Tube pre-amp with exposed tubes?

[tyne]

Hi,

I'm an electrical engineer who specializes in digital design and DSP but I'm new to the whole audio scene. I know some analog but it's not my area of expertise. I'd like to build my own stereo DAC/amplifier and use tubes in the process.

My plan is to design a DAC board taking a Toslink input, use a CS8416 to decode the SPDIF signal, connect to an AD1896 to get a constant sample rate, connect this to a FPGA to perform digital filtering for equalization, and finally use a DAC to get an analog output. The FGPA will also control the amplifier gain and decode a remote control input. This part of the design I am fairly confident about being able to design and implement though I would appreciate any feedback.

The next part, the analog amplifier, is much more of an unknown area to me. One of my design goals is to have tubes protruding from the amplifier box. I want to see their warm glow. From what I have read, it is probably easiest to use tubes for a preamp stage and then connect them to a class AB solid-state amplifier on a chip like the TDA7293. I even see some kits on eBay from China doing using 6N11 tubes but I see a couple of problems with these kits:
1) Little to no documentation or specifications
2) The tubes don't protrude high enough to expose them outside an enclosure! Perhaps I could flip some of the larger electrolytic caps to the other side of the board or build a riser for the tube?

Are there any other kits out there that you guys could recommend? Or is there a good Chinese kit out there? Do I need to use shielded tubes if I want them protruding from the amp or would a 6N11 provide enough noise immunity against 60 Hz hum? I'd rather not try to design an analog tube pre-amplifier stage myself but at the same time I would like a schematic so I can learn how it works.

Thanks!

[Pano]

You can have a look at the Tubelab section under Vendors for some great ideas and kits.
There are some other good tube amp kits in the Vendor's section, too.
And I believe some about tube buffers for chip amps if you want to go that way. Even your DAC could have a tube output section.

From your description it sounds like you want the tubes mostly for window dressing, so you don't have to be too awful picky about the project you pick. There are plenty of good projects in this forum.

[Magz]

Check out the Supratek stuff for inspiration. Some of the prettiest amps and preamps out there, with lots of visible tubes.

SUPRATEK - Home

[Miles Prower]

It depends on how much gain you require. If you don't need a tremendous amount of gain, there is no problem leaving the tubes up top and in sight. Although you're not gonna get much glow from small signal types. Though you could try a 6CB6 as a pseudotriode. This type has an unusually large cathode, and an open structure that makes it quite visible.

Tube shields should be avoided whenever possible, as these have a nasty tendency to crack tubes and let the vacuum escape.

[djgibson51]

3mm red led under the tube to highlight the pretty bits...

[DF96]

Digital gain control can mean higher distortion because the quantisation interval becomes a greater proportion of the signal level. It all depends on whether you want high quality sound, or something pretty and convenient.

[MrCurwen]

Use chassis mount tube bases to have tubes completely outside of chassis. Much better not only aesthetically but also thermally. Not a good idea to have tubes on PCB.

[Bigun]

If you want something that you can build yourself why not consider building an Aikido. You can buy the pcb's ready to go and you can learn about it along the way. Since the circuit is proven already you don't have to worry about being able to design it yourself.

I'd choose the Octal base version because these tubes in my opinion look the best. Remember that the base of the tubes should be accessible since Octal based tubes should not be pulled out of their sockets by grabbing the glass.

If later you want to re-use this for another project, you can simply transplant it. You might also want to include outputs on the chasis from the Aikido linestage so that you can use it as a pre-amp.

Aikido Octal Stereo PCB

Be aware, tubes work at voltages that can and are often fatal if you are not careful.

[indianajo]

Yeah, we get complaints all the time from new Hammond organ owners that replaced all the tubes that didn't glow and their organ still sounds like ****. I've told the electrolytic capacitor story about 200 times. I've got 4 Hammonds and most of the tubes are original. The signal tubes don't glow much, and some brands glow more than others. On the other hands, all of the signal tubes are over 40 years old.

[Gobble]

Originally Posted by Bigun

I'd choose the Octal base version because these tubes in my opinion look the best. Remember that the base of the tubes should be accessible since Octal based tubes should not be pulled out of their sockets by grabbing the glass.

Quick OT question - but whats the right way to pull the tubes? I am building a kit next week so better I find out before I make mistakes.

Thanks