JoaMat

I use LTspice to understand and to get a hint how things work. I got this table with help from LTspice. Supply voltage +/-400 V, Rupper is upper tube cathode resistor, Rlower is lower tube cathode resistor, values give offset zero for given Rupper , the resistance is the total of paralleling resistors. Looking forward your findings.

Thanks for the pin tip. This beyond my understanding. But as I see it, with 400V/22mA the Vgk will be what it will be with offset zero. 12 W with EL34 shouldn’t be much of a problem. My EL61 will also handle that, I hope.

Thanks for your comments, @simmconn. The heatsink gets warm and user is aware of it. I think he is running at 400V/22mA and so far, it seems to work. If it gets too hot than he can’t use this thing - that’s life. For pin dimension. The first version used Neutrik XLR3 pins. They were perfect, diameter slightly undersized. But to short to be soldered direct to main PCB. When looking for brass rod the best I found was 2.5 mm so, I decided to try that. 2.5 mm works with my Teflon tube sockets. The user is informed of that pins are soldered only on one side and if pins come loose, we have to find a better solution or perhaps abandon this thing. I use Addnorth HT-PLA Pro filament. Better heat performance than PLA but not as good as ABS. I’m confident the socket isn’t a problem. It might be a problem with the transistor tab cover. I haven’t thought about the cover until your post - thank you simmconn. Now the user is informed, and he promised to keep an eye on the cover.

New solid state CCS for Megatron. Now with a third pin for ground. The pins are made from 2.5 mm brass rod, which is almost 0.10 mm thicker than tube pins. PCB is a CNC milled one side 1.5 mm copper clad. So, no copper on top side of PCB. Holes for the three pins are 1.0 mm deep and you have 0.5 mm epoxy laminates left. Transistor pins are protected with heat shrink tube and the exposed metal tab is protected with a 3D printed cover. The screw holding transistor to heat sink is electrical connected to it and to the ground pin. The white base with guide pin is made of three 3D printed parts.

Fascinating chassis work. How about the scale of the altimeter? Small pointer one lap 1200 feet, two laps 2400 feet?

Please be careful with words. I do believe Mouser is a reliable company.

Happy Birthday!

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.