Increasing HV in Older Kits
The latest GK-B4 kits using the v4.0.2 PCB and above already have this has this change on the board.
This page is only intended for users with previous versions of the kit.
Background:
The limiting factors in producing HV in the previous kit's circuit are the breakdown ratings of Q2 and D2. For D2, the limit was increased by switching from the MUR160 (600V) to the UF4007 (1000V) some time ago. That left Q2, the MPSA44 (400V), as the limit. (Even though the breakdown is rated at 400V it still provides >800V due to the short duty cycle and low current involved.)
Very early on I tried a higher rated transistor for Q2 - the STX13005 (700V). I was surprised to find that there was no increase in max HV. The problem turned out to be that the STX13005 has a lower gain than the MPSA44. (Thanks to Andrey for pointing this out.) The solution was to lower the base resistor (R2) on Q2 from 3.9K to 1K.
Doing that increased the max open circuit voltage from ~750V to ~970V With a 11M Saint Gobain scint attached it drops to ~820V, with the 120M RAP47 it only drops to ~945V.) (BTW, If you're thinking about Increasing the inductance of L1, it seems to have little effect.
While this difference may not be enough to bias some PMTs for scintillation use, it does at least drive the two I have. Moreover, the additional HV will drive many of the higher voltage GM tubes, such as the LND 7616 or Victoreen / Anton 6107.
(It's possible to achieve much higher voltages by doing this mod and adding the voltage multiplier described in this schematic. I've gotten 1820V open circuit this way. However, the voltage falls to 910V when the 11M scint is attached. I guess you don't get something for nothing and the voltage multiplier creates more voltage at the expense of current.)
The HV Modification:
The modification to an existing Geiger kit is pretty straightforward - simply replace Q2, R2, and D2 (only if your kit had the MUR160) with the parts provided in the ScintCon Kit described below. The only hitch is that the pinouts are different between the two transistors. The MPSA44 is EBC (emitter, base, collector) and the STX13005 is ECB (emitter, collector, base). This is solved by crossing the base leg of the STX13005 behind the collector and insulating the collector leg with a piece of the supplied heat shrink. This is shown at left. The pic was taken from "behind" Q2. Note the yellow heat shrink.
The STX13005 is not easy to find, so if you have bought a Geiger Kit from me, just write and we can arrange sending you one.
Once you have done this mod, the instructions provided on the Adding a Scint Probe will apply to your kit.