3rd Dec 2021, 3:25 pm | #1 |
Pentode
Join Date: Aug 2005
Location: Northwich, Cheshire, UK.
Posts: 206
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Faulty EL84
Hi All,
While I wait for a substitute from Italy, this EL84 has me puzzled : the anode's on 300v, g2 on 255V, cathode at 6v (so it must be passing current), and g1 hovering around 0v, and yet no output when signal is then applied to g1 ! Is this a known failure mode for this valve ? Thanks jm |
3rd Dec 2021, 3:33 pm | #2 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Re: Faulty EL84
Capacitor short circuit across the output transformer?
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3rd Dec 2021, 3:49 pm | #3 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,431
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Re: Faulty EL84
Plus open circuit speaker or a fault in that part of the set.
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Frank |
3rd Dec 2021, 4:11 pm | #4 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Re: Faulty EL84
Yes, that's right. Also, I should have made it clear that it was the transformer primary I was referring to regarding the shorted (tone correction) capacitor - a common fault, as is the above mentioned o/c speaker, or connection to it.
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3rd Dec 2021, 4:14 pm | #5 |
Heptode
Join Date: Jul 2018
Location: Worthing, West Sussex, UK.
Posts: 992
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Re: Faulty EL84
Apart from the other suggestions, make sure with small probes you actually measure the voltages on the actual valve base pins, I have had B9A sockets go open circuit between valve and base before, or lose springiness.
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3rd Dec 2021, 5:02 pm | #6 |
Pentode
Join Date: Aug 2005
Location: Northwich, Cheshire, UK.
Posts: 206
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Re: Faulty EL84
Hi All. thankyou for those ideas. The cap' across the primary was cut out, and there is a faint hum in the speaker. I will try that last suggestion as to the valve base ofcourse. The hunt continues.....
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3rd Dec 2021, 7:24 pm | #7 |
Dekatron
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,421
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Re: Faulty EL84
Open circuit G1 on the valve or zero signal getting to the G1 pin on the valve.
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Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member |
3rd Dec 2021, 8:28 pm | #8 |
Tetrode
Join Date: Jul 2016
Location: Renfrew, Renfrewshire, UK.
Posts: 93
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Re: Faulty EL84
Hi, remember that some EL84s have pins 1 and 2 internally connected. This caused me grief on some older Ferguson radios where pin 1 was used as a handy tag for other uses. Fine sets these Fergusons, sometimes! Peter
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