Hey Les,
... Don't forget that there will still be current between anode and cathode when the grid and cathode are at the same potential. ...
Agreed. But useless as an amplifier or cathode foll0wer. And with only one triode operational, how in the world does the whole rig produce a
2 channel "stereo" output. That's what makes the circuit curious to me.
... By the way I had not noticed that your pin numbering was mirror image in post #12. ...
Post #12 just needs to be deleted -
.
... I had built things using valves from the age of about 10 (before the days of transistors) so I was used to looking at valve sockets from below when wiring them up. ...
Ditto, even to the age started. After the requisite crystal radio construct, I got this kit:
But, I'm somewhat dyslexic, and the inverse numbering of multi pin components (tubes, transistors, ICs and the like, for instance), topside to underside, caused me no end of frustration then and to this day. I learned to put a marker "dot" for a component's pin #1 on the bottom side of a chassis or a PCB for orientation. Even then I make mistakes...
bittgata,
... Also the KIS-3R33S is powered by DC, where exactly is the AC turned to DC? ...
My SWAG (Scientific Wild A**ed Guess) is the cluster of components below:
with the arrow pointing to what I'm thinking is a full wave rectifier bridge with associated components, notably the two silver topped caps. This grouping has the "look" of a circuit that is converting the "15V-0-15V" AC from the transformer PS (noted in Post #4) and converting it to the DC voltages you noted in Post #9 (≈18VDC).
And since your tests indicated what I consider an appropriate Vin for the KIS-3R33S IC, this is the basis for my conclusion that the KIS-3R33S is defective, given the Vout value you noted on the failed DAC, as compared with those values noted for the working DAC.
... To clarify a few things, maybe I didn't explain something properly or I'm not quite understanding you, the tube filaments never light up on the defective unit, there is an LED below the tube socket that lights up when the dac should turn on and start outputting an audio signal. Also I did not connect the USB to the defective unit. On the functioning unit the filaments light up as soon as I turn the power switch and I think it does so even without the USB connected but I will confirm this when I do the tests. ...
I should have been more specific about what I meant by "power up", the term implies that
any and all power sources are connected, unless otherwise specified. Since the PCB traces
appear to run from the USB connector directly to the filament pins of the 6922, it seemed reasonable that the USB was the source. But, I should note, filaments are current hogs, and using the USB's 5VDC line for that purpose, with its limited current capability (generally, 0.5A max), is not something I would think prudent.
Also, I'm sure that you appreciate the difficulty of trouble shooting any device by "back engineering" (because of a lack of a schematic, and I looked for one), which is what we're attempting to do.