Sometimes, if I cannot find anode curve data on the net I get motivated and plot then myself.  I do not have a curve tracer so the data is static.  This is not ideal but is much better than no curves:

6BM8 Pentode section triode curves:

6688 Triode connection curves:  

I did this in consideration of a triode connected 6688 as an alternative to the 5842:

For small signal applications (e.g. phono or line stages), a good operating point looks to be:

150V / 16.6mA / -1V at which, Gm looks to be around 17.6mA/V, mu 60. Rp 3.4k

This compares to a 5842:At 120V / 15mA / -1V Gm is around 20mA/V, mu 42, Rp 2.1k

206 DHT Curves:  I had two of these triodes on loan from for evaluation.  They are superbly linear but the vendor was not happy with my measurements of available power.  The anode resistance (at 400V / -26V / 45mA) is around 2.8k.  The lowest load I felt comfortable evaluating the tube into was 4.2k (approximately 1.5Rp). In reality, for most applications, 1.5Rp is too low.  Whatever, at 400V / 45mA into 4.2k , I got 1.5W at 2%, 2.2W at 3% and 2.7W at 5%.  My feeling is that this tube exhibits extraordinary potential for speaker systems having an efficiency of (say) 102dB or higher.  The vendor insisted that the tube is effectively three times a 205D.  While I did note that the shape and distribution of the anode characteristics are very similar to those of the 205, the anode resistance, emission and power measurements suggest that the design is closer to 1.5 205Ds in parallel.