22-08-2020, 10:43 AM
I have now pulled a few pictures together of my progress so far:
The first three show the electrometer valve and some of the first electronic units – The power input board and Arduino. The home-made screening for the electrometer valve is from brass sheet with an Nickel Chrome foil screen (both electrically connected). Hopefully to minimise electric and magnetic field interference. This screen is designed for direct soldering to the valve base thence to the cathode. It sits away from the valve tube glass.
Number four shows further circuitry – Anode regulator (differential with HT level and differential adjustment for the anodes – a bit overkill but does work). INA125 and INA126 instrumentation amplifier boards.
Number five shows the electrometer tube heater supply regulator. Following an earlier work with low voltage / current supplies I have worked on a constant current supply. More tricky with the 4.5V 75mA but I thought useful as the constant current, rather than voltage, supply will eliminate switch on surge. Stability is the thing with an electrometer and they can take a considerable time to settle if things are less than perfect (hence timed heater / HT switch on).
Number six is the further assembled INA125 and INA126 boards with their op amp offset circuits (for plus and minus ranging for the Arduino A to D). and the protection for Arduino input.
Number seven shows the unit cases. Originally there was only one PSU module but I thought the transformers were at their limit so now it has one case containing the mains transformer plus discrete and electronic smoothing and a second containing the main power regulators; providing outputs of -12V, +12V, +9V and +5V. The power units were separated from the main unit to limit the possibilities of heat and magnetic field interference.
The last three photographs show the completed unit in it's current state. As yet no labels have been produced.
At present I have only done basic tests but the valve circuit seems stable in that there is little drift after switch on settling. A return after zero setting within the hour finds the setting still within 1mV but, as I say, early days. This is measured at the instrumentation amplifier output (The INA125 has a basic gain of 4 and the INA126 of 5). One thing that is a little more troubling is that there is a noise / fluctuation resulting in a random wandering of output of two or three mV. This does not seem time dependant. I have been through the system shutting off various possibilities of interference but may have found the area when I tried linking the g2a input (topcap) with ground directly. This did seem to affect it and shows itself slightly differently now so it is probably an input component, connection or static error. Further work in other directions (Fet circuit) has shown that a redesign of the input may be good though.
Further down the chain I realised that I had an A to D test program running so I have verified that there is sensible data being transferred to the PC. It's too early for accuracy checks though. I do plan to change the programming to increase the heater to HT switch on time in line with other recommendations / references from 20 seconds to 1 minute (see 6BS7 datasheet, and elsewhere).
The last photograph is one of the two versions of Fet source follower I have made. One has 1MOhm and the other 100MOhm gate resistors. One interesting observation I made earlier in the week was when trying a rough test on a 6BS7 (my previous test example) for grid current. This was noted to be in the picoamp region, but the interesting observation was with regard to light sensitivity. In a reasonably lit small room I switched on the room light (pretty much straight above and consists of a high wattage (maybe 120W equivalent) bulb. This was like operating a switch with grid current instantly changing by 20mV. With my 100MOhm resistor input this equates to 200picoamp. Maybe not a great concern to most uses but a big issue for low level DC voltage or current. With low level measurement obviously the valve needs to be in the dark.
I had another interesting experience yesterday too. I had seen on the internet that someone had used a neon for gate resistor with a Fet for field detection. Their design used a meter in the drain circuit with variable source bias. I built one of my source follower circuits using this idea. Using a 2” square piece of double sided copper board on the input I tested using a voltage source and various input resistors. I was very pleased to find that with just a lash-up I could get figures in the ball park for resistors of 1GOhm, 5GOhm, 10GOhm and 60GOhm, roughly calculation the input resistance of the Fet circuit to around 100GOhm. Obviously not accurate at all but a good step in the right direction and good experience. With a reminder to earlier things mentioned I had put some heat shrink onto the neon as I was aware it was light sensitive. Interestingly there was still leakage as the room light switch test did noticeably affect things. Note 1: Beware of light interference lol. The stability of this circuit also got me things about my valve electrometer input. I am very pleased with these little Fet circuits as they are stable and good for exploration.
