12at7 preamp design

[walkman]

Hi ,

What values or Rp and Rk would be suitable for a preamp using 12at7 ?

Ive seen somewhere that 68k and 470 ohm would work well with plate voltage at 100v

Any suggestions of how to bias this tube vs 12ax7 or 12au7.

Does any one know of a setup where a toggle switch is used to select the Rk values , for each tube type, given a fixed Rp and plate voltage that will work for all?

[Wilder Amplification]

Hi ,

What values or Rp and Rk would be suitable for a preamp using 12at7 ?

Ive seen somewhere that 68k and 470 ohm would work well with plate voltage at 100v

Any suggestions of how to bias this tube vs 12ax7 or 12au7.

Does any one know of a setup where a toggle switch is used to select the Rk values , for each tube type, given a fixed Rp and plate voltage that will work for all?

According to a load line, a 12AT7 with a B+ voltage of 250, an Rp value of 33K and a cathode resistor value of around 680R will yield a voltage gain of about 37, which translates to about 32dB of voltage gain. Quiescent plate voltage should sit right at 150 with this combination.

[tubeswell]

According to a load line, a 12AT7 with a B+ voltage of 250, an Rp value of 39K and a cathode resistor value of around 680R will yield a voltage gain of about 37, which translates to about 32dB of voltage gain. Quiescent plate voltage should sit right at 150 with this combination.

Hi John, with all voltage dropped across the plate resistor, that load line's more like 33k plate resistor isn't it? (i.e. 250/33,000 = 7.5mA. For a 39k plate resistor I get 6.4mA on the y axis - not meaning to be nit-picky'n'all)

[Wilder Amplification]

Hi John, with all voltage dropped across the plate resistor, that load line's more like 33k plate resistor isn't it? (i.e. 250/33,000 = 7.5mA. For a 39k plate resistor I get 6.4mA on the y axis - not meaning to be nit-picky'n'all)

Whoops..that was a typo on my part. Sorry about that...I had originally drawn the load line with a 300V B+ which is where I got the 39K value (actually 40K but 39K was the closest "standard" value) but instead decided to settle on 250V, kept the same peak current value and posted the resistor value for the 300V load line I had drawn previously by mistake. Must've gotten the two confused somewhere lol. My apologies.

Thanks for catching that!

EDIT: I just corrected both my post along with the text on the load line to reflect the correct value of Rp = 33K.

[walkman]

Any other values of Rp that would work ok? so 12au7 or 12ax7 could also be used? The Aiken Tomcat uses a 12at7 in the preamp, what Rp , B+ and Rk does that use ? any others amps with 12at7 pre's?

[Wilder Amplification]

Any other values of Rp that would work ok? so 12au7 or 12ax7 could also be used? The Aiken Tomcat uses a 12at7 in the preamp, what Rp , B+ and Rk does that use ? any others amps with 12at7 pre's?

As far as what the Aiken Tomcat has in it I have no idea nor would I disclose that if I did. Randall Aiken is a long time member here and one of the finest amp engineers in the business. He not only builds some of the most killer amps out there, but he offers LOTS of some of the absolute best technical information on his site that you'd be hard pressed to find anywhere else from any other amp builder...all for free might I add..., which I'm sure took him LOTS of time to prepare and collaborate. I'm sure whatever he used he didn't come up with it overnight...his knowledge...his time spent designing and tone testing it until it was right. Wouldn't be fair to him to disclose his circuit designs on a public forum for those reasons alone.

Which is what you should be doing...gaining the knowledge that will aid you in designing these sorts of things rather than trying to ride on the coat tails of those who spent the years learning the required knowledge to engineer their own designs.

Not trying to be a d**k...just merely stating that Randall Aiken (along with all the other engineers out there) has spent years acquiring the knowledge he has that enables him to engineer the stuff that he does. It wouldn't be fair to him at all for him to spend the time it takes to engineer a design and tweak it until it's tonally right, only for someone to come along, reverse engineer his stuff, then post up the very proprietary information regarding his designs that took him lots of time to get right for the public to plagerize.

