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pro junior bad screen resistor

12/8/2005 6:45 PM
pro junior bad screen resistor
Maybe someone here will know this info?? I have a Fender Pro Junior amp that is malfunctioning. One of the EL-84's keeps glowing red. I have swapped in different tubes and get the same result so I'm sure its not a bum tube. It's on tube position V 3. A local tech says it's most likely a screen(grid?) resistor gone bad. He can't get to fixing until next week. So I'm thinking I could soldier in a lousy resistor if I knew which one was the offender. Going to that tube I see R20 is a 1.5k resistor. It this it?? Thanks!!
12/8/2005 6:35 PM
When you "soldier" in that lousy resistor, make sure you have discharged the filter caps...  
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12/8/2005 6:58 PM

THANKS and I will....I can solder but can't SPELL!
12/9/2005 9:29 AM
Wild Bill

Emerson, a screen resistor would not have been my first guess. The tube should not overheat at idle current/no signal even if the screen resistor was totally shorted. Screen resistors don't even have to be there at all! They are simply a protective device, in that by using them as the plate current goes up there's a voltage drop across a screen resistor. This lowers the tube gain and makes it less inclined to work too hard.  
I'd replace the coupling capacitor feeding the problem EL84 socket.  
It's most likely a lack of bias problem that makes a tube glow red. Since the other tube is fine the bias supply must be ok. That leaves either a bad connection in the path that feeds the bias voltage to the offending grid or most often, the cap coming from the driving stage is shorted or leaky. This lets the B+ from the previous stage get on the following grid, overcoming the negative bias voltage and driving the grid highly positive. The tube is now jammed full on and begins to burn out!  
Check the voltage at that grid pin to make sure it hasn't gone positive. Or just replace the couplers - it's cheap insurance!  
If the tube is fine with no signal and gets red as you crank the volume up there are other likely causes but you need to eliminate this possible culprit first before we can go on.  
---Wild Bill
12/9/2005 1:24 PM

Wild Bill,  
Thanks very much for your response! It seems this is very much over my head, as well as possibly fatal! I guess I'll take it to the tech and look over his shoulder. Thanks....
12/9/2005 6:05 PM

Perhaps when the guy said screen grid, he had in mind the control grid resistor through which the bias voltage comes? If that resistor opens, no bias. DOn't see that very often, and a bad cap is more likely.  
A burnt out screen grid resistor will turn the tube OFF, not on harder.
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