# Any amp techs?



## pat6969 (Feb 4, 2013)

I blew a screen resistor and it took out a bit of the trace. Looking at the schematic it looks like they two 1K resistors are just in series with each end attached to pin 4 of each socket. Then the B+(SCN) wire connected in between the resistors. Is this correct? Amp is a Mesa single rec.


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## nonreverb (Sep 19, 2006)

Yes you are correct...make sure the replacement resistor is 2watt


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## jb welder (Sep 14, 2010)

Screen resistors blow when the output tube they are connected to go bad. They don't blow on their own.
The tube may be shorted or only shorting intermittently, but it should be replaced or it will most likely end up blowing the resistor again.


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## nonreverb (Sep 19, 2006)

....or all the power tubes if they've been in there for some time.



jb welder said:


> Screen resistors blow when the output tube they are connected to go bad. They don't blow on their own.
> The tube may be shorted or only shorting intermittently, but it should be replaced or it will most likely end up blowing the resistor again.


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## pat6969 (Feb 4, 2013)

Thanks guys. I figured it was a bad tube, changed both out for new ones. Here is my handy work.


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## epis (Feb 25, 2012)

You did good job. If I were you, I would use Dremel to remove completely burnt part of the board. When is burnt, becomes conductive.
The hole could be filled with two component epoxy glue and eventual traces underneath the board recreated with pieces of solid core wire.
You can put the piece of masking tape on the other side of board and pure the epoxy in the hole. 5min glue works just fine and it's heat resistant.


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## Church-Audio (Sep 27, 2014)

You are relying on a mechanical connection of solder to hold these parts in place. Like others have mentioned you should remove all the toasty bits with a Dremel tool as they can and will conduct electricity ( carbon ) = conductor


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## pat6969 (Feb 4, 2013)

Church-Audio said:


> You are relying on a mechanical connection of solder to hold these parts in place. Like others have mentioned you should remove all the toasty bits with a Dremel tool as they can and will conduct electricity ( carbon ) = conductor


The burnt part of the board was contained to the screen resistor part of the circuit. All other traces are free of any residue. I made sure of this and metered every trace to make sure.


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## nonreverb (Sep 19, 2006)

Judging by the pic, you might create a parallel resistance between the left pointing resistor and the right pointing resistor. As mentioned, there is certainly enough carbon deposit on the board to create an electrical path between them. 
One problem with metering a carbonated path such as this is there isn't nearly enough voltage on your meter to create the conduction path.....400 + volts DC certainly will.


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## pat6969 (Feb 4, 2013)

Ah, I see what you mean. I will take a dremmel and clean the carbon entirely off the board between those two points. Thanks for the tips. I'm an electrician by trade so the work isn't foreign but circuit boards aren't my forte! I'm assuming if I stay away from the traces that are going to pin 4 of each socket and clean the carbon off in between I should be OK? Damn, now I have to take it all apart again! Better safe than sorry though!


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## Church-Audio (Sep 27, 2014)

Often this stuff does not conduct until its under high voltage  thats when the fun starts.......


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## pat6969 (Feb 4, 2013)

So do you think it's OK if I just use a dremmel and clean the carbon off? I'm starting to second guess it now.


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## jb welder (Sep 14, 2010)

As the other guys mentioned, it's the high voltage that can leak across high resistances.
The biggest danger would be of any high voltage leaking into the bias supply or grids of the power tubes (TSL disease).
If this occurs, the power tubes will overheat or redplate.
So however you deal with the carbonized board, after you're done you will want to monitor the bias and make sure it stays stable.


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## pat6969 (Feb 4, 2013)

jb welder said:


> As the other guys mentioned, it's the high voltage that can leak across high resistances.
> The biggest danger would be of any high voltage leaking into the bias supply or grids of the power tubes (TSL disease).
> If this occurs, the power tubes will overheat or redplate.
> So however you deal with the carbonized board, after you're done you will want to monitor the bias and make sure it stays stable.


So, seeing as how the Single Rec. has no biasing, where would I measure the mV?


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## jb welder (Sep 14, 2010)

You can monitor the bias voltage at pin5 of the power tubes. Should be approx. -48V according to schematic I have. It may fluctuate a little bit with the line voltage, but should stay fairly close.


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## pat6969 (Feb 4, 2013)

jb welder said:


> You can monitor the bias voltage at pin5 of the power tubes. Should be approx. -48V according to schematic I have. It may fluctuate a little bit with the line voltage, but should stay fairly close.


Cleaned and scraped the carbon off and played it for about 6-7 minutes at quite a loud volume. Resting, V on pin 3 of both sockets was about 456, playing was about 450. V at pin 5 was between -39 and -40 on both sockets. I have EL34's in it.


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## Church-Audio (Sep 27, 2014)

456 is about dead on. For what that amp should be. You should invest in a weber bias rite 2+2 because you can do el84 an el34 with the same probe. Gives you plate voltage and bias current draw.


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## pat6969 (Feb 4, 2013)

Cool. Does that bias voltage look OK at -40ish?

Also I would like to thank you guys for the help. I live up in northern Alberta close to absolutely nothing!! Without all your help my amp would be sitting in the corner until I made it to Edmonton, and then it would be sitting there until I made it back to pick it up!


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## jb welder (Sep 14, 2010)

Yes, that is right on the money for EL34's in that amp. They spec at -39V for EL34.


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## pat6969 (Feb 4, 2013)

jb welder said:


> Yes, that is right on the money for EL34's in that amp. They spec at -39V for EL34.


awesome. Thank you!!


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