Friend smoked champ

peteb

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The screen sees limited current, limited by the screen itself.

the cathode sees ten times the current.

I still think the problem is going to be at the cathode of the 6V6.
 

tubedude

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The screen sees limited current, limited by the screen itself.

the cathode sees ten times the current.

I still think the problem is going to be at the cathode of the 6V6.
True, but the plate is designed to carry ten times more current than the screen grid. The plate is a solid tube of sheet metal. The screen grid is a mesh of thin wire. Many tube failures are the screen grid failing.
 

peteb

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True, but the plate is designed to carry ten times more current than the screen grid. The plate is a solid tube of sheet metal. The screen grid is a mesh of thin wire. Many tube failures are the screen grid failing.

I know sometimes the screen resistor has fuse like functions, but the delicate and diminutive nature of the screen makes the screen itself vulnerable, however, I am looking for the component that smoked the inside of the chassis. And to my knowledge the amp was functioning. If the screen blows, I think that you are going to hear the difference. The results were somewhat predictable. And from what I see, it is surprising the amp was functioning up until it shut off.




here are pictures of the amp. It is my amp. My friend was playing it. He did a good job of recognizing a problem and turning it off.


can you see the problem?
 

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Maguchi

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I was jamming last night with a friend. We have been regular jamming partners for about five years. I really like jamming with him, he is a much more qualified musician than myself, but it is really hard on my gear. Usually, he travels, it’s only a mile, and I provide an amp. For some reason he is really hard on my gear. Over the years it’s always the amp he is playing that blows a fuse, a tube, or in this case the amp started smoking. I have never smoked an amp.

he said he noticed a problem all night. I plug straight in and he uses a pedal board. At the end of the night the amp was acting up, but it was still playing.

my friend saw a small amount of smoke coming out of Jack 2 and shut the champ off. I have not looked inside yet. It should be pretty obvious what was smoking. The under sized fuse, 0.5 amps, did not blow. I felt the trannies and they were both cool. i am sure the power tranny was cool because I turned the amps off for dinner. He is an actual musician and he thinks the amps should stay on, even old amps, during extended breaks. I turn amps off during long breaks. Vox says don’t leave an AC running with no signal.


what smoked? My friend thought it must be near the input because that is where the smoke came out. I think that is the only place it could come out.


all I can think of is the cathode resister fried.
Are you asking for a friend?
 

tubedude

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I know sometimes the screen resistor has fuse like functions, but the delicate and diminutive nature of the screen makes the screen itself vulnerable, however, I am looking for the component that smoked the inside of the chassis. And to my knowledge the amp was functioning. If the screen blows, I think that you are going to hear the difference. The results were somewhat predictable. And from what I see, it is surprising the amp was functioning up until it shut off.
So that's the smoke stain on the upper lip of the chassis? What does it look like under the lip? Is the volume pot smoky?
 

DugT

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I know sometimes the screen resistor has fuse like functions, but the delicate and diminutive nature of the screen makes the screen itself vulnerable, however, I am looking for the component that smoked the inside of the chassis. And to my knowledge the amp was functioning. If the screen blows, I think that you are going to hear the difference. The results were somewhat predictable. And from what I see, it is surprising the amp was functioning up until it shut off.




here are pictures of the amp. It is my amp. My friend was playing it. He did a good job of recognizing a problem and turning it off.


can you see the problem?

1641776788584.png


This looks like a capacitor blown to smitherines to me. Did I win?
 

tubedude

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What is the fuzzy yellow stuff on that wire, was it cap innards? I can't get a higher resolution image. If I click on it I can't get it on my phones screen. Looks like part of a resistor burnt also, sorta.
 

James Knox

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What is the fuzzy yellow stuff on that wire, was it cap innards? I can't get a higher resolution image. If I click on it I can't get it on my phones screen. Looks like part of a resistor burnt also, sorta.
Yes, cap innards. That is what the inside of a Sprague cap looks like. I know what it looks like cuz on one of my first builds I put the cathode cap in backwards, lol. Kaboom on startup!
 

peteb

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I can see in the picture that the cap is a 25 25 where a 50 volt cap would be more appropriate. That is part of the problem.

Looks like part of a resistor burnt also, sorta.

I can’t see that, but I believe it to be true.

the cathode resistor was recently measured at or near the stock 470 value. It now measures 300 ohms. That is the original carbon comp 1 watt Resistor. My other old champ had the cathode resistor replaced with Gray (like my screen resistor) resistor that must be higher wattage.

0.045 amps times 23 volts is 0.92 watts. Those stock resistors were under spec’ed.

I know the resistor was hot. The cathodes resistor on class A will always be hot. But I don’t think the heat of the resistor got the cap. The cap was a good distance away.

i Think the voltage got the cap.


now I can try an unbiased cathode, which I was interested in trying and hearing.

I think we already heard the unbypassed cathode. It may be that I already heard that losing the bypass cap does not really affect volume that much. We shall see.
 

tubedude

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No cathode bypass capacitor means poor attack and lower response.
I always fit 470R 3W MO ( https://uk.rs-online.com/web/p/through-hole-resistors/2142724/ ) and 25uF 63volt 105°C capacitors, ( https://uk.rs-online.com/web/p/aluminium-capacitors/7076439/ ) purely to avoid this issue.
In another light, the "poor attack" can also mean smooth compressed response, or something another player might desire. Frequency response will be wider without the cap, but signal will be lower, requiring more drive to get the same volume. Install the cap, short it with a clip lead and see what you prefer. Or put it on a switch and change it depending on the feel/response needed.
 

peteb

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Thanks tube dude.


that is a good description Of what to expect.

I don’t think you mean to short the cap? That would short the resistor.


I am going to build it unbypassed And compare it to my bypassed champ. Probably a good chance to use the ABY box.
 

Axis29

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An amp that can’t handle being dimed was either poorly designed or has something wrong with it. Especially if he was just playing guitars through it and not using some kind of pedal that self oscillates or a synth or something that’s going to hit it with a sound wave that it’s not expecting.
The amp handled it fine. It was the speakers. Weber P10Q's are not rated for that much juice. They were the speakers that were in it when I bought it... The same ones mentioned in the ToneReport article about the Vibrolux Reverb.

Anyway, did not mean to derail the thread.
 

cyclopean

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That sounds like poor product design to me. The amp either shouldn’t put out that much volume or the speakers should be able to handle it.

That’s like a stove that you can turn up to seven, but when you hit eight, it catches fire.
The amp handled it fine. It was the speakers. Weber P10Q's are not rated for that much juice. They were the speakers that were in it when I bought it... The same ones mentioned in the ToneReport article about the Vibrolux Reverb.

Anyway, did not mean to derail the thread.
 

Axis29

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That sounds like poor product design to me. The amp either shouldn’t put out that much volume or the speakers should be able to handle it.

That’s like a stove that you can turn up to seven, but when you hit eight, it catches fire.

Actually, it has nothing to do with product design... It had to do with modification after the fact.

The parts that failed were NOT stock parts.

The speakers I replaced the blown Webers with handled everything I ever threw at the amp after that.
 

cyclopean

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Actually, it has nothing to do with product design... It had to do with modification after the fact.

The parts that failed were NOT stock parts.

The speakers I replaced the blown Webers with handled everything I ever threw at the amp after that.
Ok fair enough. If the stock speakers died that would be something different.
 




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