Tubes glowing question?

[Jeff Hitman]

Got a 50 watt custom built (basically) Marshall Plexi head, uses a pair of EL34's. Looking in the grill when playing I see that as I pick a string ONE of the power tubes glows brite red/orange around the filament while the other one stays looking normal, just a constant low glow. Is this a potential problem? If so, does it sound like a bad tube or would a new bias setup rectify the problem?
Thanks,
-J

 

[WailinGuy]

Probably what you see glowing is the screen grid. It is the second grid from the cathode. (Technically, the filiment is stuffed INSIDE the cathode, which is that long skinny cylinder in the center of the tube that glows orange.) The grid closest to the cathode (which spirals around it fairly tightly) is the control grid. There is a third grid in EL34s called the suppressor grid. It is a loose spiral wire that's closest to the plate (large grey metal structure enclosing everything.)

Screen grids aren't supposed to glow under normal conditions (perhaps a small glow at max. output power is acceptable.) The fact that the screen is glowing in one tube and not the other would indicate that tube is bad and/or suffers from misaligned grids. This type of problem is not uncommon, especially with EL34s. The only other possibility (which doesn't seem likely) is that the amp's builder put a screen resistor (which acts to limit screen grid current) on only one of the output tube sockets and not the other (the one with the glowing tube).

 

[WailinGuy]

Also... try this:

Swap the two output tubes. If the glowing screen grid follows the tube, then you know it's a tube problem and not something that's going on inside the amp itself. (My guess is that it WILL follow the tube; let's see if I'm right...)

 

[Jeff Hitman]

Originally posted by Jim Salman
Also... try this:

Swap the two output tubes. If the glowing screen grid follows the tube, then you know it's a tube problem and not something that's going on inside the amp itself. (My guess is that it WILL follow the tube; let's see if I'm right...)

Thanks! Practice tonite, I'll try it out.
-J

 

[Jeff West]

Jeff- If it's happening on the power tube fed from the top of the PI, i.e., the side that has the 82K not 100K plate R on the PI, the EL34 furthest in from the end of the chassis assuming you wired it up like Marshall, then I suspect it likely may not follow the tube swap. Plus, even much colder bias won't stop it. OTOH in that case if you swap the feeds from the PI then it will track that switch. Marshalls are notorious for doing this in many cases even when sounding and operating great, especially w/EL34s, there's some controversy about explanation/implications.

Jeff W.

 

[Jeff Hitman]

OK - it does it even when I change tubes, it's the position, not the tube. Should I be concerned about imminent failure? Is this something that could lead to problems? I need to be able to rely on this for gigs without wondering...

thanks,
-J