Fuchs Mantis on the bench

[pompeiisneaks]

So I'm working on a guys amp, Fuchs Mantis, pretty cool looker inside, but I'm wondering what's what. 

He blew a fuse, so he replaced it, and when he turned on the amp, he saw arcing in a tube.  I got it and quickly powered it up to see which tube was arcing, and swapped it, and it seemed fine initially, so I suspected a tube.  I then started trying to bias it, the tubes have a nice 1 ohm resistor, all EL34 tubes with 2.2k screen resistors, etc.  Pretty typical setup, but when I setup the probes, the first half of the thing seemed pretty hot:

V1: 67.9 mA
V2: 63.9 mA

but then when I got to the second half, one was about there too, but the new tube in that same socket just was going into r unaway
V3: 72mA, 74 mA 77mA and then I shut it off

V4: 65.5 mA

Now I'm suspicious that the bias is just way too hot and the one tube in that socket conducts better than the others so it's starting to go into runaway, but I'm curious if anyone else has suggestions as to why just that one tube seems to be doing so? 

I'm sure at that point, I'm going to need to dial down the bias anyway, becuase at 70% don't I need probably around 40 to 45 mA (depending on the plate voltage I know, I didn't get htat recorded before I blew a fuse).

I was about to get some plate voltage etc and see if a different tube would go into runaway before I blew another fuse, so I never got a plate voltage reading.  From what I understand in my googling, Fuchs are a company that does dumble clones, so this is likely similar to a dumble but with 4xEL34 instead of the fendery 6L6 setup, so I'm guessing voltages are probably in that 350 to 400 range, but I'll get that measured w hen my order of fuses arrives (Getting me some of the suggested resettable fuses Sluckey mentioned to me a while back from tedweber)

[shooter]

Since the tubes are in pairs, I typically replace pairs, since the "2nd" one probably got beat on pretty bad.

Also swapping pairs to see if it follows the tubes or stays with the sockets comes in handy.

[pompeiisneaks]

Thanks I'll give that a try when I get the fuses.  I know the issue stayed with the socket as I swapped one tube out and put one of my own in, it just kept climbing in cathode current while I watched (the other one in that half was pretty rock solid.) but I don't yet know if it's something in the socket itself, or something with the high bias and the new tube was like the old.

~Phil

[Ritchie200]

Not sure about amp and topography, but have you checked screen grid and input grid resistors?

Jim

[pompeiisneaks]

I did check them on this tube, its 2.2k for the screen and I think the input was the same, both seemed identical to the others in the area.  (Going from memory though).

[pompeiisneaks]

As closure to this one, it was the bias, I shut it down to half power so I was using only two tubes, and it was still sitting near 70mA.  I dialed it down after shutting it off, and brought it back on again, current was at shutoff now, with 0mA so I slowly brought it up to where it was good for the setup, and left it at about 36mA and the customer loves it.  No idea who thought running at that high of a current was good for the tubes :P

~Phil