The AM Forum

Good evening all.

A friend brought his Valiant over to the shop due to no output.

I fired the rig up and and let it warm up for about 5 minutes. Flipped the plate on and started hearing a hiss like there was something arcing.  Shut the rig off and started inspecting things.  Found a terminal strip for the 866 had carbon build up like it was starting to burn.  First thoughts is someone powered the rig up without letting it warm up or had the rig on its side.  He confessed to the latter.

At the moment I do not have any 866 or 3b28 tubes in stock.  And would like to get these mercury vapor tubes out of here.  I have heard of folks switching these to diodes and making it solid state.

I have plenty of 1N4007's, 5404. 5408, and 6A10 diodes in stock.  I would think if you did this there should be a resistor in series with the diodes. I also have some old tubes I could use the base off of.  Is my thinking correct?
I searched a bit today to see if anyone had posted a diagram on doing this but did not find anything.

Thoughts?

Buddy,

   Your picture shows solid state replacement modules for the 866 tubes. If that is what you currently have, then just repair the carnage where it is arcing. Also remove the 2.5v filament winding from going to the 866 sockets.

Jim
Wd5JKO

Hi Jim and thanks for the reply. Yes those are replacements that I use for testing. Really did not want to give them up lol.  That's why I wanted to build a replacement in an old tube socket.  This rig belongs to an elderly gentleman  so I told him I would repair it at my cost.

The good news is that the 866 Mercury vapor tubes have a fairly low voltage drop (about 15 Volts) so changing over to a set of 1N4007 diodes (4) should work. Given the higher supply voltage these days, plate loading should be set lower, such as around 300 mA for AM and 350 mA for CW.

What type of solid state rectifier replacements are those? Any #?

Thanks, Rich

Thanks for that information, so solid state should work out fine. I will look over the circuit tomorrow. Perhaps it is not as difficult as I am thinking it is.





Rich, these came from RF Parts quite a while ago. Were like 50 bucks each.
They are  They are the FAA 1N2637 rectifiers.

Interesting post as I'm currently working on solid-stating my Valiant.     The voltage drop on the 866's is so low that I'm not bothering with a dropping resistor.   Each leg of the solid state replacement will be 4 1N5408 diodes (total of 8 required for both legs).

I chose to mount a small perf board under the chassis to mount the diodes and new filter caps -- I increased each cap from 80 uF to 100 uF.

Series two 9 volt batteries and use a voltmeter to check the voltage drop on those rectifiers, I think you will find they are quite well adapted to the Valiant.  Be sure the drop is the same on both.

I prefer the mercury rectifiers for the glow. Also xenon - have more than I could use of those. I don't mind warm up times.

Indeed. I find the glow from silicon diodes alarming.

I use a pair of 1N2637 plug in style replacements. They plug into the tube sockets with a plate cap just like tubes.
Got both for about 40 bucks on eBay.

Great. I am going to stock up on some more of these. Although I do love that glow that the original MV tubes give off I like the solid state replacements. Faster start up, no dangerous mercury to worry with.

I got this one all finished and back to the owner.  Finishing up a second Valiant now with similar problems.  And I did get a chance to record this one for a video.
Coming soon.

Got the second Valiant up and running. The owner was so happy.

https://www.youtube.com/watch?v=UubM87R4ucs