to buy expensive long plate NOS 12ax7s- I got most of mine cheap locally,

DAC is Hegel HD2 and speakers B&W CM8

Any hidden gem long plate odd ball tubes that sub for 12ax7/12at7? I like a bright clean sound but with a bit more warmth then the Tung Sols- those are pretty crystal sounding.

 

[Hi-FiGuy]

You could give Brimar 6060 tubes a try. Very nice sounding 12AT7 type, but not a long plate.

 

[jaymanaa]

If money is no object, the undispusted king of the 12AT7 is the GEC 6922, but last I looked they were over 100 each for NOS. I use green letter JAN 12AT7s in LTPs in my ST70 and am happy with them.

 

[savatage1973]

Any hidden gem long plate odd ball tubes that sub for 12ax7/12at7? I like a bright clean sound but with a bit more warmth then the Tung Sols- those are pretty crystal sounding.

You do realize that going to a 12AT7 is going to provide only 70% of the gain of the 12AX7, so you may lose the "brightness" that you want. One of my favorite 12AX7 tubes are Baldwin "greenies"--they are Sylvania long black plates with the Baldwin screening in green--there are other Baldwin labeled tubes in white or orange, but they are not Sylvanias.

 

[c_corie]

The reason I was into the idea of E180CC or 12AT7 is they are much cheaper NOS (most of them). I realize they will have less gain and maybe seem less bright- I'm not much concerned about a super bright tube but a tube with a big sense of space and decent air.

Tung Sol 12AX7 are my fave nos tube of that type.

I really liked the basic Amperex Holland 6DJ8- a little dull but still woody and a good aense of air. (In my old DH amp, also BEL 6922 were cleaner but thinner).

I want something that is clear and has a little sparkle to it maybe but not like the Tung Sols- they're great for well recorded stuff- jazz- James Taylor- modern rock-Eric Johnson- over produced metal- the issue is when you put on something like Led Zeppelin or AC/DC those same tubes make it borderline annoying sounding with the amount of zing and clarity.

 

[Palustris]

I realize they will have less gain and maybe seem less bright- I'm not much concerned about a super bright tube but a tube with a big sense of space and decent air.

It is hard to understand how one could come to the conclusion that substituting a 12AT7 for a 12AX7 will result in "less bright" sound. What will be generated is a random filter and a distortion generator. The 12AT7 does not just have lower mu than the 12AX7, but the plate resistance and transconductance are different too. Each tube type has specific requirements for plate voltage, plate load and bias that are provided by the ancillary components that the engineer has chosen to ensure the tube is operated in the most linear portion of the plate curves. When you substitute a tube type that is different from the one the engineer specified, you are creating a distortion generator. You may prefer the sound of the distortion, but it's not Hi Fi.

 

[mhardy6647]

If money is no object, the undispusted king of the 12AT7 is the GEC 6922, but last I looked they were over 100 each for NOS. I use green letter JAN 12AT7s in LTPs in my ST70 and am happy with them.

Doesn't a 6922 have a higher amplification factor than a 12AT7, Jay? And isn't the filament connection different (6V, no center tap)?

 

[jaymanaa]

Doesn't a 6922 have a higher amplification factor than a 12AT7, Jay? And isn't the filament connection different (6V, no center tap)?

I'm an idiot, I meant to say 6201, not 6922. 6922 is a good sub for the 6DJ8 and has an AF of 30/33. The 6201 is right with the 12AT7 at 60/70. Sorry about that, but the GEC is the king, or so they say. Check them out on epay, yikes! Thanks for straightening me out Doc, I'd hate to mess a fella up, that's not what we do here.

 

[mhardy6647]

I'm an idiot, I meant to say 6201, not 6922. 6922 is a good sub for the 6DJ8 and has an AF of 30/33. The 6201 is right with the 12AT7 at 60/70. Sorry about that, but the GEC is the king, or so they say. Check them out on epay, yikes! Thanks for straightening me out Doc, I'd hate to mess a fella up, that's not what we do here.

(emphasis added)

Yeah, but we still love you, Jay

on topic, I think my Bottlehead Paramours, way back when, came with a pair of NOS GE 6201s for the voltage amp tubes. I've never tried anything else, but they have -- I dunno -- a zillion hours on 'em at this point and still sound fine to me.

 

[primosounds]

I'm an idiot, I meant to say 6201, not 6922. 6922 is a good sub for the 6DJ8 and has an AF of 30/33. The 6201 is right with the 12AT7 at 60/70. Sorry about that, but the GEC is the king, or so they say. Check them out on epay, yikes! Thanks for straightening me out Doc, I'd hate to mess a fella up, that's not what we do here.

Hey Jay, i hope all is ok? I have never seen you make such an error as that and i wondered when i read the post if i was reading it correctly. Good to know it was just a "typo". I believe the tube you are referring to is the GEC A2900? Which is I believe is a type of computer tube but also the best audio 12at7? I have some for my phono preamp. There might be another designation a CV number.

 

[vinylkid58]

There might be another designation a CV number.

CV4024 is the Mullard designation for the 12AT7WA.

jeff

 

[Alan0354]

Distortion of 12AT7 is a lot higher, this has nothing to do with gain. Look at the spacing of the plate curves, 12AX7 are a lot more even spaced than 12AT7.

 

[primosounds]

I don't know the cv number for the a2900. Are you familiar with that tube? It is similar to the at7 in that it is a "medium mu" tube. I don't even know if there is any published tube data about it. A very rare tube.

