Mesa 400 tube question

kojakcurtis · Post by **kojakcurtis** » Sat Feb 10, 2007 2:20 pm

In my Mesa 400, the older 6 tube version, one of my power tubes stopped working. It doesn't glow red at all. I just replaced the tubes about 5 months ago with Mesa tubes,(it's cathode bias). Since they should be changed out in pairs, if I swap out the one that doesn't work and the one next to it with a pair of new ones would this solve the problem. If I buy new tubes and swap them and it still doesn't work, any idea what the problem could be. My amp tech is swamped right now and it'll be about a month before he can even look at it. I don't know if I can survive that long without it. When I play through it, it doesn't sound different. Any feedback would be most excellent.

jps · Post by **jps** » Sat Feb 10, 2007 2:41 pm

Try swapping that tube with another on the amp and see if the problem remains with that one tube, it sounds like the filament is bad in that tube. BTW, I doub't that your amp is cathode biased, at least for the power stage.

sharkboy · Post by **sharkboy** » Sat Feb 10, 2007 4:06 pm

I still miss my Mesa 400.

If you still have the problem after putting in new tubes (or swap positions of that tube with its mate and the same tube position is bad), the first thing to look at is if there is a bad connection to that socket. Unless you really know what you are doing, it is inadvisable to go poking around in a tube amp (high voltages stored in the caps.) It is at best a really lousy feeling to get zapped, and is potentially lethal- yes, even if the amp is unplugged.

soundmasterg · Post by **soundmasterg** » Sun Feb 11, 2007 2:33 pm

The Mesa 400's weren't cathode biased in the power section. They're actually similar in a lot of ways to the Ampeg SVT. The Mesas just don't put in a bias adjustment in most of their stuff, so its fixed bias, non-adjustable.

Those tubes work in pairs, but not with the one right next to it, but with the opposide one. So the two middle ones work together, then the next two out work together, etc. Replace the bad one and the appropriate pair and call it good. If the new tube doesn't light up, then you have a more serious problem and you should wait for your tech. Mesa tubes are usually Chinese made btw, and you can get better quality tubes for a cheaper price elsewhere. Since Mesa chooses to bias their amps the way they do however, if you want something that will be graded and tested so it biases the same as your other tubes, Mesa can do that.

jnbass · Post by **jnbass** » Mon Feb 12, 2007 12:41 am

try THAT with your 12x6L6GC 400+

kojakcurtis · Post by **kojakcurtis** » Mon Feb 12, 2007 12:10 pm

After swapping the pair of tubes, it's the socket thats messed up. when I fired the head back up it blew the fuse and the power tubes. Upon further inspection, at one point it was retubed with non-Mesa tubes, and some resistors were added to 2 tube sockets to readjust the bias, one of them being the socket that the tube didn't work in. If I remove the remains of the resistors putting it back to stock, as they are not on the original schematic and there is a service sticker on the underside stating they were added for adjusting the bias in '88,(wish I would've seen that before I retubed it), would this solve the problem. Also, how long would it take for the voltage to disipate before I could safely remove the remains of the resistors, as I've heard horror stories about stored voltage.

ken_j · Post by **ken_j** » Mon Feb 12, 2007 12:28 pm

You can discharge a cap with a resistor if you are careful. Use a resistor high enough in wattage and resistance so that it doesen't discharge too fast.

The current offering of Mesa's preferred 6L6 is the STR440. It is it Chinese. But others are or have been available. One that is favored by many Boogie owners is the STR454 made in Russia by Svetlana. Here is a link to some of their offerings. This hasn't been updated recently so the 440 is not on it. http://homepage.mac.com/mesaboogie/tubes.html
You may also find some help here. The Boogie Board There is a forum member there that goes by the user name Monsta-Tone that is a authorized Boogie technician.

kojakcurtis · Post by **kojakcurtis** » Mon Feb 12, 2007 12:51 pm

Here's a pic, the straight line is where the one that completely fried was and the arrow is pointing to the remains of the other one that's cracked. The end tube is the one that wasn't working. Like I said neither of these appear on the schematic.

kojakcurtis · Post by **kojakcurtis** » Mon Feb 12, 2007 12:52 pm

Thanks. I belong to the forum, so I'll post it there also.

soundmasterg · Post by **soundmasterg** » Mon Feb 12, 2007 1:14 pm

A cap can hold a charge for years and kill you in a second. If you want to work on amps, then you should get the proper tools to do so safely. You can get some wire, two insulated clips, some heat shrink tubing, and a 2 watt 10k resistor. Hook it up so you can clip 1 lead to ground and the other to the main filter caps positive lead inside the amp. Make sure the amp is unplugged and off before you do this! Leave the wire/resistor clip hooked up while you are working on the amp, and you won't get accidentally shocked. Make sure to disconnect your wire clip/resistor setup before you turn the amp back on or the resistor will smoke. Don't ask me how I know this!

If they added some resistors to reset the bias, then you can restore it to factory design and it should work ok. Make sure that the socket where the bad tube was isn't arcing, or hasn't arced. They look like ceramic sockets in the pic, and those don't usually arc very easily, but once a socket arcs, it is a good idea to replace it as they tend to keep arcing. Also you may have to reroute some wires or other parts to get back to factory condition. They may have done more than just adding those resistors.

kojakcurtis · Post by **kojakcurtis** » Mon Feb 12, 2007 1:37 pm

Here's the tag inside the amp.

A buddy of mine majored in electronics and said he would help make sure the voltage is dissipated, as he knows and has done it before. He loves when I call him on this stuff. The caps are ceramic.

soundmasterg · Post by **soundmasterg** » Mon Feb 12, 2007 4:19 pm

While there might be ceramic caps in the amp, the bulk of the caps in the amp will be electrolytics like the blue ones in the picture (and these are polarized too btw), or they will be smaller film caps like polyester, polypropylene, or silver mica. Some amps have paper and mylar(polyester) caps, and others also have paper-in-oil caps. The electrolytic caps are the ones that wear out with age and non-use and need to be replaced every 10 years or so for optimum health, and they are also the ones that hold the high voltage charge for a looong time. If you completely discharge one of them, and then take the clip lead/resistor off, you can watch the voltage come back up to a portion of what the original voltage was with a meter. This happens because the dialectric inside electrolytic caps have a memory and they will always go back to that state until they start to wear down as time goes on. Its good that your friend will look at it for you for safety reasons if nothing else.

ken_j · Post by **ken_j** » Tue Feb 13, 2007 4:26 am

If you don't have a schematic for this you can probably have Mesa email it to you. They will do this only for amps out of production. It is best to call Mesa's customer service. I understand that they are not as responsive to incoming emails. They happily emailed me schematics for my DC-5 a few years ago after calling.

jnbass · Post by **jnbass** » Tue Feb 13, 2007 6:42 am

caaaareful with them caps!

kojakcurtis · Post by **kojakcurtis** » Tue Feb 13, 2007 7:02 am

This site has alot of Mesa and other amp schematics for free. http://www.schematicheaven.com/mesaboogie.htm