Help for STA-35 based project

[Bonfigl]

Good morning everyone,

After building a clone of the kondo M77, which is electrically finished but I haven't built the case yet (don't worry I'll post everything in the dedicated post https://theartofsound.net/forum/show...eamp-clone-kit), I decided to build a tube power amp based on the Radford STA-35.

It all started when I read this article https://blog.ampliekit.it/index.php/...a-35-dual-mono, I decided to design the schematics on fusion360 then I generated the gerber files and finally I made the PCBs by JLCPCB.

Attached you can find the wiring diagrams and some photos of the finished board.

Everyone ready!!! I connect the power transformer, the output transformers, I connect an 8 Ω resistor as a load on the secondary of the T.U. and make tension without valves (the T.U. is produced by Toroidy.pl and I am attaching the photos)

It didn't explode !! All voltages are fine.

I decide to insert the valves, I energize and immediately the high voltage fuse blows (Vaa in the diagram), I discharge the capacitors, I change the fuse and try to burn again.

I check all the circuit and the components and they are all ok, I give voltage without the final tubes and the fuse does not burn, I have tried to change the final tubes, and with all the fuse burns (2 + 2 EL34 Tung-sol and 2 JJ) .

If I measure the voltages on the sockets of the amps without the tubes mounted and with the bias at zero I read the following values:

PIN 3 465V
PIN 4 465V
PIN 5 -59V

Do you have any suggestions why I am going crazy to solve this problem and I cannot find a solution.

You can find the wiring diagrams and photos at the following link https://drive.google.com/drive/folde...EI8U4RoYIbVnn6

Thank you so much everyone in advance

PS in a moment of desperation I mounted a 1 A fast fuse and that too blew.


Luca

[Barry]

I would suggest you check the PCB layout, to make sure it corresponds exactly with the circuit diagram.

[Bonfigl]

Good morning Barry,

Thank you for your reply, I have checked several times the empty PCB and the assembled one by comparing them with the diagram and using the multimeter in continuity mode.

Luca

[Firebottle]

I would put a much larger negative bias on the valves to make sure they are cut off, then see what the standing circuit conditions are.

[Bonfigl]

ok. in the next few days I will try to change the values ​​of some resistors to try to further lower the negative voltage.