Voltages on ECC83 in BlackBox

[ms660]

Originally Posted by John10b

Lawrence can you take a look at the bb circuit?

What am I supposed to be looking at in it?

Lawrence.

[Station X]

Originally Posted by John10b

Just a further comment, if there is indeed ment to be +ve voltage on control grid why isn’t it shown on service sheet.
Cheers
John

Very few, if any domestic service sheets show a control grid voltage. They just rely on the coupling capacitor being OK and the grid bias being equal to the voltage across the cathode resistor.

We restorers and repairers check the control grid voltage, because if it's positive a leaky coupling capacitor is indicated. The BC circuit is a special case and the grid voltage will be positive wrt to the chassis. What matters is what it measures wrt the cathode.

[John10b]

I’d like your opinion about whether or not having + 7volts on control grid is about correct according to the circuit configuration.
Cheers
John

[John10b]

Thank you Graham, so in your opinion having +7v is probably ok.
Cheers
John

[Station X]

Originally Posted by ms660

What am I supposed to be looking at in it?

Lawrence.

Look to you heart's content Lawrence!

I don't think there's anything wrong with the OP's player, but it would be nice to know the voltages across the resistors to confirm this.

[ms660]

Originally Posted by John10b

Just a further comment, if there is indeed ment to be +ve voltage on control grid why isn’t it shown on service sheet.
Cheers
John

Because the bias voltage between grid and cathode can be deduced from the info provided in the service data, and what it actually measures can vary considerably depending on the meter used (see Post#17) due to the high circuit impedance.

Lawrence.

[John10b]

Thank you Lawrence.
Cheers
John

[John10b]

Graham I will measure it later. Thank you for your feedback.
Cheers
John

[John10b]

Voltage between grid and cathode = -2volts.
Across R9, R11 = 24v.
Across R10 = 2v.
I think as you have indicated that the readings are correct and the amp is functioning correctly.
Thank you for your help.
Nick I haven’t removed V2,3, not sure if this will achieve any thing, considering what Lawrence and Graham has said, and the voltage reading I’ve indicated.
Any further thoughts, or can I put this to bed?
Cheers
John

[Station X]

Those readings are fine.

When measuring the control grid voltage wrt chassis some current will inevitably be drawn through R8 to power the meter as it were. It doesn't take much current flowing through a 2.2M resistor to give a sizeable volt drop. This probably accounts for your measuring 7V at the control grid wrt chassis.

What's the input resistance of your meter?

[John10b]

Good question the one I am using is an old Maplin 49T. I also used my new Aneng AN8002 and still had same readings.
However I just carried out the same test using my Rapid Analog Meter and I had virtually no voltage reading on the control grid!
I note the service sheet states all voltages measured using AVO8.
Perhaps I’ll redo voltage test using my Analog Meter.
Cheers
John

[Station X]

Some DMM's have an input resistance of only 1M. If you used one of these to measure the control grid voltage wrt to chassis then I'd expect a reading of 1/3rd the voltage across R9 and R11 which would be about 8V. This is not much different form what you measured.

The higher the meter's input resistance, the higher the measured voltage and vice versa.

[John10b]

I will also dig out my old AVO 7,should this give the same readings as an AVO 8?
Cheers
John

[Station X]

Try this little experiment. You'll learn much more from it than you will by reading forum posts or studying text books.

Connect a GOOD 9V battery to each of your meters in turn via a 2.2M resistor. What do you notice about the voltage readings?

[ms660]

Originally Posted by John10b

I will also dig out my old AVO 7,should this give the same readings as an AVO 8?
Cheers
John

I wrote what's shown below before I read Graham's Post.

It depends on the impedance of the circuit you are measuring, the AVO 7 is nominally a 500 ohms per volt job, the AVO 8 is a 20,000 ohms per volt job.

In other words if you were measuring a voltage with the AVO 7 switched to the 100 volts range you will, in effect, be connecting a 50k resistance across the circuit being measured, with the AVO 8 switched to the same range you will be connecting a 2 meg resistance across the circuit.

The resistance presented by the meter being the ohms per volt figure multiplied by the voltage range selected.

Lawrence.

[John10b]

Thank you both I will carry out the experiment Graham.
Thank you Lawrence, I must admit I didn’t realise the big difference between the 7 and 8.
In work we were only supplied with AVO 8.
Cheers
John

['LIVEWIRE?']

I may be wrong, but, IIRC, an AVO 7 is 1000 OPV on it's DC Voltage ranges. Lawrence's statement that the resistance applied to a circuit under test is the product of the OPV and FSD of the range selected is still true, whatever the OPV of the meter being used.

[John10b]

I note that my Rapid Analog Meter is marked as 20,000 ohms per volt.
So if i have any “suspicious “ voltage readings in the future when using my Digital Meter, I’ll double check using my Analog Meter.
Thank you for all your replies.
Cheers
John

[ms660]

Re: Post#37.

http://www.richardsradios.co.uk/AVO/avo7dia.jpg

The total resistance on the 100 Volts DC range between the -ve terminal and the +ve terminal is 50,000 Ohms (50k)

On the 1 Volt DC range it's 500 Ohms.

So far as I can make out.

Lawrence.

[John10b]

Thank you all for your feedback. I now have another question, which I feel I shouldn’t need to ask, but I’m going to anyway, after all this is what the forum is all about.
It now concerns the two EL 42 , both made by Mullard.
I noted that there is a big difference in the working temperature between the two valves ( 80 degrees C and 28 degrees C) which after checking bias voltages and cathode current ( 29 mA/ valve) puzzles me.
The amp is working correctly, as far as I can determine, no distortion or hum.
Without going into further detail why would there be such a big difference in the working temperature of the two valves in what appears to be an amplifier that is working correctly ?
Cheers
John