Mesa F100 blowing single tube, in same socket, every time

[drumguy560]

Just like the title says, if I put in a new tube, it always blows in that exact same socket. It is always socket number 2. Once the tube blows, the tube doesn't light up anymore (no heater). Replacing the tube in that socket results in blowing that tube too, gets expensive haha!

Anyhow, anyone got any ideas on where to start?

Thanks in advance!

[Steve Conner]

It takes a lot to blow the heater in a tube. If it really was blowing, there would probably be sparks, flashes, nasty noises from the speakers.

I think you might just have a loose connection in socket no.2 that is cutting the heater supply to the tube. Try taking the "blown" tube out and putting the same one back in. Wiggle it around in the socket (with the amp in standby) and see if the heater lights up again. Also, take your heap of "blown" tubes and check if they light up when plugged in the other sockets.

On most modern amps the tube sockets are soldered to a circuit board. You might have a bad solder joint there. Mesa amps have a reputation for being complicated and hard to service, so I recommend leaving the repair to an experienced tech.

[Amp Kat]

Have you tried maybe taking a voltage reading of the heaters and finding out just what the hell is going on there. You either have an open or an extremely large voltage present there that shouldn't be.

[drumguy560]

Have you tried maybe taking a voltage reading of the heaters and finding out just what the hell is going on there. You either have an open or an extremely large voltage present there that shouldn't be.

I haven't. I'm quite sure there is a large voltage and current there though because the tube lights up like a flashlight when turned off standby, and then no more heater. So it seems to me like the heater circuit on that tube is grabbing too much current. Does that sound right? I just have no idea where all of this extra current is coming from.

[g1]

What is socket #2, a preamp tube or a power tube?
How about what Steve asked, after the tube dies, will it work in any other socket?

[drumguy560]

What is socket #2, a preamp tube or a power tube?
How about what Steve asked, after the tube dies, will it work in any other socket?

#2 is a power tube. The tube does not work in any other socket. It is blown blown.

[g1]

Very strange. Are the power tube heaters not wired in parallel? Any weirdness that gets to the heater should get to the heater on the other power tube.
Any evidence of arcing on the tube socket?

[Joe L]

So with good tubes, all filaments light up ok with HV off. As soon as you turn on HV, some element in the tube briefly glows bright and then blows.

So, HV is present where it is not supposed to be. If you are using 6L6s (please don't say you are using EL34s) that leaves only the grid, cathode or filament. The other tubes aren't doing it so that leaves the cathode and grid. I don't see how the grid could open up the filaments but I don't see how HV could be on the cathode which should be grounded.

Conclusion - the cathode is no longer connected to ground and is at HV potential which draws so much current from the filaments that they melt.

Or it might be aliens.

[Amp Kat]

Yes^^^ this but you have to get a multimeter man and find out what is going on. You can't just keep making assumptions on how to cure a problem without knowing what is going on. One simply check on the Cathode and heaters and at least you'll have a clue what is happening here.

[Steve Conner]

That's a good point. It could actually be the screen wires melting because the plate connection has gone bad.

[bob p]

how about posting a set of pin voltages for us?

[Justin Thomas]

Should we get those voltages without the power tubes installed in Standby, then check again with no tubes but in play mode? Maybe a short on the board from plate or screen to heater traces that doesn't show till Standby is switched on?

Justin

[Jazz P Bass]

At this point I would go tubeless.

[J M Fahey]

Mesa F100 blowing single tube, in same socket, every time

BEGIN by changing the socket for a new one .

Before and after, measure all pin voltages , **without tube** , no signal, and post them here.

Don't plug a new tube there until we see such voltages.

EDIT: and post the Mesa F100 schematic here.
Wouldn't surprise me that they use some weird standby method.

[Jazz P Bass]

Huh!
A carbon track on the socket.
Good call JMF.
It is certainly an idea worth exploring.