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Wolfgang
Reply with quote  #31 
You took the bull by the horns, Kelvin, and I think you will have a lot of fun with this DAC and its options.
You can see in the picture (link) how the DAC can be connected directly (raw output) to a line stage which is a great feature for people who only want tubes. The off the shelf version also has this option as far as I know.

https://hifiduino.files.wordpress.com/2015/01/r2routputs.png

To your question:
To connect the DAC directly to the grid or with a small resistor value for Rg results in the best transients.
The resistor value of Rg has obviously an enormous influence on the gain in combination with NFB and I got the 6dB gain back simply by using a smaller value for Rg.

Here is my hypothetical answer:
In my originally posted circuit (no NFB) Rp (7k6)is also pretty much the output impedance for the 300B OTL input, no influence of Ra (1,26Mohms at 1kHz) because it’s so much bigger. But with NFB and 50k for the pot, Ra might become a problem as the comparatively much smaller 50k or less can load down the tube affecting the original open loop gain of 12dB without NFB ( Rf is effectively in parallel to Ra?). But I cannot take a pot that is a lot bigger than 1,2Mohm in order to stop this as I would have no controllable area anymore. Why this can be corrected by choosing a smaller value for Rg is beyond my knowledge.
There was no difference between a 100k and a 50k pot regarding gain (which would kind of support my theory), only regarding controllable area. The 100k pot started to kick in only in the last third of its range. That’s why I changed it to 50k.  I can say this: NFB volume control works and I will stick with it.
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