The first three show the electrometer valve and some of the first electronic units – The power input board and Arduino. The home-made screening for the electrometer valve is from brass sheet with an Nickel Chrome foil screen (both electrically connected). Hopefully to minimise electric and magnetic field interference. This screen is designed for direct soldering to the valve base thence to the cathode. It sits away from the valve tube glass.
Number four shows further circuitry – Anode regulator (differential with HT level and differential adjustment for the anodes – a bit overkill but does work). INA125 and INA126 instrumentation amplifier boards.
Number five shows the electrometer tube heater supply regulator. Following an earlier work with low voltage / current supplies I have worked on a constant current supply. More tricky with the 4.5V 75mA but I thought useful as the constant current, rather than voltage, supply will eliminate switch on surge. Stability is the thing with an electrometer and they can take a considerable time to settle if things are less than perfect (hence timed heater / HT switch on).
Number six is the further assembled INA125 and INA126 boards with their op amp offset circuits (for plus and minus ranging for the Arduino A to D). and the protection for Arduino input.
Number seven shows the unit cases. Originally there was only one PSU module but I thought the transformers were at their limit so now it has one case containing the mains transformer plus discrete and electronic smoothing and a second containing the main power regulators; providing outputs of -12V, +12V, +9V and +5V. The power units were separated from the main unit to limit the possibilities of heat and magnetic field interference.
The last three photographs show the completed unit in it's current state. As yet no labels have been produced.
At present I have only done basic tests but the valve circuit seems stable in that there is little drift after switch on settling. A return after zero setting within the hour finds the setting still within 1mV but, as I say, early days. This is measured at the instrumentation amplifier output (The INA125 has a basic gain of 4 and the INA126 of 5). One thing that is a little more troubling is that there is a noise / fluctuation resulting in a random wandering of output of two or three mV. This does not seem time dependant. I have been through the system shutting off various possibilities of interference but may have found the area when I tried linking the g2a input (topcap) with ground directly. This did seem to affect it and shows itself slightly differently now so it is probably an input component, connection or static error. Further work in other directions (Fet circuit) has shown that a redesign of the input may be good though.
Further down the chain I realised that I had an A to D test program running so I have verified that there is sensible data being transferred to the PC. It's too early for accuracy checks though. I do plan to change the programming to increase the heater to HT switch on time in line with other recommendations / references from 20 seconds to 1 minute (see 6BS7 datasheet, and elsewhere).
The last photograph is one of the two versions of Fet source follower I have made. One has 1MOhm and the other 100MOhm gate resistors. One interesting observation I made earlier in the week was when trying a rough test on a 6BS7 (my previous test example) for grid current. This was noted to be in the picoamp region, but the interesting observation was with regard to light sensitivity. In a reasonably lit small room I switched on the room light (pretty much straight above and consists of a high wattage (maybe 120W equivalent) bulb. This was like operating a switch with grid current instantly changing by 20mV. With my 100MOhm resistor input this equates to 200picoamp. Maybe not a great concern to most uses but a big issue for low level DC voltage or current. With low level measurement obviously the valve needs to be in the dark.
I had another interesting experience yesterday too. I had seen on the internet that someone had used a neon for gate resistor with a Fet for field detection. Their design used a meter in the drain circuit with variable source bias. I built one of my source follower circuits using this idea. Using a 2” square piece of double sided copper board on the input I tested using a voltage source and various input resistors. I was very pleased to find that with just a lash-up I could get figures in the ball park for resistors of 1GOhm, 5GOhm, 10GOhm and 60GOhm, roughly calculation the input resistance of the Fet circuit to around 100GOhm. Obviously not accurate at all but a good step in the right direction and good experience. With a reminder to earlier things mentioned I had put some heat shrink onto the neon as I was aware it was light sensitive. Interestingly there was still leakage as the room light switch test did noticeably affect things. Note 1: Beware of light interference lol. The stability of this circuit also got me things about my valve electrometer input. I am very pleased with these little Fet circuits as they are stable and good for exploration.
