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Resistor Value

NeilR

pfm Member
Last night a valve blew in my Audio Research CA50 integrated amp with a nice pop and some smoke, taking a resistor out with it. Ideally, I'd like to have a go at fixing it myself, it looks easy enough.

Can anyone help me identify the value/power rating of the resistor in the picture below? Also, does anyone know where I might be able to source a few?

Thanks, Neil

Resistor from another valve

resistor.jpg


Blown resistor

blownresistor.jpg
 
I would say that is a 30k 5% resister. Wattage? anything from 1 up to 4, I cant really be sure how big it is, go high and you cant go wrong.
 
Looking again, I think it's orange but I am not 100% sure. I guess I could borrow a multimeter from work and measure the value to be absolutely certain.

Its 15mmx5mm in size so probably 2 or 4 watts?
 
The amp is 10 years old.

Annoying thing is that the valves were replaced only 4 months ago. just long enough to be out of warranty.
 
The amp is 10 years old.

Annoying thing is that the valves were replaced only 4 months ago. just long enough to be out of warranty.

Maybe its not that high a wattage then, an easy way out is to replace both with modern high wattage items.

Have you found out what made it blow like that?

BTW Did you buy a Naim NAT05 from me when I lived in Westcliff ?
 
Is there good reason to suppose that this resistor matches the smokeless one? The remainder of the visible components and traces aren't similar.

It might be worth contact with Audio Research Customer Service.

Paul
 
Maybe its not that high a wattage then, an easy way out is to replace both with modern high wattage items.

Have you found out what made it blow like that?

BTW Did you buy a Naim NAT05 from me when I lived in Westcliff ?


No, it wasnt me.

I am led to believe that when valves blow, they usually take out the cathode resistor. The valve was glowing very brightly before the resistor popped. I couldn't get the power switched off quick enough.

I've had the same thing happen on another amp, and seen another 6550 tube go up in smoke in another forum members system. Scary.


s there good reason to suppose that this resistor matches the smokeless one? The remainder of the visible components and traces aren't similar.

It might be worth contact with Audio Research Customer Service.

Paul, each valve has the same value cathode resistor next to the valve base. I should have posted a pic of the other channel as it is a mirror image.

I'll contact Audio Research tomorrow. The last thing I want to do, however, is to take it to the UK distrubutor (Absolute Sounds), as I will get badly ripped off.
 
Resistor Value?

The shredded one doesn't look worth much to me Neil but then I'm no expert.

I'd speak to AR.
 
Here's an update :- I contacted Audio Research and told then what happened and they are sending me a schematic, a parts list and a few replacement resistors free of charge.

Not bad service considering its a 10 year old amp!

So all that's needed now is to order a matched pair of 6550s and I should be away.
 
Another update,

The schematic and replacement resistors arrived this morning and 15 min with the soldering iron had the new resistor installed. Replacement tubes installed and the amp has been back on for most of the day without issue and is sounding as good as new.

Valve amps can be a PITA, but at least thay are simple to fix!
 
By the way,

A piece of advice for valve power amp owners. Never, I mean never, leave a valve power amp unattended when it is switched on. On the back side of the PCB where the resistor had blown there was black charring and a thick PCB track behind where the resistor was had partially melted so the temperatures must have been pretty fierce. The amp was switched off less than 20 seconds after the valve blew, so god knows what would have happened if it had gone when I was out.
 
By the way,

A piece of advice for valve power amp owners. Never, I mean never, leave a valve power amp unattended when it is switched on. On the back side of the PCB where the resistor had blown there was black charring and a thick PCB track behind where the resistor was had partially melted so the temperatures must have been pretty fierce. The amp was switched off less than 20 seconds after the valve blew, so god knows what would have happened if it had gone when I was out.

Very good advice.

I once returned from a 10 minute vist to the corner shop to find smoke pouring from a Yaquin amp and the output tubes glowing white hot.
I think there is a very good case for using fuses in these applications.
Never, ever uprate the resistor wattage rating from that recommended as if the resistor continues to pass current in a fault situation......something else will fry.
 
IIRC it's the Leak power amps where a resistor unsolders itself in some fault conditions. Actually quite sensible given a single fault can make smoke without blowing the fuse.

FWIW I think actual fire would be extremely unlikely. Totally trashed amp could easily result though.

Paul
 


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