Orange OD120 Slowly Eating Power Tubes

[nateflanigan]

A few years ago I picked a 90's Orange OD120 at GC on the cheap because it was broken.  It turned out that one of the power tubes was toast, it wasn't drawing any bias current.  I put in a new set of tubes, biased it and swapped out the existing MV for a LARMAR.  I used it for about a year or so in my band with no issues, then it sat on a shelf for about a year.  I loaned it to to a friend last weekend and lo and behold, tubes were red plating amp shut down.  So, I opened it up, and checked the bias voltages, one set of tubes was reading -26v the other -3.  I pulled out the two that were reading -3 and moved one of the good tubes over to the other side.  Now both are reading -35v.


So, the question is, why is the amp slowly eating tubes?  It's not a problem that manifests itself right away, nor does it take so long that it would be the tubes dying of natural causes.  This set has roughly 100 hours I'd guess.


Any ideas what I should be looking at?

[PRR]

Tubes probably don't affect bias.

I would look for incidental issues, such as a solder joint that goes bad over time but is temporarily made good when you wiggle/remove tubes.

[nateflanigan]

Thank you PRR,
I've isolated it down to one bad tube.  If i plug it in on either of the sockets on the left, pin 5 on both tubes on that side read -2v.  If I move it over to the right side the problem moves with it.  I've moved it back and forth several times so I'm fairly certain the problem is following the tube and I'm not just activating a cold solder joint on each socket every time i move the tube.


I guess my line of questioning is...


- What problem has occurred in a tube to make it read less negative bias voltage?  I hope I've worded that right, the symptom is plain to see but I'm not sure how to technically describe it.


-What would cause that problem to occur?


Touching up all the solder joints in the bias circuit is pretty easy, and I suppose I should verify the values of the bias feed resistors.  That seems like a good start, I'd just like to understand what's happening if I can.


 

[sluckey]

Check for some resistance reading between pin 5 and pin 8, or between pin 5 and any other pin.

[HotBluePlates]

- What problem has occurred in a tube to make it read less negative bias voltage? 

Check what Sluckey said first.

If you find nothing, an extremely gassy tube could have grid current which reduces the value of bias voltage at the grid.

[nateflanigan]

Ok, so I'm not reading resistance between pin 5 and any other pin on the bad tube.  To test it a little further I plugged the bad tube into a socket and compared resistance measurements to a socketed good tube.  All the readings were the same.


I also noticed that the logos on all of the tubes have been blackened a bit from heat.

[sluckey]

Sometimes a bad tube is just that... a bad tube. Not necessarily anything in your amp that's 'eating' tubes. Tube failure is probably the #1 problem in tube circuits. Applying power to a tube circuit will eventually cause the tubes to die. 

[nateflanigan]

Right, it could be that simple.  It seems fishy though, being that when I bought the amp it was experiencing what seems to be the exact same issue.  I wish I could remember exactly what order things happened in when i first sorted out the amp.  The way the pcb is set up there are two pads that feed the bias voltage to the first tube in each set, then there's a wire connecting pin 5 of the first tube to pin 5 of the second.  At some point in my first run of trouble shooting that wire connecting the grids burnt up.  I replaced it, put in a fresh set of tubes and all was well.  The bad tube this time around did not start out on the same side that has the replaced wire.