DIY mini T2 Build Thread

[JoaMat]

3 hours ago, Shawn said:

BTW there is a wire cut in your back here but I believe it's the software problem.

 

Yes, a small problem with the 3D viewer... only a 1200 USD software.

The LT1083 thing looks nice - is that your own design?

[JoaMat]

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.

Edited March 18 by JoaMat

[Shawn]

16 hours ago, JoaMat said:

Yes, a small problem with the 3D viewer... only a 1200 USD software.

The LT1083 thing looks nice - is that your own design?

Not original from me. I found it on Alibaba and made some changes to it.

Here is the schematic I drew it under 10 mins. Well... not perfect but readable.

The size is 96mm*35mm. Still can shrink it down by removing the heatsink and attaching it to the chassis. What is the size of your DC-DC converter?   And your pics remind me I have a bunch of 20B-V4 sitting in my garage for yrs. How do you convert the circuit to adapt 20B-V4?😳

AC Out should be DC Out.

Edited March 19 by Shawn

[JoaMat]

 Tanks for the schematic.

5 hours ago, Shawn said:

What is the size of your DC-DC converter?

The DC-DC converter itself is 62mm x 26mm. Board with rectifiers, electrolytics and converter module is 91mm x 33mm per section.

5 hours ago, Shawn said:

How do you convert the circuit to adapt 20B-V4?

No changes to the circuit more than the tube change and its filament arrangement.

[Shawn]

11 hours ago, JoaMat said:

 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.

The size is impressively small. What is the heat generation of it?

11 hours ago, JoaMat said:

No changes to the circuit more than the tube change and its filament arrangement.

Excuse me for one more question, JoaMat. Here are the socket views of 6CA7 and 20B-V4.

To convert the 6CA7 to 20B-V4:

1. Pins 1 & 2, Pins 6 & 7 will be supplied by 5VDC instead of 6.3VAC.

2. Each 20B-V4 needs to be powered by separate supplies.

3. The Pin 8 which is CT on 20B-V4 remains the same since it's the Cathode on DHT.

4. Disconnect Pin 4

Please correct me if I'm wrong. Don`t really want to massive and fry those expensive tubes.

Don`t know what to do with Pin1(G3). And there is a 220Ohms resistor between Pin 3&4 and 22KOhms between Pin 2&5, Do I need to change it?

 

 

[JoaMat]

Below is my way to describe how to prepare mini T2 for EML 20B-V4.

1. Cut traces to Pin1 (G3).
2. 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.
3. Remove 220R resistors.
4. Remove 22K resistor
5. Find a solution to put 22K resistors from Pin5 (G1) to resistor string between 120K and 300K resistors. This is probably the trickiest part.
6. 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. 

[JoaMat]

On 3/17/2025 at 11:25 PM, JoaMat said:

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.

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. 

Edited Thursday at 10:35 AM by JoaMat

[Shawn]

12 hours ago, JoaMat said:

Below is my way to describe how to prepare mini T2 for EML 20B-V4.

1. Cut traces to Pin1 (G3).
2. 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.
3. Remove 220R resistors.
4. Remove 22K resistor
5. Find a solution to put 22K resistors from Pin5 (G1) to resistor string between 120K and 300K resistors. This is probably the trickiest part.
6. No heater wires between tubes of course. 

Here is the draft based on your description.

Regarding No.5, Should I pull the right-hand side(picture below) Pin5(G1) also to the resistor string between 120K and 300K resistors? Also Could you double check if there is anything I missed? Ignored the blue traces that go to KSC2690A and O-.

I will be working on the new layout after making sure it is correct.

1 hour ago, JoaMat said:

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. 

You know what? You are at another level of speed. I was thinking about your description and gonna say the zero jumper is the solution and you did it!

Edited Thursday at 11:41 AM by Shawn

[JoaMat]

1 hour ago, Shawn said:

Should I pull the right-hand side Pin5(G1) also to the resistor string between 120K and 300K resistors?

Yes.

1 hour ago, Shawn said:

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.

Edited Thursday at 12:37 PM by JoaMat

[Shawn]

Done. Credit to JoaMat.

Instruction again for 20B-V4 each channel：

1. Remove JP1~JP3(JP1 on the back which is connect Pin 1 & 3).

2. Remove two 220 ohms resistors.

3. Remove the right-hand side 22k ohms resistor.

4. Wiring the two pads which are marked as 20BV4.

5. Separate heater power supply. 

Based on layout ver.23. 

Hope this instruction can help someone keep investing in their mini T2.😊

 

Edited Friday at 07:02 AM by Shawn
22k fixed. Picture updated

[JoaMat]

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.

 

Edited Thursday at 11:59 PM by JoaMat

[Shawn]

7 hours ago, JoaMat said:

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.

 

Edited and fixed.🤞

I will work on the CNC enclosures next week. The drilling hole patterns will match any layout versions from .2~0.23.

Just send the Gerber to the factory, Can`t wait to listen to it.

Edited Friday at 07:11 AM by Shawn

[Shawn]

Okay. After multiple checks for the occasional humming issues for a week.

I finally learned the lessons. Separate each channel into individual space connectors. The two EL34 heater cables can`t go through the same connector(or be too close). The two 460V rails may have interference with each other, but I don`t have the knowledge to explain it.