Why is this amp blowing tubes???

mrelusive · Post by **mrelusive** » Thu Jan 31, 2013 5:40 pm

A friend left an Egnater Tourmaster 4212 at my place for me to check out. I have replaced the preamp tube for the second gain stage at least three times. It'll sound fine, but you can't get much volume out of it... What up wit dat?!

soundgardener75 · Post by **soundgardener75** » Thu Jan 31, 2013 5:53 pm

Check the fuse maybe?

zach · Post by **zach** » Thu Jan 31, 2013 5:57 pm

soundgardener75 wrote:Check the fuse maybe?

the only thing i can think of is that something is allowing too much current through that tube...that points to a short or shorting cathode resistor or bypass cap. i'd suspect a cap before the resistor, but with the values that most companies use both should be stupid over rated. could be a bad solder joint somewhere, or something wrong with the socket as well.

gb74 · Post by **gb74** » Thu Jan 31, 2013 5:59 pm

well a bad tube would cause the fuse to blow but not the reverse. My first guess would be a cold solder joint that is causing the amp to short out and killing the tube. It could also be a bad cap or resistor that is sending too much current to the tube.

Finally, it could also be severe bad luck, but since it happened 3 times in the same spot, I doubt it.

At any rate, this seems to be a case for a tech, it will be hard to figure this one out with basic troubleshooting.

Zozobra · Post by **Zozobra** » Thu Jan 31, 2013 6:17 pm

Pretty much what zach said. Most obvious thing would be blown plate load or cathode resistor. If the plate load has failed open then the load line will shoot straight through the max dissipation curve and cook the plate. A leaky coupling cap could put a lot of DC on the grid but that isn't always a problem depending on what the stage is doing. A bum grid leak resistor could screw the bias too, causing it overheat the plate.

legion · Post by **legion** » Thu Jan 31, 2013 6:20 pm

I would open it up and look at the connections of the tube socket itself. make sure none of the solder lugs are touching the chassis. there have been a lot of sub par 9 pin preamp tube sockets made and used.

elder_things · Post by **elder_things** » Thu Jan 31, 2013 7:10 pm

Like these guys have said, probably a surrounding component. Would be a good idea to make your voltage checks.

mrelusive · Post by **mrelusive** » Thu Jan 31, 2013 7:40 pm

ah shit... well, i'll prolly call around tomorrow and see if anyone can work on it locally

zach · Post by **zach** » Thu Jan 31, 2013 8:21 pm

Zozobra wrote:Pretty much what zach said. Most obvious thing would be blown plate load or cathode resistor. If the plate load has failed open then the load line will shoot straight through the max dissipation curve and cook the plate. A leaky coupling cap could put a lot of DC on the grid but that isn't always a problem depending on what the stage is doing. A bum grid leak resistor could screw the bias too, causing it overheat the plate.

think real hard about why an open plate resistor won't hurt a tube :cop:

since he's not reporting weird noise or blocking distortion, i doubt it's a faulty grid leak or leaky coupling cap.

also, we don't know the nature of the failure...is the filament still lighting? have you tried the "bad" preamp tubes in other spots in the amp to eliminate socket specific issues?

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