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Old 22nd March 2005, 10:54 AM   #1
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Default el84 help

i have a pioneer sm-83 amplifier. My problem is with one of the el84m. When music is playing one of the tubes pulses along with the signal and the sound distorts in the affected channel.The other tubes are ok. Does anyone know what the problem could be? Many thanks.
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Old 22nd March 2005, 11:52 AM   #2
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What do you mean by pulses along with the music?

Have you tried putting the good valves from the other channel into the channel which is affected?
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Old 22nd March 2005, 08:07 PM   #3
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hi there........I ain't got schematic on hand..... decoupling caps could be on their way out.....bit of a stab in the dark..

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Old 22nd March 2005, 09:38 PM   #4
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I did a search, and came up with the schematic, which can be found here.
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Old 23rd March 2005, 04:31 AM   #5
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Default el84 help

hi jason,
i have voltage on the green wire of the output transformer for one valve that seems all ok but the the red wire connected to the other valve is showing negative very low volts. does this mean i have a shorted transformer? . theres still volume coming out of the affected channel but its distorted. Any help would be much appreciated .
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Old 23rd March 2005, 05:21 AM   #6
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Hi,

Sorry, I can't really be of much help - the colours of the transformer leads aren't marked on the schematic.
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Old 24th March 2005, 08:54 AM   #7
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hi, all
the wires i was talking about are connected to the plates of the el84s , one valve has the right voltage at the plate 450v and the other, negative millivolts,would this indicate a short in the transformer.
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Old 24th March 2005, 08:57 AM   #8
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A short? Probably not.
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Old 24th March 2005, 09:03 AM   #9
Giaime is offline Giaime  Italy
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If you have few millivolts on the plate, check all connections (sockets, wires, these things) between the plate and the output transformer. Check also by swapping tubes around. If the plate pin (with every tube) remains very low, you probably have an open transformer. You hear sound because the other half of the push pull (the other EL84 in that channel) is still working, but probably in AB class so the waveform is heavily clipped, and the sound is distorted. But the question is, how did you burnt that transformer?
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Old 24th March 2005, 09:05 AM   #10
Giaime is offline Giaime  Italy
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And what about that "pulsating" EL84? I think you have a burnt screen resistor that has burnt the screen grid in that tube. Or just lost bias, check decoupling caps to the EL84 grids.
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