Lazy Sunday 6SN7 Preamp

[firefly]

My lazy sunday project, I really had nothing to do so I went rummaging through my tube boxes & things. I found a lot of things but no hi voltage transformer. Then I remembered this link, about low voltage 6sn7 operation:
http://greygum.net/sbench/sbench102/TubeMisc/6sn7lv.gif

I made a few calculations, a schematic sketch, quickly threw everything together and hooked it up on the main stereo.
1. I didn't expect it to work right away but it did...
2. I didn't expect it to sound like it did...
3. I didn't expect to like it that much...

I think I was expecting a "tubey" kind of sound, warm and smooth. In fact it sounds crystal clear, detailed without a hint of hardness. Maybe a bit "thin" at the bottom end but it could be normal at that voltage. But the wonderful mid-high range...! Makes me wonder... how would it sound with "tipical tube operation" values?
I need to work out the circuit a bit!
Any ideas, improvements?

[GloBug]

That's pretty cool. Wish I could offer advise.

[firefly]

Ok... let's say HT 300V, plate resistor 33k
Bias? -6V/4mA... -8V/3,2mA, using an LED string?
That would give an anode voltage of approx. 165 to 190V, still quite far from max. ratings
Anyone ever tried different ratings "looking" for best sound?

[v4lve lover]

why dont you get a neon lamp . pass tube and a pentode make a nice stabilized supply here are some examples hope you had german Verschiedene Roehren-Netzteile und ihre Schaltungen

google translate also works fine as long as you choose english their algorithyms are made for that

[ruffrecords]

The 6SN7 is a very linear tube. I use it a lot in a mu follower configuration running at just under 5mA. It will drive +20dBV into a 10K at no more than 0.4% distortion which is almost entirely 2nd harmonic.. At normal operating levels the sound is very clean.

Cheers

Ian

[firefly]

v4lve lover thanks for the link, I didn't know this site! There are some huge beasts there reminded me how it was turning on my 8xEL34 Conrad-Johnson in july
A friend of mine owns an AudioNote Ecc82 preamp and if I remember correctly it used a 6X5/Ecl82 regulated PSU, in fact I paralleled the 6sn7 because the AudioNote was wired like that.

Thanks Ian, so no need to push the tube too hard. I already am astonished at how clean these tubes sound at 60V... amazing!

[firefly]

Ian, I've been playing with the mu follower idea a bit... seems interesting, but I have no clue about the "right" voltages for each stage what do you suggest? From what I've seen online the 6sn7 is quite flexible...
I have some 6ns7 and 6cg7 around, building a preamp with one of each it could run for a few years
thanks, Flo

[ruffrecords]

I typically run both stages at the same operating point with a quiescent current of between 4 and 5mA. I use 1K cathode bias resistors, the bottom one decoupled by 220uF, and a 10K resistor between the stages. A 220K grid bias resistor for the top tube is fine and the cap from bottom anode to top grid of 100nF is fine. ra turns out to be around 10K, so the plate load turns out to be about 20 times the 10K in between resistor so its about 20*ra which gets you close to the intrinsic distortion of the 6SN7/6CG7. This circuit will drive 20V rms into 10K at about 0.4% THD most of which is 2nd harmonic - 3rd is about 20dB below that and 4th 20dB further down.

You will need to elevate the heaters to about 75V to avoid exceeding the hk voltage limit.

Cheers

Ian