The CCS works really well... It dropped the voltage to one of the 5691 tube correctly as calculated. However, when I put both 5691 tubes in, one of them had a B+ of 115.8v while the other is 106.8v... a 9v difference between the tubes! Not what i expected from these superbly built tubes. And according to jacmusic.com, these are the best ever 6SL7 variants.

So I measured the cathode resistors just to be sure the voltage drop is correct and that it is not due to the CCS. One tube measured 1.058v and the other measured 1.048v. On the spot as far as I'm concerned! (The cathode resistors are Kiwame 820R and the Rset on the LM317 should be some Dale 1% 1k resistors. But this just shows how accurate the CCS is!) This can only mean that even the 5691 can vary from tube to tube! And the ones I have are matched to 58/60 and 58/68 according to the guy I got them from...

Well... anyways, next would be to put these stuff together with the Sabre32 dac and the Placid power supply.

I'm doing the DN2540 cascode with LM317 so that i can set the current to exactly what i want. In this case, it is to load the RCA 5691 red base tubes with just 1.2mA of current per tube. This should be sufficient for voltage gain from the Bufallo DAC into the BCF, more importantly, I can draw from the same B+ rails that powers the BCF.

With the previous MJE350/BC557C and LED version, I could not get the resistors for the B+ correct. Quite frustrating when this has worked for me before...

Schematic is the same as Figure 13c from Jung. Rset is 1k to give 1.25mA.