[Docere]

Hi all,

I "contribute" here occasionally and though that you might be interested my latest amp: 6C6 directly-coupled to a 2A3.

My first SET amp is a classic Radiotron SE2A3 design popularised on the JE Labs site years ago - a really nice amp. I wanted see what would happen if I took the topology of 6C6 driving a 2A3 and did it a little differently. Thought of the design as a modulated power supply and current loops, used an atypical operating point for the 2A3 (only dissipates 10W), and threw a fair bit of custom nanocrystalline and Super Hi-B core at it.

Still breaking in and I have a few minor things to tweak... broadly, it has a similar presentation to the classic (tonally dense and flowing), but is rather different in significant ways.

One channel inner workings almost done:




20kg of right channel in the system... needing a clean! 




Feel free to ask questions.

Cheers.

[FullRangeMan]

This is a big amp really, caps are big too.
Smart way to attach he parts, much better than Turret post.
What are the OPTs?

[Docere]

Thanks for the kind words FRM!

Yeah, it is big. Close to 40kg of stupidity in total.  But hey, once I started down the path it seemed a shame to stop. 

Sharp eye re component mounting - reduces the amount of dissimilar conducting material. Components are held in place by their leads held in place through a hole in a non-conductive post. Unfortunately, I was a bit too clever with some with the posts also providing support for capacitor mounts... will make changing out some resistors difficult, but not impossible, if needed.

I posted a while back on the transformer thread, but to summarise:

• Output: J&K Audio Design custom wound L3 on nanocrystalline core, potted. >45Hy at my operating point, wide bandwidth with clean roll-off. Really nice - not lean sounding like some.
• PS choke: As above (wound like an output transformer but heavier). 7Hy at 10 Ohm DCR (makes for a fast-recovery PS).
• Power: J&K Audio Design custom wound L4 on Super Hi-B core, potted.
• Filaments: J&K Audio Design custom wound on Z11 core with flux band.

Cheers.

[FullRangeMan]

Wow that is a really special amp! A transformer to filaments help a lot lower the noise floor. There is a lot of refinement in these transformers, Hi-B already good so imagine SHi-B and Nano C
Congratulations for this first class machine, it will last a century.

[Docere]

Thanks again FRM. You are right, this really is good enough to be a lifetime amp. Built with reliability in mind - lots of ventilation, reliable over-specified parts etc.

You mentioned capacitors earlier in the thread - I was a bit rushed and did not explain why I chose them. Thinking of this amp in terms of current loops, the cathode circuits are important: AuriCap XO for input cathode, ClarityCap CMR in the output (large!). A bonus of direct coupling is the ability to run smaller output cathode bypasses because the cathode resistances are greater. This makes capacitors like the CMR possible, just. The last capacitor in the output stage power supply is also in a current loop and I might need to work on this - not sure it is up to the job sonically.

I still have a little voicing to do: drop 2A3 filament voltages slightly (currently a little high, but within tolerance), maybe try partial Ultrapath (use a nice copper foil cap) in the output stage and increase the capacitance on 6C6 screen bypass. I think I need to let it (and me!) break in before doing this though... not sure how long that might take.

Cheers.