Valve Jr problems...possible bad tube?

[cortmarshall]

Well, I bought some JJ's from eurotubes for my Epi Valve Jr, and had them in for about a week, they sounded tons better than the stock sovteks, but a few days ago i turned it up some and suddenly the output dropped to about half, the bass disappeared, and it started crackling. I Tturned it off then back on and kept it at low volumes and it was fine(prolly a bad idea). Today it started doing it again, it cuts in and out between normal and half output and because of the crackling at first i thought it was the speaker so I took it apart again and tried the speaker with a different amp and it worked fine... So then i hooked it back up turned it on and looked at the tubes, which now glow, idk if this is normal or not the sovteks didnt but i havent looked at the JJ's till now. I was extremely careful putting them in, this was my first tube swap! and only handled them with a tissue because i heard fingerprints mess with them and bttr to be safe then sorry. Used contact cleaner on the pins per Eurotubes instructions ran them in and out etc. The el84 is what seems suspect to me though, it glows orange in the same places the preamp tube does, center section top and bottom, but red in some spots and theres even a slight blue glow in the very center when the problem occurs. It doesnt smell too good either, kinda like somethings burning. I have opened up the amp before and cleaned it out but i was very careful about what i touched and whatnot. Thanks in advance, and pics!

[cortmarshall]

Re: Valve Jr problems...possible bad tube?

First is under normal operation, second when the problem occurs. Notice the blueish purplish glow in the center and the side(Don't know what its called) glowing red.

Edit: Sorry for the slightly blurry pics. Cameras auto focus doesn't seem to agree with me.

[wixedmords]

Re: Valve Jr problems...possible bad tube?

CM,

Put the Sovtek EL84's back in and see if the issue continues.

If you don't have them, you'll need a backup set of tubes for troubleshooting issues. It is just about rule 1 with tube amps.

You can also move the El84's around from either set of tubes and find out if the odd glowing follows the tubes or stays with that socket.

Good luck.

[wixedmords]

Re: Valve Jr problems...possible bad tube?

Unintentional double post. Oddly I hit the button once.

[karpathion]

Re: Valve Jr problems...possible bad tube?

CM,

Put the Sovtek EL84's back in and see if the issue continues.

If you don't have them, you'll need a backup set of tubes for troubleshooting issues. It is just about rule 1 with tube amps.

You can also move the El84's around from either set of tubes and find out if the odd glowing follows the tubes or stays with that socket.

Good luck.

+1,000.

Technically, you could test the voltage drop across the filament in the tube. If it doesn't show 6.3V across it "or close", it is bad. But, I may be wrong.
I just wouldn't go sticking my hands inside of a hot amp. It wouldn't end well.

But then, I'm only up to diodes, transistors, and FET devices in my studies. I wouldn't know, I'm still a rookie.

[glassman]

Re: Valve Jr problems...possible bad tube?

Being a single ended amp, you only have one EL84 to work with. The condition you describe is called "redplating"...simply put, the tube is over dissipating. In a single ended amp such as yours the issue is typically a bad tube; a short between the control grid and the cathode will cause the amp to sound as you describe and can be intermittent as well. The cause is usually mechanical within the tube...as in caused by rough handling. EL84's are rather fragile tubes and the grid wires can deform with a significant impact.

You need to replace that tube. The tube vendor may replace it if it's a recent purchase; just describe what it's doing to them.

[cortmarshall]

Re: Valve Jr problems...possible bad tube?

Ok great information. so it's nothing with the amp. Just the tube. That's good. I'll contact Eurotubes, and be more careful next time. Thanks