Usagi pre-amplifier



topology:  preamp signal: UX226 sound quality: TBD
output: OPT rectifier: UX280 or AZ1 nostalgia factor:  10/10
gain: 6dB (2x)   hernia scale:  9/10





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My signature dish, the 26 preamp :)   It had to happen now that I am in the building mood again.

Why the name Usagi? Well, I wanted to name the new power amps Yojimbo for obvious reasons and the not so obvious one of this being a Sakuma style amp. Sakuma being a bad-ass Japanese amp builder, reminding me of the Samurai movie named Yojimbo. So what should go in front of Yojimbo; Usagi ofcourse!!!!  Google it ;)


A lot has changed, a little less set in my old ways and ready to try some new things. Yet at the same time knowing what I liked and what I want to do the same again.

I'll share more blabla when I have the willpower. For now, here's a schematic of the signal section, at least the first brain-fart.


Concept of the signal section. 

The filament uses Rod Coleman's DHT regulator board by way of filament bias (thanks Thomas, love your stuff!). No caps, low cathode resistor, 4:1 step-down, tvc volume control. Hot-diggity!

I will play a bit with the cathode resisor to change the bias point a bit, but the 1A current source that runs through it means there will be around 10V bias leaving just a bit over 160V as the anode voltage. We'll have to see what the B+ really generates after we have built it to know where the tube will operate.

I've added a 150R grid-stopper and a 250K grid leak resistor to avoid any nastiness when the volume control switches between windings and momentarily has no connection at all. I'll have to test if it is a make-before-break switch, that would mean I can probably remove the 250K and 150R .



The "Raw DC" supply will be in a seperate chassis from the signal section, mainly to keep the size within reason, but also to keep transformers as far away as possible from the tubes. The "raw" supply is just passive supply giving around 16V and these will used on all six tubes in the pre/power amps.  

Check this project for the passive part of the filament supplies.

Inside the signal chassis there will be a few small caps, followed by a Coleman regulator for the filament of the 26.




The preliminary B+ supply per channel. I'm using an old Tribute power transformer for this as it has two pairs of windings that seem to be exactly what I need (guess they were in a previous 26 preamp). Optionally I have a different transformer on hand that will give me a lot more voltage which would allow for a HV regulator circuit; I'll need to see if the calculated 3-4mV ripple left on this passive supply will be audible or not. I also have a 340Vct with 5Vct Bartolucci transformer that was intended for an earlier  26 preamp I built in combination with the 80 as a rectifier which is also a possibility. Hence the "preliminary" status for the B+ supply; lots of options.



Let me know what you guys think and if you've built a 26 preamp yourself.


Lundahl LL2763 Datasheet: Click here