Good evening, ladies and gentlemen, it's time once again to play Guess That Tube. In this week's edition, we're going to present a distortion spectrum of the Mystery Tube connected as a common cathode amplifier with no feedback. Compared to the fundamental's reference level (-15dB from full scale on the graph, 8.5V RMS output), second harmonic is about -70dB and dominates the 0.03% THD. Third and higher are below -100dB (0.001%). Several samples were tested with very similar results.
Can you Name That Tube?
Can you Name That Tube?
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George you are more courageous than I, I waffled and did not post my suspicions.. 
I use the D3A in my own designs and strongly suspect you may be right based on limited measurement. I got something like 0.05% at 10Vrms on my Amber. (No functioning FFT capability until recently when of course I was distracted by another more pressing project for a client.)
This seems like both a reasonable level of thd and output amplitude for a D3A with about 110mVrms of signal on its grid, CCS load and IR LED bias..
I use the D3A in my own designs and strongly suspect you may be right based on limited measurement. I got something like 0.05% at 10Vrms on my Amber. (No functioning FFT capability until recently when of course I was distracted by another more pressing project for a client.)This seems like both a reasonable level of thd and output amplitude for a D3A with about 110mVrms of signal on its grid, CCS load and IR LED bias..
In that thread SY promised to reveal his phono stage. It is stated that "Compared to the fundamental's reference level (-15dB from full scale on the graph, 8.5V RMS output)". I am not sure if the reference level is 8.5 V RMS or the output level is 8.5V RMS. At 8.5 VRMS OUTPUT this isn't a phono stage. At 15 db below 8.5 volts it could be.
Maybe, I kinda cheated just a little. I have seen an early version of SY's phono circuit. In fact I built a perf board version of it that worked quite well on DC power supplies. My plan was to mount it inside the turntable base. Before that ever happened I lucked into a high end E-MU recording system on Ebay that had a phono stage built in. I hooked it up so that I could rip my records into the PC at 24/96. I still use it for my phono stage.
Of course we know that SY and Pete are collaborating on a crossover network and an entirely new "mystery tube" might have been discovered.
These work really nice too.
The THD from a 12AT7 is mostly second harmonic, but I have never been able to get .03% out of one, even with a CCS load.
I've also seen SY's phono stage design, we shared designs as we discovered that we were working on phono stages at the same time using the D3A in the front end. Mine differs substantially from his design in that it uses 5687 based gyrator loads, and the second (and last) stage uses a 5842 which I deem to have low enough rp to drive a 100K load directly without any help.
Basically IR LED bias triode connected D3A loaded by a gyrator driving a passive EQ into a fixed biased 5842/417A also into a gyrator load.. Other than that IR LED there is no sand anywhere in the signal path.. That IR LED works far better than a RC bias network with any cap I could find, and saves some additional circuit complexity and another coupling cap in the path.
It's been built by a few others mostly in Asia where it seems my few current fans all reside..
The thing is so quiet that the input referred noise is actually below the noise floor of my FFT analyzer.
In Windows all that characters are available, use Control Panel -> Regional and Language Options -> Languages -> Details... -> Add
However, the problem will be a keyboard map, I can type both English and Russian without looking at the keyboard, but when I look I can't tell which Russian letter is where...
6Ф12П may be spelled like 6F12P, it's pentode is equal to 6Ж51П, i.e. 6J51P.
When we tested my Pyramid-V in your laboratory it was exactly signature of that 1'st pentode in 6Ф12П
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