ECC81 mu-follower problem

[merlin el mago]

I have noise in both channels, I do the measurements and the voltage between the two tubes in place of 85V I have 175V after this measurment I tested the heater elevataion and voltage is ok 76V, can be the two tubes are damaged?

 

[Rundmaus]

Your measurement would mean there is a voltage drop of only 8.7V across the 33k resistor, meaning a plate current in the lower triode of approx 0.2mA. Is that the intended operating point?

Rundmaus

 

[kevinkr]

There has got to be a wiring or component value error.. There should at least 3mA flowing through that resistor.

 

[Rundmaus]

Please measure the voltage drop across the 470R cathode resistor to confirm the plate current problem in the lower triode.

Does the heater of the lower valve light up?

Rundmaus

 

[merlin el mago]

Please measure the voltage drop across the 470R cathode resistor to confirm the plate current problem in the lower triode.

Does the heater of the lower valve light up?

Rundmaus

I have a red LED and measures both channels 1,8V

the 4 heaters have light up

 

[merlin el mago]

Your measurement would mean there is a voltage drop of only 8.7V across the 33k resistor, meaning a plate current in the lower triode of approx 0.2mA. Is that the intended operating point?

Rundmaus

I measures 175V instead of 85V

 

[kevinkr]

Filaments wired for 12.6 operation with 6.3V applied?

 

[merlin el mago]

Your measurement would mean there is a voltage drop of only 8.7V across the 33k resistor, meaning a plate current in the lower triode of approx 0.2mA. Is that the intended operating point?

Rundmaus

No, 33K resistor have to drop from 183,7V to 85V not 175V

 

[merlin el mago]

Filaments wired for 12.6 operation with 6.3V applied?

6.3V

 

[merlin el mago]

Enclosed circuit used

 

[Rundmaus]

Do the other voltages on the schematic measure as they should? Voltage across the 560R cathode resistor of the upper triode would be interesting.

Does the red LED light up?

Rundmaus


EDIT: Is the red LED a standard type or did you accidentally stumble upon a low-current LED?

 

[merlin el mago]

Measured resistor and caps and are OK.

 

[Rundmaus]

If the LED lights up and there are only 8.7V across the 33k resistor, either there is a wiring error or the LED is a low current type.

But if there is no plate current in the lower triode, there shouldn't be plate current in the upper one either - which would show up in the other voltages being wrong.

Please, give a readout of the voltages around the 560R and check if the LED lights up.

Rundmaus

 

[merlin el mago]

Do the other voltages on the schematic measure as they should? Voltage across the 560R cathode resistor of the upper triode would be interesting.

Does the red LED light up?

Rundmaus


EDIT: Is the red LED a standard type or did you accidentally stumble upon a low-current LED?

Voltage across 560R, left channel 0,881V, right channel 0,939V

The LEDs are 20mA, yes both red LEDs are light up.

 

[merlin el mago]

B+ 280V

upper cathode left 188,8V
upper cathode right 195V
lower plate left 135,8V
lower plate right 138,7V

 

[merlin el mago]

Red LEDs pic

 

[merlin el mago]

Your measurement would mean there is a voltage drop of only 8.7V across the 33k resistor, meaning a plate current in the lower triode of approx 0.2mA. Is that the intended operating point?

Rundmaus

Voltage across 33K
Left channel 51,8V
Right channel 55,2V

 

[tinitus]

.... I tested the heater elevataion and voltage is ok 76V, can be the two tubes are damaged?

quote from Valve Wizard

Heater elevation: Because the upper cathode is at a high voltage it is usually necessary to elevate the heater supply to avoid exceeding the maximum rated heater-cathode voltage (Vhk). For the ECC81 this is 90V, so we would need to elevate the heater by at least 100V.

I have no idea what it means, but thats what it says
only found the figures to look different
or are you saying your measured heater-cathode voltage is 76V, and ok being below 90V

 

[Rundmaus]

Merlin,

the voltage measurements seem inconsistent:

In the first post you measured 175V where there should be 85V (between tubes = plate of lower tube). Then there are only 8.7V across the 33k resistor.

Now you say that the lower plate is at 135V, leaving approx. 50V across the 33k. Thats 1.5mA plate current. Consistent with 0.9V across the 560R resistor, again approx. 1.5mA plate current.

A bit on the low side, but definitely better than the 175V reading before.

What meter (low internal resistance?) are you using?

Rundmaus

 

[Rundmaus]

Heater-cathode leakage might be a problem, as the circuit significantly exceeds the specified V_hk limit if the heaters are elevated by only 76V. See tinitus' post.

Rundmaus