BY8410 EHT Diodes

Thanks for the excellent post Trev, I've got some of these too and your experience will help me avoid the issues you encountered.

I believe Stan builds these into little valve enclosures, maybe he will post his efforts as I remember them looking very good indeed.

Chris

Hello Chris
I would like to post the procedure to assemble a solid state EHT rectifier but I cannot find out how to add a picture or a PDF file, all I get is Web Errors.
Regards Stan.

Hi Stan,

Its quite simple, all you need to do when creating a new post is click on the "attach file" just below the box you enter text into. That will launch a dialogue box which will allow you to search your computer for pictures or files.

You have the choice to load the whole picture or thumbnail it in the posting. If you load a pdf just ignore the resolution and thumbnail just upload it.

Have a look in the library you will see what pdf's look like, I just logged in as a test user with the same rights as yourself and I managed to upload pics and pdf's so it should work for you.

Hope this helps

Chris

The BY8410 is a Fast High Voltage Soft Recovery Diode. It is rated at 10kV working voltage and 12kV Peak Inverse Voltage at 5mA, the diodes are designed for encapsulation but if well insulated with a shrink sleeve do not cause any problems. Please see PDF data sheet attached.
I have been using these diodes to replace valve EHT rectifiers for some time and have not had any failures so far. To replace the EY51 or U25 wired in type of EHT rectifier I solder two diodes in series and insulate the joint with a small piece of shrink sleeve, then put a short length of red and black PVC sleeve on the ends of the assembly to denote polarity. The complete assembly is covered with piece of shrink sleeve to hold the red and black PVC sleeve in place and insulate the assembly. This can be wired across the original valve EHT rectifier leaving it in place, the cathode is wired to either of the heater connections and the anode to the single connection to the EHT over-wind. If there is a fault with the heater winding on the line output transformer, it can safely be removed, as it is no longer required. I have noticed that the EHT is somewhat higher using the silicon diodes due the lower forward volt drop of some 42 volts at 5mA. If you look at the EY51 EHT rectifier characteristics an estimate of the forward volt drop is around 1kV at about 1mA which gives and internal resistance of around 1Mohm. This causes considerable voltage drop as the cathode ray tube beam current increases and variation in the width of the picture, with this in mind I have started to put a resistor in series with the diode assembly to match the characteristic of the valve. Adding a series resistor saves having to increase the drive to the LOPT stage to compensate for the reduction in width due the increase in EHT and also assists to reduce the reverse current as the diodes turn off by integrating the charge on the CRT final anode capacitance.
The plug in type EHT rectifiers EY86, DY86, DY802 and U26 type rectifiers can also be replaced with the BY8410 diodes, please see attached picture of the stages of construction.
In this case I use three diodes in series, the anodes and cathodes being joined together as close as possible by griping the lead of the diode with pin nose pliers as close to the body as possible and forming a small hook to join each diode together and soldering the joint. Again, the joints are covered with a short shrink sleeve and the polarity denoted with a red and black PVC sleeve on the free ends of the assembly, another piece of shrink sleeve is placed over the complete assembly to hold the red and black PVC sleeve in place and insulate the assembly. The cathode connections of all, the plug in types of EHT rectifier is all the same and are all joined on a B9A plug base and the diode cathode is soldered to these connections. A small piece of 20mm plastic conduit or 22mm plastic overflow pipe is then glued over the base. A 100k resistor of the old carbon composition type is then soldered to the diode anode and the free end of the resistor is threaded through the top cap as removed from an old EHT rectifier, this is soldered in place and the top cap glued to the PVC tube.
I have made several of these replacement rectifiers both with various values or resistors and with out resistors in series, and have found that a 100k resistor is the best compromise value which works in most sets. The rectifier without a series resistor can be used to waken up a tired tube, as it will allow five to ten times the normal beam current to flow and cause considerable overload on the LOPT stage if the contrast and brightness are mal adjusted.

Click to download file

Click to download file

Hi Stan
Thanks for that great description. Those B9A type rectifiers of yours are excellent' I would never have thought of building them like that.

Frank

Hello Frank
The only problem is the B9A plugs which are hard to find, the rectifier on the left of the picture is made using the old glass EHT rectifier base glued into the PVC tube. I have not had any problems with corona using this double insulated type method of construction.
P.S. Sorry about the picture, it was too big for the forum so I converted it to a PDF.
Regards Stan.

very nice idea beats using micro wave diodes
rob t

HI Stan
There is nothing wrong with the picture it shows everything perfectly clearly.
I was wondering about the B9A  bases as I cant remember seeing them for sale anywhere, I have plenty of duff valves though, I knew they would come in useful some day.

Frank

Hello Frank, Trevor.
I still have a few EHT diodes left if you need any more, I am pleased to see them put to good use. They are also suitable for rebuilding the old selenium stick Tripler’s and in most cases will fit inside the old stick, with the selenium discs removed. I have used them to rebuild the Thorn 1400, 1500. 2000 and 3000 EHT Doublers and Tripler’s.
Regards Stan.

Hello Trevor
I can put some in the post for you or I could take them up the David’s for you pickup the next time you are down in Newcastle.
Regards Stan.

Hi Stan
That's a very kind offer. I have enough yet to keep me going for a while.
Thanks again for the offer though.

Frank