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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 12th May 2019, 10:11 pm   #1
leslie5555
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Join Date: May 2015
Location: Reading, Berkshire, UK.
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Default Black box EL42 valve voltage measurements

Hi all
I have a Pye black box MK2 Amp with the ecc83 ez41 and two el42 valves.
I have checked the voltages on the two output valves as follows

Pin 2 = 291v
Pin 3 = 15.7v
Pin 5 = 298v
Pin 6 = 0v
Pin 7 = 15.7v
Pin 8 = 0v

Other valve
Pin 2 = 291v
Pin 5 = 297v
Pin 6 = 0v
Pin 7 = 15.7v
Pin 8 = 0v

Also the HT is 267v on one tag and 300v on the other

Are those measurements what should be expected?

Les
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Old 12th May 2019, 10:54 pm   #2
martin.m
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Default Re: Black box EL42 valve voltage measurements

The EL42 pin connections are as follows:
1 and 8 heaters
2 anode
3 and 4 internal connections
5 g2
6 g1
7 cathode

The two EL42 cathodes are connected together in this model of Black Box. The voltages you have seem about right.
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Martin
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Old 12th May 2019, 11:59 pm   #3
PJL
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Default Re: Black box EL42 valve voltage measurements

The HT is a bit high and the cathode voltage a bit low. However, the voltages don't tell you a great deal as the valves share the same cathode bias resistor so it is impossible to tell if they are well matched and taking roughly the same anode current.

You can roughly check the current of each valve by measuring the voltages from HT to each anode then (with power off) measure the resistance of HT to each anode and use ohms law to calculate the current. It is 'rough' because the resistance of copper wire changes with temperature and part of each winding is also carrying the screen current.
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Old 13th May 2019, 10:35 pm   #4
leslie5555
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Default Re: Black box EL42 valve voltage measurements

Thanks to both of you for your help.
Les
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