I smell smoke

[rickenrocker]

I just replaced the 12AX7 (power?) tubes in my AC15CC1, which solved a loud popcorn popping that was going on, but I started to smell smoke after a minute or so. Is this normal for new tubes or am I frying something?

I turned it off right away and now I'm afraid to turn it back on.

[elysrand]

You should not smell smoke, ever. Unless there was some foreign material on the tube itself burning off, then usually a true "smoke" smell in indicative of a resistor or other part conducting too much current, thus overheating, and therefore in danger of imminent failure soon after.

A 12AX7 is not a power tube. With true power tubes, you will often have problems switching types unless the bias is readjusted. Not readjusting bias can result in overheating in power tubes.

There is an outside chance that one or your new tubes came out of the box already defective, and something is shorting out inside the tube. Perhaps it would be a good idea to take those new tubes to the nearest tube tester and test them out. In the meantime, you could reinstall the old tubes for awhile.

[soundmasterg]

Are you sure that you put the new tube in the correct socket in the amp? The AC15 uses 12ax7's in the preamp and phase inverter positions, and uses EL84/6BQ5's in the power amp position. The sockets are the same for both tubes, so you could conceviably put the 12AX7 in the socket intended for the EL84, and this would cause a problem and smoke a resistor, or possibly worse. Tubes are usually quite forgiving devices, but not when you use them incorrectly. I'd suggest to take the amp to someone who can check it out and make sure nothing is damaged. It is cheaper to do this than to possibly damage the output transformer if you have the tubes in the incorrect sockets.

BTW, when you replace power tubes on an AC15, since it is a cathode biased amp, you almost never have to "set" the bias as the very nature of cathode bias allows the tubes to set their own bias that is almost always in the correct range for the tube. Just make sure you replace the tubes with the correct ones for the socket or you can cause damage.

Greg

[rickenrocker]

I pulled them from the right place, the tubes i pulled were matching numbers and I accidentally even got the brand the same, they look identical as the tubes I took out.

[soundmasterg]

Ok, so the next step is to turn it on again and see if you have the same problem, and this time watch closely to make sure the heaters are lighting up inside all the tubes. If they are, then see if it plays and sounds ok. If you see or smell smoke, then turn it off again and take it to a tech to get checked out. Aside from something burning off one of the tube envelopes, then something inside the amp such as a resistor has smoked, which means there is something else wrong with it to cause that.

Greg

[rickenrocker]

I think it's one of the new tubes I got. I'm taking it in anyway, it's under warranty, at most I think I have to pay for new tubes they put in.

[rickenrocker]

I was gonna take it in tomorrow (1hr drive and then they have to ship to another state ) but the EL84s I ordered showed up so I gave a whirl. Everything sounds great!

I'm trying a different brand of tubes, I wish the amp hadn't been out of order for so long, I'm having a hard time remembering what the original ones sounded like