14-08-2020, 02:26 PM
Thank you for the offer of 6BS7's Geoffrey. I still have my original though and do have three others (I think I bought four at the time) so will be Ok. As implied by Andy's experiences, these valves were originally specially selected for electrometer use. The datasheet gives an example circuit. I have only used the one of mine. which I used in a cathode follower, testing to check voltages across a resistance box made up using ten 10MOhm resistors. The figures came out as may be expected rather than the incorrect values which were given when using my (10MOhm) digital meter. I have had somewhat a fixation on linearity which makes a, somewhat more intrinsically linear, cathode follower more appealing rather than focussing more on stability and reproducibility. In tests I have used one circuit with a Russian 6н16б double triode with a 6ж1б CC tail which was useful experience.
The unit I am working on, which is complete, but needing some fettling (as Andy would say), has working front end undergoing tests has two channels, using an эм6 double / split tetrode electrometer valve feeding an IN126 instrumentation amplifier on one channel and an IN125 instrumentation amplifier (with direct input) on the other. It also provides voltage references. The unit has an Arduino micro which provides switching and timed operation of electrometer valve heater and HT for stability at the moment. Future intention is to link readings to a PC via processed via a 16 bit A to D (already fitted). On both channels output is available directly post instrumentation amplifier before any processing for the micro / PC. I can see that some of the work may well be useful here but I wasn't intending to go through the complete system as it has been and will be very involved plus also not of too much relevance to most vintage radio etc. I am happy if anyone has queries or wishes to liaise if they are doing similar things as I know information / experience is difficult to get (Keithley handbooks and site are very useful). Like any extreme measurement some issues are very expensive - valves are a minor cost; high value precision resistors dictate a bank loan lol - I have accumulated some NOS which should be sufficient for my needs though. These will be less precise but are making me think about ways of checking them, which is good.
Andy - I also use a voltage reference for most of my CSS, usually using a TS431 (~2.5V) reference as I bought a bag (50) some while ago and use them often. The circuit would otherwise probably be similar to yours. Mostly I am using my 60V supply but electrometer valves usually often anode voltages below 10V (at least mine [эм6,эм7]) so these are more transistor levels. It did make it easier with interfacing to the instrumentation amplifier (on +/- 12V). I most always use semiconductor CCS but (as indicated by my results above) valves can be more effective in simple form if voltages are high and currents low. My take on the Fet CCS source follower was that a fixed resistor would besimpler and less noisy / more stable but use of a variable source resistor should perhaps enable output zero setting (the circuit boad is all surface mount and not cramped at one and a half by one and a half inches).
Tracy
The unit I am working on, which is complete, but needing some fettling (as Andy would say), has working front end undergoing tests has two channels, using an эм6 double / split tetrode electrometer valve feeding an IN126 instrumentation amplifier on one channel and an IN125 instrumentation amplifier (with direct input) on the other. It also provides voltage references. The unit has an Arduino micro which provides switching and timed operation of electrometer valve heater and HT for stability at the moment. Future intention is to link readings to a PC via processed via a 16 bit A to D (already fitted). On both channels output is available directly post instrumentation amplifier before any processing for the micro / PC. I can see that some of the work may well be useful here but I wasn't intending to go through the complete system as it has been and will be very involved plus also not of too much relevance to most vintage radio etc. I am happy if anyone has queries or wishes to liaise if they are doing similar things as I know information / experience is difficult to get (Keithley handbooks and site are very useful). Like any extreme measurement some issues are very expensive - valves are a minor cost; high value precision resistors dictate a bank loan lol - I have accumulated some NOS which should be sufficient for my needs though. These will be less precise but are making me think about ways of checking them, which is good.
Andy - I also use a voltage reference for most of my CSS, usually using a TS431 (~2.5V) reference as I bought a bag (50) some while ago and use them often. The circuit would otherwise probably be similar to yours. Mostly I am using my 60V supply but electrometer valves usually often anode voltages below 10V (at least mine [эм6,эм7]) so these are more transistor levels. It did make it easier with interfacing to the instrumentation amplifier (on +/- 12V). I most always use semiconductor CCS but (as indicated by my results above) valves can be more effective in simple form if voltages are high and currents low. My take on the Fet CCS source follower was that a fixed resistor would besimpler and less noisy / more stable but use of a variable source resistor should perhaps enable output zero setting (the circuit boad is all surface mount and not cramped at one and a half by one and a half inches).
Tracy
