I had purchased a couple of matched pairs of 2SK369BL jfet devices from Alan Wright. In addition to this, I was curious to try the 6GM8 (ECC86) car radio tube; in this case as the screen grid and plate elements of a cascode. My first implementation of a hybrid cascode was in the Calrad Rebuild project. The jfet provides the cathode (source) and grid (gate) elements. I used a well filtered tap from the DC heater supply to set the bias voltage on the screen grid at 6V, the resulting cathode / drain voltage being in the 7V region.
This unit is a rebuild of my 'Friday' MM design. The RIAA equalisation is as usual for me, an all-in-one arrangement, feeding a 6N1P second stage for an overall gain of 60dB at 1kHz. The gain of the cascode is 52dB; This is 6dB higher than that of the Calrad version due to this pair of fets having a higher transconductance combined with a higher value load (35.7k vs 27k). (The gain of a cascode is approximately the Gm of the input device times the load resistance.)
The unit already had a PSU with separate series regulators for each channel. I modified the PSU to increase the output impedance. In case you are wondering if I have lost my mind, I maunder on a bit on this issue in the power supply article. I used the 150V gas reference tubes to provide separate supplies for the cascode front end. I provided 100n capacitors across the gas tubes to kill the HF noise, with a further 22n capacitor across each supply located right on the front-end wiring.
This unit having very low input capacitance is particularly well suited to high source impedance cartridges such as many MM types or the high output MC Sumiko Blue-Point Special*. I quite liked this cartridge already but with the cascode, it is a great second to my Benz Micro LP2. In fact, now I am listening to this new phono-stage with my LP2; The sound even with no break-in, is causing me to envision a MK11A version of the WE416 design, using a cascode and eliminating the input transformers. (The gain is high enough that no input transformer is required. Transformers are excellent coupling devices but no device at-all is best!)
Click to see the schematic:
*There is more information on this aspect on the phono-stage section of the Calrad rebuild page.