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Sudden tube failure?
Sudden tube failure?
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Old 29th January 2018, 12:47 AM   #1
6V6dude is offline 6V6dude  Australia
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Default Sudden tube failure?

My first ever tube failure, I wonder what happened to the tube.
Single ended EL84, 280V on grid and 310V on anode. I turn the amp on but get no sound and after a while smoke comes out.
I've found the tube to be faulty, on power up the grid dropped down to 30V, burning the drop down resistor before it. Changed tube and all is well.
But what could have happened to the tube to develop this load so suddenly? It's an amp head that never gets moved, no vibrations, nothing. I am baffled.

Last edited by 6V6dude; 29th January 2018 at 12:53 AM.
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Old 29th January 2018, 02:30 AM   #2
FullRangeMan is offline FullRangeMan  Brazil
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You never knows what a tube suffer before it came to your safe hands.
Your tube is inexpensive, now imagine you how a expensive tube amp owner feels in this situation.

Some current production tubes as 300B 211 845 etc can cost 1000USD each or even more and usually they dont last much some 1000 hours, 1 dollar per hour, 2 per Stereo.

How much hours this tube had run in your amp?
What was the brand?
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Last edited by FullRangeMan; 29th January 2018 at 02:33 AM.
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Old 29th January 2018, 04:13 AM   #3
VaNarn is offline VaNarn  Australia
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Sudden tube failure?
Running the EL84 as a Class A pentode would be partially determined by the O.P. transformer impedance. The plate/anode supply voltage would typically be 250 V and the screen grid ( G2) between 210 to 250 V. An internal short is the most likely cause of the failure, especially if the EL84 is running at its maximum dissipation rating or higher. The grid bias (G1) should be around -6.5 to -7.5 v.
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Old 30th January 2018, 02:46 AM   #4
6V6dude is offline 6V6dude  Australia
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Well I am running it pretty hot as you can see from the readings. It also has 15ohm OT running 4 ohm Jensen speaker. It works hard but makes for great sounding bass amp for recording. I've build this few years ego as a guitar amp but one day plugged bass in it and since than it's only used for that.
I think the tube was Mullard but markings are worn off so I'm not 100% sure. But it's not as thick as Sovtek so it's not a modern tube. I've replaced it with Sovtek now and still sound the same so I'm happy with that.
But yes, maybe the old tube shorted from the excessive heat it runs at.
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Old 30th January 2018, 03:29 AM   #5
FullRangeMan is offline FullRangeMan  Brazil
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You abused the tube
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Old 30th January 2018, 03:41 AM   #6
20to20 is offline 20to20  United States
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Quote:
Originally Posted by 6V6dude View Post
My first ever tube failure, I wonder what happened to the tube.
It has a 15ohm OT running 4 ohm Jensen speaker. I am baffled.
..... me too.
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Old 30th January 2018, 07:47 AM   #7
6A3sUMMER is offline 6A3sUMMER  United States
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Agreed, the tube is being grossly overloaded. And heat causes grid wires to sag (and short)

The 150 Ohm cathode resistor has 8.55V across it: that is 57mA.
The plate current plus screen current is 57mA.
308V-8.55V = 299.45V.
True the screen does not have 308V, is a little bit lower at 286V;
But let us look at the approximate plate dissipation plus screen dissipation:
299V x 0.057A = 17Watts. That is there even before there is a guitar signal present.
The EL84 plate is rated for 12Watts, the screen is rated for 2Watts (14Watts total).

The rated maximum current rating for the tube is 65mA.
The output transformer is very small, more suited for a lead guitar. It may have
low primary inductance, and the small core may saturate, so it would be a more difficult load at bass frequencies.
And the 4 Ohm speaker on the 16 Ohm tap is a very severe load.
If you play a loud note with such a heavy load, the tube will have a much higher
average current than 65mA. And it will be (perhaps nicely) distorted.
Therefore it sounds good, but will not last long this way.

Guitar amps are designed according to sound good as an instrument amp; and to give certain sound effects. But to get that sound, they often break the maximum ratings of the tubes (and output transformer current versus frequency), just like this guitar amp.

Hi fi amps are more generally designed for accurate sound, and reliability (or at least hopefully so).

"All generalizations have exceptions"
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Old 30th January 2018, 10:13 AM   #8
6V6dude is offline 6V6dude  Australia
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I agree with all you've said. It is indeed overloaded in every way to the limit of what the tube and OT can take. But I designed it to for the sound I'm getting from it. No way I'd take this on the road, but for my purpose running it for an hour at a time at home it's fine. Well, it's been running for several years till the tube shorted, as mentioned possibly because the plates sagged. That's a great theory actually. I wonder if the Sovtek can take it as long as the old tube did.
But my home SE 6V6 hifi amps run moderately at about 270V - just to show I'm not really that crazy all the time :-)

Last edited by 6V6dude; 30th January 2018 at 10:16 AM.
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Old 30th January 2018, 10:22 AM   #9
JMFahey is offline JMFahey  Argentina
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You abused the tube but even if you didnīt, parts wear even under normal use, tubes specially.
Notice a tube is the only major component which is not soldered in place but is plug-in instead, that alone should speak volumes.
And *everything* has a "service life" which is statistically determined.
Everything eventually fails.
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Old 30th January 2018, 11:05 AM   #10
JonSnell Electronic is online now JonSnell Electronic  United Kingdom
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8.55 volts on the cathode is a bit too low for my liking.
I would replace the 150R cathode resistor with 180R. Keep the dissipation down a little.
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