Loose EL84 cathode

[tdlunsfo]

This is on a Epiphone valve standard. The symptoms were a very touch sensitive chassis. Opened her up and found a lot of heat damage around the EL84’s, bad solder joints, etc. Not finding the smoking gun, I touched up all the bad solder joints and fired it up. Same problem. Checked the tubes.. all good. Flipped the amp over and all of a sudden the noise stops but the tubes start red plating (likely was doing this before but not while on the dim bulb tester). I checked the bias and it climbing into the 70 ma range. I went back and looked at some pics I took and noticed something touching the getter of one of the EL84’s, pulled the tube and turned it upside down and the cathode drops through the tube and hits the getter (explains why It tested fine on the tube tester). This is the cause of the noise but my question is does it also explain the runaway on the tubes? I have never experienced this one before and was wondering if anyone else has. I plan to throw a new set of EL84’s in it tomorrow to verify, utilizing my bias tester.. of course. Thoughts?

[tdlunsfo]

Checked the tubes again this morning and they are gone. My theory is the loose cathode caused the other tube to take the load and runaway.

[Enzo]

I would think not so much took the load as maybe the short in one tube killed the bias to both tubes.

[tdlunsfo]

I thought about that as well. Looking forward to trying the new tubes tonight to see. I must say though..that was tricky. Never seen that before but then again I've only been doing this a few years.

[g1]

I have a lovely unused NOS GE6550 (groove tube). The getter halo is broken loose and rattling around in the tube.
I can't force myself to throw it out as I know it would work fine in an upside down chassis that doesn't get much vibration.

[Justin Thomas]

Funny, every time I get a really "nice" old tube with that orange paint in an amp, that particular tube is the first one I need to toss due to loose parts, audible noises, horrible microeconomics, or it starts redplating...

Justin

[tdlunsfo]

Well I got the new tubes in and they are also redplatting. Bias probe is around 48ma per tube. 370v plate to cathode and 13v cathode to ground. I am at 18w a tube and 158% dissapation. Surely Epiphone didn't let it leave the factory like this... I don't have voltages for this amp but they don't seem unreasonable. The 120ohm cathode resistor may be a little low? Cathode Cap tests good.

[g1]

Do you have 0V at the power tube grids?

[Jazz P Bass]

Have to wonder what the difference is between the Valve Standard & The Valve Senior. (voltages are absent on either schematic)

The Valve Senior uses a 250 ohm/ 10 watt cathode resistor.

[tdlunsfo]

30mv on pin 2 control grid on both tubes. Moves around a little.

[tdlunsfo]

Valve Standard shows a 120ohm 10W resistor. I may play with some different values. My calculations show I need the 250ohm.

[tdlunsfo]

Looking at this some more and it appears I need likely need to lower my B+ to the EL84's. 380v seems a little high.

[The Dude]

Remember to subtract your cathode voltage from plate voltage to get actual voltage across the tube (plate to cathode). If you indeed go with a 250 ohm cathode resistor, your cathode voltage will go up and you will be in safer range for plate voltage. Of course, your plate voltage may rise slightly with less conduction, but I wouldn't worry about plate voltage until you get tube current in a reasonable range.

[tdlunsfo]

Ok. I threw in a 300ohm resistor and the bias is down in the 25ma range and about 10 watts per tube. Probably could actually stand to lower it a little to get the 12 watts. They are happy now but its still puzzles me why Epiphone would send the amp out with the tubes so hot. Only thing I can figure is they originally had 6P14p's in it. I can't read any tube markings on the originals so not entirely sure. It did raise the cathode to plate voltage to about 384 but the bias voltage went up to 16v as well. Alas, i think it is good to go now.