Fried Bias circuit? cracked power tube? buwah?

[cjl]

Apparently I fried the bias circuit by continuing to play my dual rec after I noticed the slightest (and I do mean slightest) crack in the third position power tube near the base of the tube.

It worked fine with no signs of any problems for at least 2 or 3 weeks after I noticed the crack. No blue glowing, no fuses blown, no hot running tubes, nothing. Then it happened.

It would blow fuses, so I decided to change out the power tubes. Got a matched quad of ruby 6L6GC mstr power tubes from dougstubes.com and I swapped them out myself since the dual rec has a fixed bias.

So I turned it on and the tubes were glowing blue for a few minutes as I left it on standby for about 15 minutes. Then I realized that they are running REALLY hot and then when I finally plugged in my guitar...THERE WAS NO SOUND.

NOOO!!!!!!

So I try like 47 different things just to be sure its the amp and not something else, and sure enough, this is what I was told by the amp repair guy I know:

"If you played it with the crack in it, you probably fried the bias circuit. Since there was a crack, the vacuum tube drew oxygen into the circuit which it isn't supposed to do and thus, fried the bias circuit. It will probably take about 3-4 hrs and $200 to fix when all is said and done."

Anyone know anything about frying bias circuits and what entails a repair?

 

[cjl]

bump

 

[xarkon]

If there was a crack in the base of a power tube (probably due to heating and cooling cycles), that opens a potential base for a short circuit between two pins. Hence, a voltage from one gets applied to a second.

There are circuit components - bias resistors, filtering and tone shaping capacitors, etc. - connected to these pins.

Resistors have two major specs (beyond the method of their construction). These are resistance value and power handling capability. (I'm ignoring resistance tolerance here.)

Capacitors are rated for capacitance and voltage. (I'll leave aside electrolytic capacitors, which are used for filtering, and have specific polarity.)

Basically, if a capacitor sees a significant overvoltage rating, it can completely open, or completely short. It's been a LOOOONG time since I worked on much circuit design or repair, so I don't remember the default failure mode.

If a resistor sees a significantly higher voltage than what was intended, the problem is a little more complex, because it depends on the circuit configuration around it. However, to simplify, it's likely that much more current will be running through the resistor due to the higher voltage drop from one end of the resistor to the other. Power = voltage drop * current. So, the resistor's power rating is exceeded, the resistor smokes, and it's all over.

Depending on the failure mode, other components not directly involved in the original failure might get to go along for the ride. Hence, if I were looking at this problem, I might want to disconnect various parts of the circuit and ensure that nothing else was toasted, because I wouldn't want the amp coming back to me a week later - or complaints that it just didn't sound the same. That work takes time.

Hope that helps.

Dave

 

[stokes]

"If you played it with the crack in it, you probably fried the bias circuit. Since there was a crack, the vacuum tube drew oxygen into the circuit which it isn't supposed to do and thus, fried the bias circuit. It will probably take about 3-4 hrs and $200 to fix when all is said and done."
If he made that statement without opening the amp up and doing some testing,which to me sounds like what he did,I would be leary of that repairman.There are many things that can or cant happen with a cracked tube,and without opening the amp up and checking it is impossible to say point blank that the bias circuit is fried.Of course if he did diagnose it properly,then forget what I said.

 

[cjl]

anyone know any good mesa repair shops in the chicago area? i have no idea where to take this thing and i need to get it fixed asap.

Thanks.

 

[fishyfishfish]

That sounds like a chunk of change to just solder in a resistor. Mesa charged me $60.00 after my amp took a huge beating from a 220 volt power supply. I don't want to sound like an A-hole, but get a second opinion.

 

[mikey383]

....after my amp took a huge beating from a 220 volt power supply.....

Ouch!!!

What's the story behind that?

 

[sixtonoize]

double post

 

[sixtonoize]

"If you played it with the crack in it, you probably fried the bias circuit. Since there was a crack, the vacuum tube drew oxygen into the circuit which it isn't supposed to do and thus, fried the bias circuit. It will probably take about 3-4 hrs and $200 to fix when all is said and done."

That is the LEAST scientific diagnosis I've ever heard. That's like saying "I plugged my amp in, and it blew. Therefore, my guitar was out of tune."

Really, if I were you, I'd pull your amp out of the chassis, and have a look around. If you've smoked resistors, they should look burned. What I'm guessing happened is that the crack DID allow oxygen to enter the tube, which caused it to draw too much current, which in turn burned out the bias resistor(s). However, you also could have fried your screen or grid resistors just as easily, which would still result in no sound, and have NOTHING to do with the bias circuit.

If this IS the case, it'll probably take about 20 minutes (and less than a dollar in parts) to fix.

If you don't feel comfortable checking it out yourself, I'd take it to a tech. NOT, however, the tech that quoted you $200.

 

[Elpelotero]

$200 for a circuit sounds a little stiff. Call around the authorized mesa repair centers on the website. Call Mesa itself.