That being said, the 12AX7, while it would work in the place of a 12AT7, probably wouldn't work very well in a circuit that's optimized for a 12AT7. The 12AT7 has a higher current drive than a 12AX7 in exchange for gain factor. Their characteristic curves are WAY different. They will bias up different in the same circuit, which means their input/output symmetry will be way different. This may/may not yield acceptable tonal results as "acceptable tonal results" is a very subjective matter and depends on who you talk to. For this reason alone you cannot calculate tone. You can, however, build up a circuit, tweak the values of said circuit until it sounds right, then reverse engineer your own circuit with the math, voltages and load lines, which will enable you to learn all the different "on paper" response curves which best suit your own tonal tastes.

[walkman]

What sort of reply is that ????? of course i'm asking out of interest to learn , and perhaps try to incorporate into a DIY amp of my own ... asking what values of Rp and Rk is hardly asking someone to part with trade secrets. Even if you were to tell me does that mean that i could actually build an amp the way he could? No, and if i could do you think i'd need to ask if i could ?? There is far more to building amps than component values and being able to read a schematic even if that is what most beginners struggle with. I am learning slowly that it is grounding and layout are just as important, and yes would prefer to study the wisdom of experts.

Anyway that said ... i do understand in some way the difference between 12ax7 and 12at7 thats why i'm asking for suggestions, and refer to the Tomcat as an example , i don't know of any others that use 12at7 in the preamp . Do you ?

So your saying that there can be no compromise in values that will provide a reasonable tone for both types ?

Have you ever designed a 12at7 preamp for guitar tone ... besides what you have presented from the specifications , have you tried it out?

I have some 12at7s , after reading about the Tomcat i'm interested and would like to try it out , im looking for some pointers, some guidance .

I appreciate your replies regarding the plate load, but you must understand that people asking questions like mine aren't generally or always looking to rip-off others hard work.

[tubeswell]

@ walkman - Merlin Blencowe's site has a nice (freely available) chapter on setting up triode gain stages (from his first book), which gives you the basics of DC load lines. From there you should be able to get an idea about how to work out what plate load and bias point to use with the relevant 12AT7,12AX7, 12AU7 etc plate characteristics chart to get the optimum gain from those tubes for a given HT/B+ voltage.

[walkman]

Hi tubeswell, thanks for your reply.


if we look at a 12ax7 , then for a fixed b+ say 250v , then a lower Rp , as low as 47k will result in a compressed tone, and a high rp say 220k will result in hi gain but more brittle tone.

Likewise a lower Rk, of 680 ohms will produce higher gain , and 2.2 k lower etc

what i'd like to know is what if say a 68k Rp will result in a brittle / thin sound for 12at7, and how compressed a 12ax7 will sound with that Rp.

Apparently 12at7 are not commonly used except for phase inverters and reverb drivers ( or as little power amps , like a reverb driver).

I will just have to wire it up and give you my subjective assessment

cheers

[Ken Moon]

I will just have to wire it up and give you my subjective assessment

...or, you could read the well-written and absolutely free resources that were mentioned above (Randall Aiken's and Merlin Blencowe's websites), and learn how to do the loadlines in about 1 hour, even if you've had no more than elementary school math training.

Then you'll be able to dispense with parrotting those popular, but meaningless, generalities about certain components and their effects, and replace that with the warm knowledge that you don't need these crutches anymore.

And then, when you do experiment, you'll know what to look for, and understand what you see.

Really - it's easy - do it today!

All the best,

Ken

[Tonewood]

I find Merlin's "Load Line Plotter" found under Downloads at the following link very useful.
The Valve Wizard

[Wilder Amplification]

if we look at a 12ax7 , then for a fixed b+ say 250v , then a lower Rp , as low as 47k will result in a compressed tone, and a high rp say 220k will result in hi gain but more brittle tone.

Likewise a lower Rk, of 680 ohms will produce higher gain , and 2.2 k lower etc

what i'd like to know is what if say a 68k Rp will result in a brittle / thin sound for 12at7, and how compressed a 12ax7 will sound with that Rp.

Apparently 12at7 are not commonly used except for phase inverters and reverb drivers ( or as little power amps , like a reverb driver).

I will just have to wire it up and give you my subjective assessment

cheers

As opposed to a variable B+?

How can a load line explain what kind of "tone" you'll get out of it? Answer...YOU CAN'T!!! You cannot "calculate" tone.