 

[Alan0354]

You need to also be very careful swapping the 12AT7 in place of the 12AX7. The bias is totally different. 12AX7 is designed to run at much lower current. Depending on the circuit, the bias might be totally off if you just switch the tube without changing the resistor value around it. I don't know about the 12AT7 being less bright, I don't see any particular reason for that if it is designed right. But that MIGHT happen if the bias is totally off when blindly switch one in place of the X7.

Without looking deeper ( I can't either as I don't even know the circuit), in general, the 12AT7 uses lower resistor values for both the plate and cathode resistors. Like 12AX7 commonly use 100K for plate and about 1.5K for cathode. You have to use a lot lower for 12AT7.

 

[Poultrygeist]

I really like the military designated CV4024 ( made by Mullard ) in my Bottlehead amp. Compared to others these are still fairly affordable.

 

[Alan0354]

I would try slightly changing the cathode resistor ( changing the operating point just a little) and listen to it to see whether it sounds better( personal taste). My bet is changing the operating point a little might give the change as much as rolling tubes.

For example, if the cathode resistor is 1.5K, replace it with a 1.3K in series with a 500ohm pot. Start out with the same total value of 1.5K so you have the exact same operating point as before. Mark the position of the trim pot so you can go back to the original value. Then adjust the pot up and down and listen to it as see whether you get what you want. If that works, save you a lot of money buying expensive tubes and roll. It it doesn't work, you waste $2 for the pot and new resistor.

Changing the cathode resistor change the plate voltage, that change the characteristics of the tube and likely the sound a little. Maybe that's all you need.

 

[GordonW]

Distortion of 12AT7 is a lot higher, this has nothing to do with gain. Look at the spacing of the plate curves, 12AX7 are a lot more even spaced than 12AT7.

Depends on what part of the plate curves you're working in, and over what needed output voltage.

I've done plate plots/sims, and found out that in some cases where high output voltage is required- the 12AT7 can result in a distortion curve that's more consistent with gain (less change in distortion level with change in voltage- i,e more consistent tonal quality over different volume levels), and less peak distortion at any voltage, than the 12AX7. Ask a 12AX7 to produce 30V AC of output- and the 12AT7 may well do better, biased right. OTOH, if you only need 1V output- then the 12AX7 is almost undoubtedly always going to win that one...

Also, there's also the lower output impedance of the 12AT7. That can overcome Miller capacitance in following stages better, as well, too...

Regards,
Gordon.

 

[Tubologic]

I don't know the cv number for the a2900. Are you familiar with that tube? It is similar to the at7 in that it is a "medium mu" tube. I don't even know if there is any published tube data about it. A very rare tube.

The A2900 was a designation only used by G.E.C/M.O.V and is indeed their industrial version of the 12AT7, with very similar specs. No other companies ever produced this reference, all my exemplars have the green G.E.C decals just like on the famous KT66/88. A wonderful tube but good luck finding one at a decent price.

Datas were published and can be found here:

http://tdsl.duncanamps.com/pdf/a2900.pdf

 

[Tubologic]

It is hard to understand how one could come to the conclusion that substituting a 12AT7 for a 12AX7 will result in "less bright" sound. What will be generated is a random filter and a distortion generator. The 12AT7 does not just have lower mu than the 12AX7, but the plate resistance and transconductance are different too. Each tube type has specific requirements for plate voltage, plate load and bias that are provided by the ancillary components that the engineer has chosen to ensure the tube is operated in the most linear portion of the plate curves. When you substitute a tube type that is different from the one the engineer specified, you are creating a distortion generator. You may prefer the sound of the distortion, but it's not Hi Fi.

I fully agree with all the above. This should be read (and reread) by all indiscriminate tube swappers/rollers.

If you really need a distorsion generating device in your Hi-Fi (?) set-up try to get an old APHEX Aural Exciter , they are much cheaper than N.O.S tubes and at least the effect is controllable.

And (BTW) the E180CC is neither a substitute for a 12AX7 or a 12AT7 and has different electrical characteristics. It was puposely designed for computer use and counting circuits (low cathode interface) which doesn't guarantee any sonic advantages like high linearity or low microphony.

 

[Alan0354]

Depends on what part of the plate curves you're working in, and over what needed output voltage.

I've done plate plots/sims, and found out that in some cases where high output voltage is required- the 12AT7 can result in a distortion curve that's more consistent with gain (less change in distortion level with change in voltage- i,e more consistent tonal quality over different volume levels), and less peak distortion at any voltage, than the 12AX7. Ask a 12AX7 to produce 30V AC of output- and the 12AT7 may well do better, biased right. OTOH, if you only need 1V output- then the 12AX7 is almost undoubtedly always going to win that one...

Also, there's also the lower output impedance of the 12AT7. That can overcome Miller capacitance in following stages better, as well, too...

Regards,
Gordon.

I drew the load lines for both 12AX7 ( top) and 12AT7 (bottom).



1) I drew RL=100K for 12AX7, I make the operating point at 200V( typical). You can see the region between A and B is very linear, that is the spacing between the plate curves is very even.

2) I drew RL= 22K (red) and RL=100K(green) for the 12AT7. You can see the spacing of the plate curves at operating point of 200V is very uneven. The spacing on the left side of the loadline at 200V is much wider than the right side. There is severe distortion.

Yes, miller effect can be less for 12AT7 ONLY IF you change the plate and cathode resistor to 22K and 330ohm as in my drawing. If you just replace the 12AX7 without changing the resistors as what OP want to do, it will not help at all.

If you look at the uneven spacing of the plate curves of 12AT7, I don't think there is a value of plate resistor that can produce low distortion like 12AX7.

Actually, I am using 6SN7 for my preamp tube instead of X7 or T7. It is more linear, just all around better tube. Problem is it will not fit in the small socket.