JoaMat

Welcome to HeadCase @EchoRoot Check below two links: https://drive.google.com/drive/folders/1r3g2TAtBUaBdiMorTWX7yYgeJ7maQbYW?usp=sharing https://drive.google.com/drive/folders/152thxfZafBmisG0CKCv6X82V-Ch9TjMH?usp=sharing

I believe you should move 22K resistor right tube to between G1 and 20BV4 pad. Then wire the 20BV4 pads together for bort EL34 and 20B-V4.

Yes. Upper left tube: from pin1 (G3) is a blue trace connecting Pin3 (A) - must be cut, but you probably have done that. Really an exciting project.

Done some more work. Added a couple of zero ohm resistors round the output tubes and a few other changes. If 20B-V4 tube remove the zero ohms and 220R resistors. Pcb layout is more fun than Sudoku.

Below is my way to describe how to prepare mini T2 for EML 20B-V4. Cut traces to Pin1 (G3). Cut traces at heater pins connected to resistor string 120K, 300K, 300K, 300K (one per channel). Purpose of the sting is to create -400V potential for Pin5 (G1) via 22K resistor. Remove 220R resistors. Remove 22K resistor Find a solution to put 22K resistors from Pin5 (G1) to resistor string between 120K and 300K resistors. This is probably the trickiest part. No heater wires between tubes of course. I’m sure you can modify your mini T2 to take the 20B-V4 tubes. But, maybe one should make a new board for the Emission Labs tubes… Not much heat produced by DC-DC converter supplies. No heat sinks needed.

Tanks for the schematic. The DC-DC converter itself is 62mm x 26mm. Board with rectifiers, electrolytics and converter module is 91mm x 33mm per section. No changes to the circuit more than the tube change and its filament arrangement.

mini T2 DHT – for Emission Labs 20B-V4 tubes. Here with 2A3 and adapter from octal to 4-pin. The tube filament is injected to the adapter from 9VAC via rectifier and step-down DC-DC converter. Trimmers for voltage and current limit. I use it for 2.5V to 6.3V heaters.

Yes, a small problem with the 3D viewer... only a 1200 USD software. The LT1083 thing looks nice - is that your own design?

Glad you found a solution to the hum problem. I don’t have any empty board left – last one sent to Finland a week ago Below print screen from my PCB software, silk screen disabled, version .24.

I’ve had more than a couple of OPA197 failing. Original I used OP27 in offset servo position and they failed on a regular basis. OPA197 seems a bit more robust, but some casualties there too. When offset servo fails output voltages go high, almost all the way up to +HV (in my case anyway) and it has, so far, always been Op-amp problem. I’m working on a new version of a mini T2 board. STN9360 replacing KSA1156 and also moving offset servo up on tube side of the PCB – making it easier to replace a faulty OPA197. No problem with balance servo… knock on wood. Any ideas regarding a more reliable offset servo are more than welcome. Edit. I mixed up the balance and offset servos in the text above – now it’s correct.

Sil-Pad 477-4862 under bracket at stn9360

There is about 1/4 W each KSA1156. Below STN9360 (in red) replaced A1156. Bare copper (no solder resist) under the bracket. Put thin Sil-Pad to isolate bracket from the copper. Time will tell… if this is a good idea. And 2SC3324 is now 2SC2713 on silk screen, Thank you.

Yep, thanks a lot Shawn. It seems that KSA1156 has vanished from Mouser and other places. Mini T2 has a bunch of them. Today I’ve replaced one channel’s KSA1156 with STN9360. Surface mounted device, 90th between pins same as A1156. I soldered them in standing and power ON. Worked great – after two hour I measured something like 60+ degrees Celsius. Perhaps one can put the STN9360 on a small PCB board for cooling purpose. How about a board with aluminum substrate?

ssdynalo mini with 2sa1349 and 2sc3381 as input devices – on the small boards marked with red arrows.

Below STN9360 replacing 2sa1468 on a Kevin DIY T2 board. I bent outer pins slightly outwards to fit into the to220 pads.

Another alternative is STN9360, 600V, 0.5A, 1.5W in sot-223. Has been used in Megatron before (less than 100mW). Solder it in standing instead of ksa1156.

STTH512FP should work. Mouser has plenty of them.

Thank you. For daily use heat sink certainly needs to be grounded. We also need some cover for the transistor tab and leads, which otherwise are exposed for any 5-year engineer. I think it’s solvable. Better also an isolated mosfet... if available?

The heat sink is not grounded and the two Gothenburgers has been warned. Those drop in devices are only for testing purpose and should be handled as most dangerous - able to kill you. Absolutely no children or pets around.

I’ve made a solid state CCS for Megatron to replace the top EL34. Seems to work all right. I think a modified Megatron with solid state CCS will simplify the building of Megatron considerably.

Happy Birthday!

Happy Birthday!

A few words regarding filament supplies I’m using with 2A3 and 300B tubes. 9 VAC => unregulated 11 VDC => XL4015 DCtoDC step down converter... ... with trimmers for output voltage and current limiter. It’s easy to set voltage to correct voltage depending of tube and also to trim current to achieve a sort of soft start. So far it works ok.

No output transformer. There are output electrolytics to headphone. Schematically it looks something like this. Reason for three KSC2690A in CCS is to distribute the heat, so no heat sink needed. At the moment I only use two transistors in CCS due to insufficient transformers. I don’t know how to optimize the circuit, but I’m happy with the result.

Aerial picture of a balanced amplifier for dynamic headphones. Only light from LEDs and tubes. ISO-25600, F4, 1/13 sek. And in day light. 4 x 2A3, 4 x 12BH7, 8 x switch mode filament supplies, 2 x GRHV and 4 x transformers. DIY is still fun.