SOLVED - help G7 first build - no plate current

[ghiatorino]

Hi,
this is my first time i build a tube microphone from scratch, and first time working with vacuum tubes. Nothing exploded, so far so good.
I have some troubles making the EF86 work the right way, in attachment there's a picture with voltage readings, i summarize my measurements here:
(loaded psu)
Heater: +6,3 V . Current draw is 193 mA
Plate: +177 V on XLR, 173 on socket pin 6 (i am expecting a much higher voltage drop here...). Current is 0,18mA (should be around 1mA). The same current is entering in the 470K voltage divider.
Capsule backplate voltage: +85 V (measured between the 470K voltage divider)
Cathode: +8,2 V on socket pin 3 (i think the problem is here)
Pattern voltages are ok.
I hooked up a couple of dummy loads (280K, 30K) on PSU from B+ to ground to check PSU current capability, up to 10 mA can flow with no problems.
Double checked components values, build a second PCB with new components. Capacitors are at least 250V. 2 different brand new tubes tested.
When i plug the mic the tube heats up, but no plate current is flowing and no voltage drop on 100K plate resistor. Seems to be in cutoff region? am i totally wrong? what's happening?
Any suggestions on what's going on would be appreciated, this thing makes me wake up at night
Many thanks
t.

 

[Khron]

Are both cathode resistors ok? As in, the correct values (1.6k & 220). Or, worst case, i don't suppose you have a second tube to try out, do you?

 

[ghiatorino]

Thanks Khron
yes, cathode resistors are correct and yes, i tried 2 different brand new tubes, JJ and Svetlana...

 

[Khron]

Is the tube socket ok?

 

[ghiatorino]

think yes, i measure voltages directly on tube socket and on mic pcb to check the socket status. Also make a continuity test between pin sockets and pcb.

 

[Matt Nolan]

Apparently you have over 4mA of current in the cathode circuit yet no current in the anode circuit. Something must be wired up wrong somewhere, or you have a hidden short somewhere.

 

[Matt Nolan]

Can you measure the voltage at G1?

 

[Pip]

Check Ground/Earth integrity.

 

[ghiatorino]

Matt, Pip thanks for jumping in,
Since the dummy load tests went ok (to my poor knowledge in tubes) i am searching for the short or wrong connection on the microphone pcb.
I will measure this evening and let you know,  should i expect 0 Volts on G1, right?
By the way there's a thing i still can't understand:
Out of the PSU i have the heater ground running on a separate wire from 0V ground (xlr pin 7 and pin 1). Once the 7pin xlr is hooked up to the mic the 2 traces are connected together on the pcb, so, being at the same potential, can i use both for 0V reference to measure voltages around?
then the ground plane...
Since i have a 7 contacts cable, the xlr pin 1 is also the shield, connecting mic chassis to ground via the female xlr contact pad shorted to pin 1.
Then back into the psu i have the two xlr's pin1 connected together, tied to chassis via the contact pad on the 3 pole xlr.
Then chassis is connected to earth ground.
Is everything correct?

 

[ln76d]

Yes, you can use both for 0V reference to measure voltages.
Common gorund connection is inside microphone.
Check again your cathode resistors. To get 8.5V at cathode, resistor need to be high.
This also would give you no current at the plate.
Also ground connection on cathode resistor.
You can skip 220ohm resistor with a jumper and use 1.8k instead 1.6k - in my opinion sounds better.

 

[ghiatorino]

solved,
one of the legs that connects main pcb to tube socket was loosed from the inside, making random connection to the filament.
Many thanks to everybody