UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 5FP7 project starting with the focusing coil.

AdrianH · 28th Oct 2021, 1:09 pm

I do have a magnetic unit, obtained when I got the chassis which was after the project started. If I need to use it I will, but still like the idea of using the focusing coil. Basically I have as per 'a' in the page an open field, unless I can machine some iron to fit the coil I have, it will have to stay that way.

I have been working through some used EL84's I have on my home made static tester, varying the screen volts from 250 down to 150 volts and mapping the anode current against grid voltage with Va at 250 volts and plotting the results.

Click image for larger version

Name: EL84plots.jpg
Views: 41
Size: 48.7 KB
ID: 244451

I have also wound a single ended transformer to try and drive the scan coil. I had a spare EI core and bobbin that quick calcs showed it should be capable of approx 25VA and EI-60 with a stack height of 30mm. I have assumed the ra of the EL84 to be 4k5 Ohm and done a 9:1 n ratio to drive the existing wound scan coil. If I have it correct approx 23mA should give me the 204 mA required in the coil with approx 14 Volts across the scan coil and 126 Volts sawtooth.

Click image for larger version

Name: scandrivertfmr.jpg
Views: 43
Size: 63.8 KB
ID: 244452

With a few pot's in the cathode resistance of the EL84, I may be able to vary the shape of the scan or even add a bit of negative feedback?

I used a figure of 14 TPV for the windings and stacked all the E's together and used a bit of High temp masking tape used for powder coating as an insulator between the E's and the I's on the end. I managed to completely fill the bobbin which either means I have gone way OTT or that I am crap at winding a bobbin.

Next is to work building up a small test board.

Adrian

28th Oct 2021, 1:09 pm	#32
AdrianH Octode Join Date: Mar 2020 Location: Blackburn with Darwen, Lancashire, UK. Posts: 1,572	Re: 5FP7 project starting with the focusing coil. I do have a magnetic unit, obtained when I got the chassis which was after the project started. If I need to use it I will, but still like the idea of using the focusing coil. Basically I have as per 'a' in the page an open field, unless I can machine some iron to fit the coil I have, it will have to stay that way. I have been working through some used EL84's I have on my home made static tester, varying the screen volts from 250 down to 150 volts and mapping the anode current against grid voltage with Va at 250 volts and plotting the results. I have also wound a single ended transformer to try and drive the scan coil. I had a spare EI core and bobbin that quick calcs showed it should be capable of approx 25VA and EI-60 with a stack height of 30mm. I have assumed the ra of the EL84 to be 4k5 Ohm and done a 9:1 n ratio to drive the existing wound scan coil. If I have it correct approx 23mA should give me the 204 mA required in the coil with approx 14 Volts across the scan coil and 126 Volts sawtooth. With a few pot's in the cathode resistance of the EL84, I may be able to vary the shape of the scan or even add a bit of negative feedback? I used a figure of 14 TPV for the windings and stacked all the E's together and used a bit of High temp masking tape used for powder coating as an insulator between the E's and the I's on the end. I managed to completely fill the bobbin which either means I have gone way OTT or that I am crap at winding a bobbin. Next is to work building up a small test board. Adrian __________________ Asking questions and learning, or trying to! Youtube EF91Valve Last edited by AdrianH; 28th Oct 2021 at 1:14 pm.