6688, whether you speak Mandarin, or Cantonese, has connotations of “easy” and “prosperous”. So a tube that bears this number, naturally attracts the Chinese in me to view it as more than “a vacuum tube”. If all tubes are equal, clearly this fella is more equal than others.
I actually played with this little fella a longgggg time ago… via the Grounded Grid Gain Stage, back then using it as an I/V convertor for the then Monica2… Looking back, that was likes ages ago…
Anyway, here in 2017, we have moved far ahead and the GG stage was sitting there collecting dust, so I gutted everything there, and added in some other components to finally make a headphone amplifier worthy of our time and attention.
The circuit is really simple.
Nothing special here.
You know I’m a fan of the Cree Schottkies at the tube cathodes.
You have seen parallel feed as well.
This is the first time I’m using this rubber wood chassis, as well as 2-chassis construction.
Also, first time I’m using Cinemag iron.
Cinemag CML-2394 420H (!) @20mA
Cinemag CM-13107 10k : 600/300/150/32 *50% Nickel
Rat’s nest, as usual. I have a reputation to maintain!
The 600 ohm taps go to the Output RCA while the 32 ohm taps go to the headphone jacks. Well, if I have headphones of various impedances, I’ll wire in a switch but no need here.
The tuned RC is optional. Panasonic metallized polypropylene capacitors for power supply here.
After the initial run in period, I have modified the schematic few times before arriving at current iteration. Schottkies still better than a few RC combos I tried at the cathode.
There shall be more modifications…
Okay, how’s the sound?
Marvelous! Even via audio output from my Mac, the spaciousness, the airiness… makes you want to listen and listen some more. So far don’t even detect any fatigue-ness at all.
All in, highly recommend you get the Cinemag irons. First time I’m using this irons and I believe the sonic quality is from the irons alone. The other tricks, I have played many times before.
And it’s so versatile. Could be used as a line amplifier (which I haven’t tried yet).