My test rig for DHT/IHT stages (and even Pentodes) has evolved over the years. Lately I settled with some nice modifications to allow testing the majority of valves I have. I use a modular socket system, nothing fancy and can add/remove a source follower stage at the output. There is also a screen regulator in case pentodes are submitted under the mercy of the jig.
Here is the simplified diagram. I added a nice fixed bias supply formed by a SMPS PCB board which delivers up to 400V, however the output is dial to about 100V. Then I use a Swenson Regulator to knock down the noise by about 100dB. A simple pot provides the voltage needed between 0 and -100V. It can be tweaked for whatever range you need. The pot is 20T wirewound so allows a fine adjustment on the bias:
For the curious builders, 
here is the rig mounted on a piece of floor board:
