Ever since my Koss ESP95X took me down the electrostatic rabbit hole - I've been very curious about the earlier closed Koss electrostatics. I read of one person converting an ESP9 to Pro Bias, but he used a lot of equalizer corrections. The Koss ESP9 circuitry included tone correction. But the Koss ESP6 didn't require tone correction, or so I was told, so I bought a pair of "working" ESP6 off EBAY - greatly overpaying at $80. I was going stir crazy during the COVID lockdown and needed a project that wouldn't cost an arm and a leg.
I first tried to connect them to my Sabaj D5 amp dac directly and jacked the volume to full. The right side sounded distorted, low volume - and I couldn't get ANY sound out of the left side. Figuring my D5 couldn't provide enough power - I cut into the cable and connected the headphones to my 200W speaker amp, making sure to zero the volume before I powered up. The end result: the right side started to sound good but would distort - and the left side crackled and sounded super distorted.
Time for the nuclear option.
Caution - I don't recommend this unless you know what you're doing. And maybe not even then. Spritzer earlier in this thread commented that these aren't designed for Pro Bias - and will probably die over time. I didn't want to pull my normal bias amp out of the closet so I decided to live dangerously.
I opened up the headphones by only undoing the exposed outer 4 bolts under each earpad and took out all of the circuitry and wires. I jerry rigged a cable by splicing wires to the male end of my Koss extension cable - I used 6 strands of 20 gauge wire. On the other end I soldered terminal connections - once I realized the Koss ESP 6 didn't use soldered connections to begin with. This way I can completely disassemble whenever I want - by just unbolting the cable from the positive, bias, and negative terminals. In the picture below, bias is blue - and I don't think the other two matter because these were single ended to begin with. I made the top, yellow, positive. Please disregard the electrical tape - I just used it to hold felt as an initial test rig. I'll clean things up when I'm satisfied.
I didn't want to damage my Stax SRM 717 amp. So I connected to my Koss ESP95X amp. To my surprise, it worked. The right side was super clear, unfortunately - the left had even worse crackling and distortion. It was hurting my ears. I had not touched the drivers (the middle 4 bolts on each ear cup, under the electrical tape). You don't need to remove the electrical tape and touch these 4 bolts - unless you think you have dust/debris between driver and stator. Thinking that might be the issue - I investigated and found the results of picture number 2.
I think the gel from the original fluid pads had leaked between driver and stator - fusing them.
I packed away my project and after a couple months decided to try again. For $60 I got a cleaner pair and this time both sides worked and sounded pretty good against my Sabaj D5 amp. At volume maxed, the high end and low end distorted a bit - so I went for the nuclear option again.
This time - everything worked and sounded good but not great, when assembled as shown in the third picture. I used generic pads meant for the HM5 - and I need to find a headband that doesn't double as a midieval torture device. When I do - I'm next going to try to leave them open and I'll report back how they sound that way. Eventually I'll try to make them look presentable as well.
Update - I modified the headband to reduce the clamp, and left each cup open. Over the drivers I only left the two layers of felt. Over this I put a layer of speaker cloth, and otherwise they were fully open. Unfortunately the sound did not improve. There was now no low bass, as opposed to the previous shy quantity. Compared to modern electrostatic these sound slow and slightly echoey. I'll next remove the felt altogether but I'm not expecting any miracles. Its been a fun project but I have a couple $100 headphones that sound better.