Blown Preamp Tube?

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Blown Preamp Tube?

Postby Rusty the Scoob » Wed Mar 11, 2009 11:08 am

My beloved bass amp finally gave me its first problem ever last Saturday. I hadn't noticed that the cooling fan was turned off so it got hotter and hotter and finally went into thermal shutdown during that middle section of Morning Dew where Phil drops enough bombs to sound like the apocalypse. :lol: My keyboard player and drummmer (the two closest guys) both smelled something burning but I'm hard of smelling and didn't notice anything.

When it cooled down the next morning it turned back on and seems to mostly work, but with a hint of distortion that wasn't there before. It's not a blown speaker or tweeter because I tried it with an extension speaker, and it's not my bass because I tried two different basses. I'm hoping it's just a partially burnt 12AX7, since it's a tube preamp with a solid-state power section... anyone had similar symptoms? The tube is around 7 years old.
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Re: Blown Preamp Tube?

Postby SarnoMusicSolutions » Wed Mar 11, 2009 12:53 pm

It's not a tube. If someone smelled burning, it surely isn't a preamp tube. My guess is an overheated resistor near your output power transistors. Just look inside at the big resistors and you'll probably see at least one brown one. Very likely it's in the power amp section. That's where the real heat comes from.

Brad
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Re: Blown Preamp Tube?

Postby waldo041 » Wed Mar 11, 2009 3:20 pm

bummer!

like brad said, it's probably in the power amp section. thats most likely where the thermal protection circuit is also because of the heat generated. that said, if you have a preamp out tap on it, tap it to another power amp to eliminate the power section completely.

good luck.

peace,
waldo
"Tone is in the instruments. Technique in the hands. Do what you will." ~ quote from some guy at the TGP forum
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Re: Blown Preamp Tube?

Postby Rusty the Scoob » Wed Mar 11, 2009 3:49 pm

Ooh, that's a good idea, even as a test. I could use either the DI or the effects-loop output and if it sounds clean I'll know it's not the tube or anything in the preamp section. Thanks!
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Re: Blown Preamp Tube?

Postby Rusty the Scoob » Sat Mar 14, 2009 6:03 am

Looks like you're right, Waldo and Sarno. One of the resistors in the power amp section has little brown burned spots on it. I'm tempted to find another ceramic resistor with a similar value and solder it in, but I'll take it into the shop. (they agreed with your assessment based on my description, BTW... you guys know your stuff!) I have a minor mod I've been wanting to do forever so now's my chance.
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Re: Blown Preamp Tube?

Postby waldo041 » Sat Mar 14, 2009 6:14 am

scoob,

that little brown spot was what your bandmates smelled! lol

seriously though, i would have them up the wattage of that resistor and ensure it is a good quality flameproof version. in up'ing the wattage i mean if its a 1/2 watt resistor, change it to a 1 watt or 2. they will be a little larger but should be able to handle the heat a little better if the fan goes awry again.

while it's in the shop have that fan checked out also. i dunno if it's heat activated or if it's always on. but i do know they make super quiet ones nowadays that should fit right in.

happy to hear it's on it's way to recovery.

peace,
waldo
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Re: Blown Preamp Tube?

Postby strumminsix » Mon Mar 16, 2009 5:11 am

waldo041 wrote:while it's in the shop have that fan checked out also. i dunno if it's heat activated or if it's always on. but i do know they make super quiet ones nowadays that should fit right in.

happy to hear it's on it's way to recovery.



+1 I'd go for either always on or variable speed. Always on if it's a head, variable speed for combo since if you ever record, and I assume you would mic it since you called it your "beloved" then sometimes fans can act finicky!
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Re: Blown Preamp Tube?

Postby Rusty the Scoob » Mon Mar 16, 2009 9:53 am

I think the fan is on/off and triggered by temp... I've never really noticed when it turns on or off that I can think of, but I have noticed it being on, if that makes sense... I wonder if I could have an LED added to the front that lights up when it turns on or something.

I'm not too worried about the noise from the fan, the DI on this amp is the best part. I know I switched the fan off by mistake, and should have noticed when the "internal speaker" offswitch was off, since the two switches are right next to each other, so I have no reason to believe it's not working properly. I'll definitely ask him about the fan when I talk to him next, he said he'd run it under load for a good hour to make sure it's back in working order and no components go off spec. Of course it's out of it's chassis now so it's got a lot more cooling!
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