19-12-2020, 12:14 AM
Having picked up some green CRT's ex scope and Radar types I would like to be able to try some of them out on my home made TV. As using Electrostatic deflection, I think it will pose a few issues to me along the way and in fact this EHT generator may well have to be negative generating so as to keep the deflection plates within a reasonable range, even possibly mixing +Ve and -Ve EHT?
Anyway, the plan initially is to use a circuit from the Practical Television circuits book, but use 7 pin valves such as 6D4 and EL91. The circuit calls for a LOPT of which I did not have so thought I would make my own rather crude affair.
A former was machined out of Nylon approx 50mm diameter.
This, going right to left in the picture, was then wound with a 200 turn feedback coil, 500 turn primary coil and 5 x 600 turn secondary coils to make up 30000 turns in total for the overwind.
I have a few 500pF high voltage caps from a radio rally of many years ago, so used two to form the EHT smoothing capacitor. Rather than use valve rectifiers. I used a 16 KV, 5 mA diode for the EHT and a BYX10 to rectify the feedback winding.
Although I like using valves, the EL91 would appear limited to 16mA anode current at 250 Volts, so saving the extra current by not heating a couple of rectifier heaters seems worth while.
I also use three ferrite aerial rods to form the core, this may be over the top, I will attempt to do more testing tomorrow.
Testing is a bit of Heath-Robinson. I have 19 x 10 Meg Ohm resistors in series and use a Digital volt meter that has a 10 Meg Ohm input resistance, so in total a 200 Meg resistor across the output as a load giving a 1/20 Voltage reading on the DVM. Not sure on accuracy as all resistors are 5% tolerance.
But it indicates a peak of 3,200 Volts at present across 200 Meg. The feedback pot allows for dropping the output down to around 1000 Volts. If, as I did by mistake just put the DVM across the output then it indicates a maximum 1080 Volts into 10 Meg so 1 mA current.
With the 200 Meg resistance at 3200 Volts that indicates to me 16 uA (micro Amp). So perhaps I should be able to do a simple line graph to indicate what the max available is for the typical 5uA the CRT's I have require?
HT to the unit has been set at 250 Volts, max current drawn is 19.4 mA So think I am OK with the 16 mA EL91 anode current.
It seems I like building death traps.
Adrian
Anyway, the plan initially is to use a circuit from the Practical Television circuits book, but use 7 pin valves such as 6D4 and EL91. The circuit calls for a LOPT of which I did not have so thought I would make my own rather crude affair.
A former was machined out of Nylon approx 50mm diameter.
This, going right to left in the picture, was then wound with a 200 turn feedback coil, 500 turn primary coil and 5 x 600 turn secondary coils to make up 30000 turns in total for the overwind.
I have a few 500pF high voltage caps from a radio rally of many years ago, so used two to form the EHT smoothing capacitor. Rather than use valve rectifiers. I used a 16 KV, 5 mA diode for the EHT and a BYX10 to rectify the feedback winding.
Although I like using valves, the EL91 would appear limited to 16mA anode current at 250 Volts, so saving the extra current by not heating a couple of rectifier heaters seems worth while.
I also use three ferrite aerial rods to form the core, this may be over the top, I will attempt to do more testing tomorrow.
Testing is a bit of Heath-Robinson. I have 19 x 10 Meg Ohm resistors in series and use a Digital volt meter that has a 10 Meg Ohm input resistance, so in total a 200 Meg resistor across the output as a load giving a 1/20 Voltage reading on the DVM. Not sure on accuracy as all resistors are 5% tolerance.
But it indicates a peak of 3,200 Volts at present across 200 Meg. The feedback pot allows for dropping the output down to around 1000 Volts. If, as I did by mistake just put the DVM across the output then it indicates a maximum 1080 Volts into 10 Meg so 1 mA current.
With the 200 Meg resistance at 3200 Volts that indicates to me 16 uA (micro Amp). So perhaps I should be able to do a simple line graph to indicate what the max available is for the typical 5uA the CRT's I have require?
HT to the unit has been set at 250 Volts, max current drawn is 19.4 mA So think I am OK with the 16 mA EL91 anode current.
It seems I like building death traps.
Adrian
Learning as I go!
Youtube EF91 Valve
Youtube EF91 Valve
