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'73 Super Reverb: Smoke from the preamp section

79stratman

Member
Messages
870
Hey guys,
Just a shot-in-the dark here: I have a BF'd '73 SR. The other night, I experienced a burning-odor and light smoke emminating from the preamp-side of the chassis. A quick peek before shutting the amp down, appeared to indicate the first 6L6 starting to glow (but not full-on, deep cherry red glow).

I was using a Weber Impedance Matcher set at 2 ohms out of the amp>matcher> 8 ohm load to the JBL D-130F I was experimenting with.
Is this a contributing factor, or simply an isolated component failure issue?
Or, are there other potential circuit issue which may be developing?

I'm not very tech savvy, only building a 5E3 kit so far. Some more knowledgable friends indicated the possibilty of a gird coupling cap failing.
Would that be the +437v cap to pin 3?

Any other potential areas to investigate? I haven't pulled the chassis yet, so am unaware of any obvious signs of failure. It's likely that I may only spot something obviously overheated, but am willing to try to investigate further.

I downloaded a schematic, though it's only minimally-helpful at my present ability level.

Thanks
 

phsyconoodler

Member
Messages
4,305
Check under your cap can and look for one of the caps expelling it's insides.It will have a greasy,oily residue that spews from the vent hole on the capacitor.
Using a two ohm transformer into an 8 ohm load is not a good way to use an attenuator.
 

79stratman

Member
Messages
870
Check under your cap can and look for one of the caps expelling it's insides.It will have a greasy,oily residue that spews from the vent hole on the capacitor.
Using a two ohm transformer into an 8 ohm load is not a good way to use an attenuator.

Understood...but, the Weber Impedance Matcher is not an attenuator, but rather a means to match amps and speakers of various outputs and loads. That said, my tech warned me against use ANY type of attenuator or impedance matcher. Others I know and trust have used both without incident for many years.
 

soldersucker

Member
Messages
2,724
Pull the chassis and take a pic and sniff around.They are usually the easy ones to figure out not like intermitents.
 

plexi67

Member
Messages
359
does this amp have a hum balance pot? if it does, a bad tube will smoke those. As others posted, coild be from the cap can,screen grid resistor and other things. should be easy to sniff out or see.
 

79stratman

Member
Messages
870
I can't see an impedance 'matcher' causing the condition you are experiencing.
Did you check the amp yet?
__________________

Pull the chassis and take a pic and sniff around.They are usually the easy ones to figure out not like intermitents.
__________________

does this amp have a hum balance pot? if it does, a bad tube will smoke those. As others posted, coild be from the cap can,screen grid resistor and other things. should be easy to sniff out or see.




No, my schedule hasn't allowed the chassis-pull/inspection today. If things go well, possibly tomorrow...

It's encouraging to hear that it may be easy to sniff out!

As far as the hum-balance pot...I honestly don't know. I've only got a few hours on the set of NOS tubes, but it's certainly possible that the tube might be bad. My gut instinct says it's something else causing the issue.

I truly appreciate everyone's kind assistance! Thank you so much for giving some hints at the potential problem!!!
 

phsyconoodler

Member
Messages
4,305
Those are just some of the amps I make.The latest ones haven't found their way onto the website yet.

Get that chassis out of there so we can find your problem already!!!!!
 

79stratman

Member
Messages
870
Those are just some of the amps I make.The latest ones haven't found their way onto the website yet.

Get that chassis out of there so we can find your problem already!!!!!

Yes...yes...I apologize for the delay! LOL

I observed a longitudinal split in the 470ohm 1W resistor which bridges pins 4 & 6 on the first output tube. That is the only OBVIOUS component issue I could find. I have done no testing of values. This was strictly a visual (and olefactory) inspection. The only place I could smell a residual burnt-offering smell was at this output tube socket. The smoke I witnessed emminating from the area of the Vibrato Channel inputs were most-likely the smoke's path of least resistance for exiting the chassis.

So, not being a tech...is that the screen resistor which went bad?

If it is strictly a problem with ONLY the resistor being fried, it appears to be an easy replacement, which I could handle. If there are other components associated with this type of problem, and may be suspect, then it's a different ball game.

Again, I have some concerns, since the same output tube socket is the one which redplated a few years ago. The output tubes have been replaced, but not sure this resistor had ever been replaced. It appears to be original, though my tech is heavily-stocked with NOS parts. If he did change something, it appears old...as well as, the solder joints (from what I can tell).

Not sure why this would be a recurrent problem? The first redplating incident years ago, was seriously hot at the time, whereas this occurrence had only a hint of the tube glowing a bit brighter at the time of shutdown.
 

smolder

Gold Supporting Member
Messages
14,522
the screen resistor is not the problem. But it is an indicator of a problem. By it blowing, it may have saved some other components. I'd advise that you get this to a tech. The tech can check the system, make sure you have a reliable rec and power tubes and send you of with a clean bill of health and confidence. Most will also explain the situation so that you feel comfortable with the process and the fix.

At the least you will probably need one or two power tubes and they need to be bias'd... and you need the the experience and equipment (a tech?) to do that.
 

79stratman

Member
Messages
870
OK, that's what I was looking for. I was hoping for a simple solution, but had an inkling it may have been and "indicator" of other problems, but not having the knowledge-base, couldn't confirm anything. I've been very happy with my local amp tech/builder, so I will get it over to him soon (even if I might have to endure the verbal tongue-lashing for using the Impedance Matcher, against his advice...LOL).

Thank you very much for the confirmation!
 




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