Plate / cathode voltage are too high.

[SparkleBear]

Hey all,

I’m building my second revision of a tube mic. The first go around it went perfectly but this build the plate voltage and cathode voltages are measuring way higher.

On the first build I got 68v at anode, .8v at cathode and .2 at the grid.

On this non working build I’m getting 118v at the plate, 16v at the cathode and 0v at the grid.

If I tap on the capsule electrode on body I’ll hear the tube open for a moment before it closes down. I’ll also heat it work for a moment in the process of the tube warming up.

I double checked the layout in kicad and nothing seemed to change.. not sure what’s up.

Tried with multiple tubes… all same results

Any ideas?

 

[Khron]

Time to start testing the passives then. Is the plate resistor really 100k, and the cathode resistor 1.6k?

Maybe also consider replacing R2 as well.

 

[Drosselmeier]

Salve. Check the compliance of the installation of the circuit and replace the tube.

 

[dmp]

Tube is nearly off - tube socket defective? Poor solder joint?

 

[ruffrecords]

Check the value of R3, maybe you accidentally fitted 16K. Check C3 not shorted.

Cheers

Ian

 

[Khron]

If C3 was shorted, that would pull the cathode to ground, not lift it to 16v.

 

[oscarhuang]

R3 maybe not connect to ground.

 

[rock soderstrom]

Tube is nearly off - tube socket defective? Poor solder joint?

Yes, check the tube socket, it looks a little odd, the pin clips are inconsistent in height, see photo.



In any case, measure R3 through the socket, i.e. from above with the tube pulled out. Still 1.6K?

Check solder connections at the socket and especially the connection between main PCB and tube daughterboard!

 

[SparkleBear]

Thanks for replies everyone… I’ll check the build again when I’m back from the New Years trip! HAPPY NEW YEAR TO ALL!

I also have a hunch it is something with the daughter board connection or the tube socket. I purchased a replacement and will disassemble in the next few days to discover the issue.

Thanks again!

 

[SparkleBear]

R3 maybe not connect to ground.

MYSTERY SOLVED!..

Turns out that the little traces that connected the ground pad to the ground plane got damaged and so the cathode bias resistor was NOT grounded. Made a bridge with the ground plane and it fired right up!

Thanks for the help y'all!

YAHOO