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Old 25th March 2013, 06:35 PM   #11
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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?

Last edited by Rundmaus; 25th March 2013 at 06:36 PM. Reason: Idea.
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Old 25th March 2013, 07:03 PM   #12
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Measured resistor and caps and are OK.
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Old 25th March 2013, 07:12 PM   #13
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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
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Old 25th March 2013, 07:26 PM   #14
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Quote:
Originally Posted by Rundmaus View Post
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.
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Old 25th March 2013, 07:39 PM   #15
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B+ 280V

upper cathode left 188,8V
upper cathode right 195V
lower plate left 135,8V
lower plate right 138,7V
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Old 25th March 2013, 07:44 PM   #16
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Red LEDs pic
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File Type: jpg DSC00269.jpg (212.9 KB, 257 views)
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Old 25th March 2013, 07:52 PM   #17
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Quote:
Originally Posted by Rundmaus View Post
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
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Old 25th March 2013, 08:26 PM   #18
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by merlin el mago View Post
.... 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
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Old 25th March 2013, 08:34 PM   #19
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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

Last edited by Rundmaus; 25th March 2013 at 08:34 PM. Reason: typo
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Old 25th March 2013, 08:37 PM   #20
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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
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