DIY 12b4a preamp

[je2a3]

I purchased a transformer coupled 12b4a preamp. The transformers were wound by Dave Slagel specifically for this amp. It sounds very nice, super detailed and revealing. I don’t have a schematic and I don’t know which preamp design it was made from.

So far the GE 12b4a has had the least amount of noise of the tubes I’ve tried. This uses a 5y3 rectifier and I prefer the sound using a 6106 military spec 5y3 rectifier.

Nice! 👏 👍 😊

 

[MrEd]

I like this pre a lot, the noise has come about recently and i have a couple sets of tubes on the way.
Pretty sure that will put things right again.

 

[RPMac]

Yes, maximum signal to cover up the noises that are there, then attenuate that mixture after.

The big downside to this is a variable output impedance, but if you're driving amps with high input impedance without a ton of cabling, it's not the end of the world. This can be further mitigated by the cathode follower I tacked on. Without the cathode follower, you have to load the tube down with a lower impedance potentiometer, but the 12B4A is still up for the job under these conditions. You also have to consider that the maximum signal you can feed into the preamp is limited by the 12B4A cathode bias voltage, but it's sky high here so there's no issue.

Sonically the distortion spectrum is more constant in these kinds of designs compared to a typical tube preamp with the volume control up front.

Paul, if I stream only and use the volume control in the app, will I get the best S/N ratio and dynamics by setting the BeePre and Stereomour 100K ladder-steps at max(or bypassed)?

 

[paul_b]

I shot you a PM to avoid thread jacking.

 

[RPMac]

👍

 

[MrEd]

I shot you a PM to avoid thread jacking.

No we would not want you to take over @mhardy6647 's job. 😅

 

[paul_b]

Putting an autoformer after the 12B4 is also a great strategy. I'd want to see 50H or so of primary L on the autoformer, but that's a pretty low if you're using something like the Slagle autoformers. A CCS in place of the plate loading resistor will help keep the load seen by the 12B4 nice and high.