Looking for DIY Warm tube preamp (to pair with pass amp)

[kodabmx]

6CG7/6FQ7 are a fantastic choice for the front end of an Aikido and more affordable than most 6SN7 types, they are the 9 pin equivalent and have very low intrinsic distortion. An all 6CG7 Aikido or an Aikido with 6CG7 front end, with 6922/6DJ8 or 6N1P output would be a very low distortion, capable preamp that would be especially affordable with 6CG7/6N1P.

Still too much gain for most setups.

It's so true dude...

I use 6N1P or 6N3P for my preamp "line" gain, and 6N6P as the follower after. Generally, 1V in will make 11V out.
My older source might have a 0.775V output (tape), while I design my stuff for 1.228V. I learned the output of modern studio gear is a lot hotter (3.1V!) so I now have specific padded inputs for specific devices to make them all about the same level.
If there's a quiet tape, I'll need the gain to make power 🙂
My amplifiers want about 4 volts for full power I think. I need to measure it again since I've changed so many things including gNFB ratio since my last measurement.

 

[Lingwendil]

As soon as I finish prototyping the Salas line preamp PCBs with 6AQ5 and 6P43P-E I'll have two variants that each have a gain of about x6- should be good for most stuff I'm likely to encounter. I do want to put together a CCDA with 6CG7/6N1P soon though with strategically chosen parts values to allow me to roll different varieties in with simple changes. would be handy when building zero-gain Pass type mosfet output stages. That way anything from 10 to nearly 100x isn't all that difficult to provide off one circuit.

 

[kodabmx]

Ya I cheat in that regard - I attenuate.

For rolling, I'd use relays to set cathode / plate resistances.
I like CCDA, but replacing the resistor with another tube makes "rolling" easier. That's why I like the Aikido style. I just omit the "mojo" because I'd rather make clean power to begin with.

If you put ~125V across the tube, a 240R cathode resistor works for almost anything except 6N2*. 6N1, 6N3, 6N5, 6N6, 6N14, 6N23, 6N24, 6DJ8, 6AQ8 and others I forget will all work within an acceptable range.
6N2 will work if you can provide some current to it's grid, but 240R will make the bias voltage under -0.5V and distortion will go up a lot...

Of course the best is to dial it in exactly for the tube type, but it's an audio amp. It doesn't need laboratory test equipment specs 🙂

 

[wanderer47]

Thanks for the feedback guys.

For now I have ordered a bottlehead moreplay and gonna see how that pairs. The impedence ratio is about 15 to 1 in the SE input of pass so hoping will be ok. From what I've read its a warmer sounding preamp.

I think the ATCs are in general a fairly neutral speaker and I think I just want to warm them up as much as I can. Tube amps will be tough since they are not very sensitive but at some point i could get a 50+ wpc tube amp that helps too.

 

[Lingwendil]

Time to build up a nice push pull setup 🙂

 

[oltos]

"you can roll the 6SN7s to tune it"

You can most effectively adjust the "warmth" by adjusting the operating point of the voltage amplifying stage, i.e. by adjusting the value of cathode resistor. The higher is 2nd harmonic distortion, the "warmer" is the sound.

I always assumed that popular driver tubes like the 6SN7 were at least somewhat warm intrinsically. But I guess that's why even if you use that tube in the Aikido or any preamp, you'd have to raise the bias voltage to increase 2nd harmonic distortion. and thereby increase the "sweet and warm" sound.

 

[oltos]

The OP asked about the SP14. It's a relatively easy kit to build. The board is designed to run 6SN7s at a certain operating point and includes the power supply with tube rectifier and regulated DC filament supplies. It's extremely well thought-out and, IMO, is about as good as an octal Aikido gets. I built an octal Aikido from scratch years ago, point-to-point. Anyone who's built a scratch preamp know how hard it is to get it quiet. All that is taken care of here. It's also eminently upgrade-able. You can start with the basic design and upgrade components later as your budget allows.

Would you say that this preamp sounds warmer-but just as clean (BUT not bright), with accurate 3D lifelike mids but no bloopy bass-like your Aikido preamp? If yes, did you not have to raise the bias on the 6SN7 tubes at all for any degree of satisfying warmth (assuming you'd like sound of that sound)?