richard parker
Active member
One of the two EL34s in my Cornford is redplating. Could this simply be a tube gone bad ?
Redplating
- Thread starter richard parker
- Start date
Edgecrusher
Well-known member
Re: Redplating
Probably if it was a voltage problem you should see it in all the powertubes. But by red plating you mean that the actual plates inside are glowing molten? Not just the filaments but the actual plates. Does the amp sound any different?
Probably if it was a voltage problem you should see it in all the powertubes. But by red plating you mean that the actual plates inside are glowing molten? Not just the filaments but the actual plates. Does the amp sound any different?
guitfiddle
firstlessonologist
Re: Redplating
Switch the two tubes and see if the red plating stays in the same tube, or in the same socket. If it moves with the tube, then it's just a bad tube. If the other tube does it in the same socket, then there's a problem in that socket.
Switch the two tubes and see if the red plating stays in the same tube, or in the same socket. If it moves with the tube, then it's just a bad tube. If the other tube does it in the same socket, then there's a problem in that socket.
Chickenwings
Alnico 6/8
Re: Redplating
a toasted screen resistor would be my first guess.
a toasted screen resistor would be my first guess.
MetalManiac
Li'l Junior Member
Re: Redplating
yYou could switch the tubes , but you might burn another tube. Redplating can cook a tube in short order ( get it..'short order cook'..lolz). Take it to a qualified tech ; (qualified being the operative word) it needs service. A screen grid resistor could be bad, but could be be other things as well.
I recapped a 2xEL84 amp, and it was redplating, and so having met one of the finest techs I have ever known, He serviced the amp. Heres what was wrong
1) Replaced rectifier diodes including single diode in bias supply
2) Caps in bias supply I had reversed! Negative side of the cap goes to voltage, positive side goes to ground. This will cause these caps to smoke and the amp won't ever be able to bias up or work correctly.
3) Rewired the power supply - rerouted and replaced wires as needed and ended up rebuilding most of this. It had a couple of shorts and "miswires" that were corrected.
4) black wire on power cord was frayed, I spliced in a new black wire and resoldered it to the circuit breaker part of the amp.
The biggest problem was noticing was the heavy handed soldering I did Some of the solder blobs were so big that they were shorting out where they shouldn't be and in some spots, the wire insulation was burned so bad the inside wire was exposed. I always replace badly burned wires and did so wherever needed in this amp (or I splice in a good piece of wire if possible). Just need to be more careful with the soldering iron, and pick your angle of attack carefully and not be afraid to move wires as needed to get safely into a spot without nicking the wire. Even if it means soldering left handed, need find a way to get in there without burning the wire insulation!
So you see, you dont just assume its a fried screen grid resistor.
yYou could switch the tubes , but you might burn another tube. Redplating can cook a tube in short order ( get it..'short order cook'..lolz). Take it to a qualified tech ; (qualified being the operative word) it needs service. A screen grid resistor could be bad, but could be be other things as well.
I recapped a 2xEL84 amp, and it was redplating, and so having met one of the finest techs I have ever known, He serviced the amp. Heres what was wrong
1) Replaced rectifier diodes including single diode in bias supply
2) Caps in bias supply I had reversed! Negative side of the cap goes to voltage, positive side goes to ground. This will cause these caps to smoke and the amp won't ever be able to bias up or work correctly.
3) Rewired the power supply - rerouted and replaced wires as needed and ended up rebuilding most of this. It had a couple of shorts and "miswires" that were corrected.
4) black wire on power cord was frayed, I spliced in a new black wire and resoldered it to the circuit breaker part of the amp.
The biggest problem was noticing was the heavy handed soldering I did Some of the solder blobs were so big that they were shorting out where they shouldn't be and in some spots, the wire insulation was burned so bad the inside wire was exposed. I always replace badly burned wires and did so wherever needed in this amp (or I splice in a good piece of wire if possible). Just need to be more careful with the soldering iron, and pick your angle of attack carefully and not be afraid to move wires as needed to get safely into a spot without nicking the wire. Even if it means soldering left handed, need find a way to get in there without burning the wire insulation!
So you see, you dont just assume its a fried screen grid resistor.
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