Bad 12AT7?

[Dr. Toobz]

I've hardly used my Stereomour since building it (and changing a few parts to use 45 tubes), but recently, I have developed a problem that I think is just a bad tube, but I'd like an opinion. After few minutes of play, I start to get loud, random squealing, static and popping sounds in both channels that does not vary with volume or changing the source selector switch. The sound is equal in both channels. Replacing the output tubes has no effect at all. However, running the amp with the 12AT7 removed from its socket solves the problem. I've double checked my socket wiring under the driver tube and everything looks ok, and there's no hum or noise to otherwise suggest a bad joint somewhere. Re-seating the driver tube sometimes makes the problem go away, but my theory is that the lack of noise really has to do with the tube having cooled down again vs. being reseated. Could it be that I have a dud 12AT7, with only 10 hours on it? This wouldn't be the first bad EH tube I've run across.....

Sadly, I have no other 12AT7's in the house right now, and no 12AU7s to drop in, either, as my Crack uses 6CG7's instead.

[Paul Birkeland]

The sound is equal in both channels.

This usually suggests a problem that is not the tube itself, but rather something awry that is joint to both channels (like the power supply).

When you get this noise, is the 12AT7 still glowing properly? 

Do any of the LED's go out when you get this noise?  How are the voltages? (in particular, the voltages on the metal tabs of the large transistors on the C4S board)

[Dr. Toobz]

The 12AT7 glows and heats as normal. I did check the LED's on the C4S board, and they light up as usual. I'll have to dig out my meter later to provide voltages.

Perhaps I need to go back over the C4S boards and re-flow the joints? My thinking is that if the problem were earlier in the circuit, such as on the main power supply board that provides the B+ to both the 45's and 12AT7's, I would hear problems with just the 45's plugged in as well, correct?

[Doc B.]

Sounds like a tube pin contact problem to me. It would be worth giving the 12AT7 pins a good cleaning to see if that helps. Also check the grid stopper resistors attached to the socket to be sure the leads haven't come loose in the body of the resistor.

[Dr. Toobz]

Good call - I think that may have been the problem. I've played the amp for an hour now with no noise or issues - all I did was re-seat the tube. It's possible that when moving the amp, the tube may have popped out a bit, which is probably made worse by the fact I have a tube shield base on that socket for both cosmetic and practical purposes (my last residence had RF issues). EH tubes seem to have thick diameters, and have trouble fitting in the shields, so I can only surmise that over time, the tube probably "scoots" further from the ceramic socket.

Keeping my fingers crossed that this was the issue! Sounded like a 50's sci-fi movie soundtrack for a while there......

In the meantime, I think it would be wise to order some spare 12AT7's, likely some NOS 6201's or something.

[Dr. Toobz]

Just as an update, this problem with the 12AT7 would come and go, despite re-seating the tube. I checked all of my connections under the hood, and couldn't find any evidence of cold joints or any other obvious explanations. Today, I received two Sylvannia "Gold Label" JAN 12AT7WA tubes I snagged on eBay. They have triple mica layers and are from the 50's, as far as I can tell. Anyway, in addition to much improved sound, the popping went away. It's almost as though the EH tube was arcing or something - or at least that's what it sounded like. When I plugged it back into the amp, it hums more than the NOS tubes, has a lower gain, and almost sounds a bit distorted. I'm hoping this was just bad luck and not something wrong with my circuit.....if it happens again, I can only surmise that it's something common to the whole amp (e.g., in the power supply) and probably a resistor or something that gets hot as the amp is run for a while.

By the way, the NOS 12AT7 are amazing! Definitely worth the $30 for a pair.