JoaMat

Happy Birthday!

Thanks @Shawn. I have ”gathered” some info from three sets of 300B, one 2A3 and one 6A3. To me it seems that they behave quite similar and need about 7K – 7.5K cathode resistors. Your 50K trimmer (Grid Pot) will probably make it easier with the “fine tuning”. I guess you have put trimmers on the tube side of the board for easier access.

After paralleling with a 3.3K resistor an increase of a few mA seems reasonable, but what happened to the offset?

Glad you find a way to adjust the offset. With EL34 as CCS, +400V and offset close to zero, I believe 1.5K gives ~23mA and 1.0K gives ~34mA.

Well, High Rollers. You can’t debate an angry person, waste of time.

How much offset can you trim away with the trimmers? I guess one need about 15K to get rid of +10V offset (with 2 x 500K resistors in the voltage divider).

Yep, that's what I've seen.

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I’m using a single cathode resistor. When you are using two 3300R resistors, as on your board, I guess that is equivalent to one 1650R. If you follow silk screen you will get -100V or so on output. You probably need to double resistor values. Maybe someone out there can tell more about this.

Thanks. XGL6020 inductors are probably excellent. With TPS62913 I use Wurth 2.2uH 4.4 x 4.1 mm, which works all right. I don’t change anything when switching between 300B and 2A3 (except from filament voltage). Both tubes need 3.6K cathode resistors @20mA. EL34 needs 1.6K and EML 20B needs 0.6K. I use small daughter boards with resistors and jumpers to change cathode resistance.

With help of above Mini Hot Plate I managed to build another three small regulators. Here they are on a “motherboard” with rectifiers and electrolytics to achieve 12VCD. The trimmers are for adjusting output voltage, 2.5V for 2A3 and 5V for 300B. Now I want a regulator for 6,3V and 7.5V. The buck converter above is limited to 5.5V, so might try LMQ66430. @Shawn What inductor did you use with LMQ66430?

I used a Mini Hot Plate when soldering the buck converter and inductor. First a thin layer of solder and then up on the plate. 150 degrees Celsius for a minute before up to 250. Rest of components are hand soldered. Thanks for the bridge tip. I have some small Schottky bridge rectifiers (CDBHD240-G) I intend to try first.

Thanks to @Shawn, who found the device used in Elekit filament supply module, I now have a working DHT switched mode filament regulator ”prototype” (in red circle). Board size 23mm x 15mm (0.9in x 0.6in) Soldering looks awful, but remember my eyes are 69 years old and my hands are not that steady.

Happy Birthday, Kerry!

That was fast! Thanks a lot.

I don't what chip it is. Marking might be???? Very hard to see. 26QH 288 A3HL Including in curcuit is also dual comparator 2093B and .05R sense resistor. Good luck and looking forward a report.

Nice idea. If you can implement an auto detect function as well – like Elekit has done with TU-8900. Power module… filament supply in black square, and the tiny - 2mm x 2mm - step-down converter.

You won’t find them boards at Kevin’s library. Custom made.

Check this link - lots of good stuff. There you find gg.zip .

Congratulations to the T2 build. Please, allow me to point out… the crossed/twisted legs of TTA004B/TTC004B that replace 2SK79/2SK216. Great work. No updated boards needed.

Did this today today – in full dayligt. Replaced EML 20B-V4 with 2A3 tubes in a Megatron with solid state constant current sources. Changes are filament voltage and cathode resistors (from 640 ohm to 3600 ohm. Only light is from blue and red LEDs and some light from the tubes themself.

Happy Birthday!

Thanks for comment and link to ROHM document, @simmconn. Very much appreciated. That test had obviously no chance to be successful.

Since I don’t know how to calculate the thermal resistance between to-252 and heat sink out into free air I made this small test. I got a clear answer. I do know Ohm's law though.

Thanks for notifying. The more eyes the better and… No, I missed that damage. With L-bracket it looks like this. Clearence between +400V area and ground is 40 mil. So, I doubt that there has been an electric short. I suspect that, for a reason I don’t understand, the current rushed and 10m90 quickly over heated. At the same time the PSU gave up and all went silent. I guess it’s heat from 10m90s that caused the damage. But that is a 100% guess. Anyhow, I think I stick to to220 version of 10m90s and forget this mishap.