However, based on your responses it seems as if you just wanna forget about the "electrical" aspect of things and skip right to the "tonal" aspect of things. That I have a big issue with. Why? You ever hear the expression "You give a man a fish and he eats for a day. Teach a man to fish and he eats for a lifetime."? Well if you skip to the tonal aspect of things without learning the electrical aspect, you miss the "how" and "why" of things and find yourself repeatedly having to ask for the answers all the time. However, if you'd simply take the time to learn the electrical aspect of things, the "how" and the "why", and how the electrical aspect relates to the tonal aspect (and you're more than welcome to ask questions along the way for stuff you don't understand...we're more than happy to help as long as we can see that you're putting forth the effort to learn), you'll find that you no longer have to ask "tone questions" and will be able to figure this stuff out on your own.

Of course these things take time to learn...you won't learn it overnight. But once you do learn it and the lightbulb comes on, it will all make sense and you will thank us for encouraging you to actually "learn" this stuff.

This forum isn't about "free answers" or "free information". It's about "learning" and we are more than happy to "teach" those who are willing to "learn". Not give out information that took us time and years to learn for free. Might sound a bit "hard ball", but it gets really irritating when people seem to think that those who took the time to gain the knowledge and experience they have are somehow obligated to offer it up for free to those who didn't.

[GregS]

if we look at a 12ax7 , then for a fixed b+ say 250v , then a lower Rp , as low as 47k will result in a compressed tone, and a high rp say 220k will result in hi gain but more brittle tone.

Likewise a lower Rk, of 680 ohms will produce higher gain , and 2.2 k lower etc

what i'd like to know is what if say a 68k Rp will result in a brittle / thin sound for 12at7, and how compressed a 12ax7 will sound with that Rp.

Apparently 12at7 are not commonly used except for phase inverters and reverb drivers ( or as little power amps , like a reverb driver).

I will just have to wire it up and give you my subjective assessment

After planning your triode stage with the load line method mentioned earlier in the thread it is a good idea to get additional parts with values around what you planned. When you build your amp then you can experiment to get it the way you want.

I think the reason 12AT7's aren't commonly used in commercial designs is because of their lower gain than a 12AX7. Probably the idea is to get the required gain with as few amplification stages (tubes) as possible to lower production costs. I'm planning to try 12AT7's in a future design.

Greg

[raiken]

Any other values of Rp that would work ok? so 12au7 or 12ax7 could also be used? The Aiken Tomcat uses a 12at7 in the preamp, what Rp , B+ and Rk does that use ? any others amps with 12at7 pre's?

It uses a 47K Rp and a 1.5K Rk for the first stage, bypassed with a switchable 0.1uF for the bright cap. The second half of the 12AT7 is used in the second stage boost circuit, so it is rather unconventional, with different plate and cathode circuitry.

I can tell you that the 12AT7 produces one of the most "open", "airy", and "chimey" tones I have ever found (that's all the adjectives I can think of at the moment). I tried my best to redesign the Tomcat to use 12AX7s, but no matter how I designed the circuit, even down to simulations that gave the exact same gain and clipping characteristics, it still lacked the "openness" of the 12AT7 front end.

The problem is that there are no decent non-microphonic, non-noisy 12AT7s on the market. I would have to go through literally a hundred just to find ten that were usuable even in a head. Combos were a nightmare. NOS tubes were the worst, even supposedly "screened" ones from the online tube sellers and the exotic 12AT7 equivalent mil tubes. The best I found were the JJ ECC81s, but I still had to sort them with about a 90% reject rate. Some were acceptable until I kicked in the gain boost, then the noise and microphonics would show up to ruin the party.

Randall Aiken

[mooreamps]

The problem is that there are no decent non-microphonic, non-noisy 12AT7s on the market. I would have to go through literally a hundred just to find ten that were usuable even in a head. Combos were a nightmare. NOS tubes were the worst, even supposedly "screened" ones from the online tube sellers and the exotic 12AT7 equivalent mil tubes. The best I found were the JJ ECC81s, but I still had to sort them with about a 90% reject rate. Some were acceptable until I kicked in the gain boost, then the noise and microphonics would show up to ruin the party.

Randall Aiken

So, what do you with the rejects ? You got any you might want to unload ?

-g