Some success today.
I made two Bifilar wound coils and it seemed to be a lot better, the first one is shown here, in great close up, and what a mess it is!
But it works, the second one was a bit better to look at. The board now has the input coil, with a 6K8 resistor across the tuned secondary, this feeds the grid of the first EF91, it also has a tuned anode coil again with a 6K8 resistor across the coil. The secondary of this feeds the grid of the 2nd EF91. Anode again tuned coil, but at present no resistor across it, the secondary of this going to a CV4007 diode to rectify the output.
My test set can tune filters, but has no means of showing a DC voltage response like you could with a Wobbulator/Polyskop so before I connected up the rectifier valve I did a quick test to see how it was looking, unfortunately using a 10:1 scope probe for the input to the test set which has a 50 Ohm resistance, so levels mean nothing.
During the day I measured the output from the Hedghog II ad measured them at -45 dBm into 50 Ohm!
So as a last test after connecting up the diode, detector I fed the IF strip with a -45 dBm, 90% A.M. modulated signal using a 20KHz tone and was able to recover a 3.5 Volt peak to peak waveform, off the detector. That means if the tuner has gain then things should improve and I will have a large enough signal to feed the sync and video stages.
The next item to do on the list with this is add two more coils acting as traps for the sound carrier and see how effective and sharp I can make them.
If successful I will have to be on the lookout for more Westminster coil formers as I am struggling to find a shop that still sells anything new that would be suitable.
I am going to try for an absolute max of 2Mhz bandwidth on video, I do not thing even that would be required for a small DG7-5 tube, but would appreciate comments.
Adrian
I made two Bifilar wound coils and it seemed to be a lot better, the first one is shown here, in great close up, and what a mess it is!
But it works, the second one was a bit better to look at. The board now has the input coil, with a 6K8 resistor across the tuned secondary, this feeds the grid of the first EF91, it also has a tuned anode coil again with a 6K8 resistor across the coil. The secondary of this feeds the grid of the 2nd EF91. Anode again tuned coil, but at present no resistor across it, the secondary of this going to a CV4007 diode to rectify the output.
My test set can tune filters, but has no means of showing a DC voltage response like you could with a Wobbulator/Polyskop so before I connected up the rectifier valve I did a quick test to see how it was looking, unfortunately using a 10:1 scope probe for the input to the test set which has a 50 Ohm resistance, so levels mean nothing.
During the day I measured the output from the Hedghog II ad measured them at -45 dBm into 50 Ohm!
So as a last test after connecting up the diode, detector I fed the IF strip with a -45 dBm, 90% A.M. modulated signal using a 20KHz tone and was able to recover a 3.5 Volt peak to peak waveform, off the detector. That means if the tuner has gain then things should improve and I will have a large enough signal to feed the sync and video stages.
The next item to do on the list with this is add two more coils acting as traps for the sound carrier and see how effective and sharp I can make them.
If successful I will have to be on the lookout for more Westminster coil formers as I am struggling to find a shop that still sells anything new that would be suitable.
I am going to try for an absolute max of 2Mhz bandwidth on video, I do not thing even that would be required for a small DG7-5 tube, but would appreciate comments.
Adrian
Learning as I go!
Youtube EF91 Valve
Youtube EF91 Valve
