Triode-Strapped Curves for the 6BM8 Pentode

The 6BM8 contains a high mu line-level triode and medium power pentode (Pa ~7W) in one convenient glass envelope.

It’s also known for sounding rather good, and was popular in 60’s vintage Japanese amplifiers like the Sansui 250.

I thought it might make a nice input and driver tube for an upcoming SET project. This project calls for the pentode to be run in triode mode, but I couldn’t find any curves for this configuration online. Consequently today I had a go at making my own. The below curves are for a brand new Electro-Harmonix 6BM8 and I have included Vgk increments of 1 & 2 volts. The measurements were made using a simple unregulated supply and cheap DMM’s, so shouldn’t be considered definitive. However, they are more than adequate for determining a suitable operating point. I hope someone else finds them useful.

At a glance the tube looks pretty linear below 200V. Over the next few days I will determine the optimum operating conditions and implied performance and post here for reference.