Help! My EL34B based amp arced at full volume.

jerrynsr · 26th August 2012, 04:01 PM

My 2 year old EL34B based amp arced on the right channel for the first time. I was measuring the db output with my little dbmeter I was about 10 feet away. I heard one static crackle, one of the EL34B tubes on the right channel arced between two pins in the front of the tube. I was not close enough to see what pins they were. But close enough to see the arc then the bottom of the EL34B glow brightly for a second. I was close enough to shut down right away. I am driving 4 ohms speakers off the 4 ohm taps. Everything was sounding great at 90db. After two minutes I turned the amp on again and listened at 30% volume. Sound is fine, bias voltage is fine on all 4 EL34B tubes. I listened for 30 minutes, no arcs and the EL34B apperars to be operating normally. I have a 1/16" gap between the bottom of the final output tubes and the sockets. Should I force the tubes to be flush with the sockets ?

Have an of you seen this before ? I used to see this when a cap is failing on an old tube radio, but when it started arcing it didn't stop until I replaced a dead capacitor and rectifier tube. My 55w EL34B amp stopped arcing as soon I lowered the output volume and input signal level.

DF96 · 26th August 2012, 04:45 PM

Valves should usually be fully inserted into their sockets, to ensure good contact. It is possible that you have seen the effect of parasitic oscillation.

jerrynsr · 27th August 2012, 02:32 AM

I've been listening to tunes through my el34 based amp with the wife. Volume is at 30%
. Its been 1hr now and no problems. It still sounds great!

26th August 2012, 04:01 PM	#1
jerrynsr diyAudio Member Join Date: Sep 2011	Help! My EL34B based amp arced at full volume. My 2 year old EL34B based amp arced on the right channel for the first time. I was measuring the db output with my little dbmeter I was about 10 feet away. I heard one static crackle, one of the EL34B tubes on the right channel arced between two pins in the front of the tube. I was not close enough to see what pins they were. But close enough to see the arc then the bottom of the EL34B glow brightly for a second. I was close enough to shut down right away. I am driving 4 ohms speakers off the 4 ohm taps. Everything was sounding great at 90db. After two minutes I turned the amp on again and listened at 30% volume. Sound is fine, bias voltage is fine on all 4 EL34B tubes. I listened for 30 minutes, no arcs and the EL34B apperars to be operating normally. I have a 1/16" gap between the bottom of the final output tubes and the sockets. Should I force the tubes to be flush with the sockets ? Have an of you seen this before ? I used to see this when a cap is failing on an old tube radio, but when it started arcing it didn't stop until I replaced a dead capacitor and rectifier tube. My 55w EL34B amp stopped arcing as soon I lowered the output volume and input signal level.

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26th August 2012, 04:45 PM	#2
DF96 diyAudio Member Join Date: May 2007	Valves should usually be fully inserted into their sockets, to ensure good contact. It is possible that you have seen the effect of parasitic oscillation.

27th August 2012, 02:32 AM	#3
jerrynsr diyAudio Member Join Date: Sep 2011	I've been listening to tunes through my el34 based amp with the wife. Volume is at 30% . Its been 1hr now and no problems. It still sounds great!