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.
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.
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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?
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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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.
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.
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.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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"
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"
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
But my home SE 6V6 hifi amps run moderately at about 270V - just to show I'm not really that crazy all the time
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You abused the tube but even if you didn´t, 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.
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.
Most agree that the tube is operating near the meltdown zone at idle.
But only one pointed out the obvious.
Turn off the lights in the room and observe the output tube. Is there any glow coming from the screen grid wires at idle......Then plug in the bass dial it up to 11 and thump away for a while.....it there a glowing grid now? Does the glow coincide with the loud, or the silence?
A class A amp will burn the most power in the output tube during silence. It cools off when putting out power....but not if it's OPT is saturating
If you like the sound this way, just be prepared to replace the tube more often.
But only one pointed out the obvious.
Turn off the lights in the room and observe the output tube. Is there any glow coming from the screen grid wires at idle......Then plug in the bass dial it up to 11 and thump away for a while.....it there a glowing grid now? Does the glow coincide with the loud, or the silence?
A class A amp will burn the most power in the output tube during silence. It cools off when putting out power....but not if it's OPT is saturating
If you like the sound this way, just be prepared to replace the tube more often.
6V6dude,
Take a look at the plate of the one that broke; if it sagged you might be able to see that.
But it is more likely that the screen grid sagged, and you will not be able to see that, unless you take the tube apart very carefully.
As I suspected, you were willing to let the tube life shorten, to get the kind of sound you wanted. That is a valid design criteria for guitar amps.
A fisherman may use a very low test line in order to see if he can land the big fish by playing him. If the line breaks, he is willing to replace the line, and try playing the fish next time.
Take a look at the plate of the one that broke; if it sagged you might be able to see that.
But it is more likely that the screen grid sagged, and you will not be able to see that, unless you take the tube apart very carefully.
As I suspected, you were willing to let the tube life shorten, to get the kind of sound you wanted. That is a valid design criteria for guitar amps.
A fisherman may use a very low test line in order to see if he can land the big fish by playing him. If the line breaks, he is willing to replace the line, and try playing the fish next time.
Been there.....raced that. I planned on a teardown every year, but the usual suspects were OK (turbo itself, rings, bearings, valves and valve seals). The real wear items were the timing belt, alternator, and air conditioner clutch. None of these liked being run at 7200 RPM on a motor designed for 5500 max. I also killed a transaxle just about every year.
You might experiment with a resistor in series with the screen grid. I have found that it has little affect on the tone when the amp is NOT pushed into hard clipping, but a big effect on the tone at the edge, and into clipping. Also try wiring the preamp tube's B+ supply after all or part of this resistor, with a small bypass cap.....it's value affects sag and recovery time.
What works for you is all about how you play, and the sound that you like.
Relocated to Instruments & Amps, home to all things guitar related. 
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