Resistor popped...need help for replacement in Yaqin amp - diyAudio
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Old 3rd July 2004, 05:26 PM   #1
nubbins is offline nubbins  United Kingdom
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Default Resistor popped...need help for replacement in Yaqin amp

Here's my Yaqin Integrated stereo tube amp. 50w+50w into 6ohms.

Click the image to open in full size.


Ooops...it's naked.

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It's recenlty popped a resistor and I have no idea what to replace it with as I can't find a similar matching 20ohm 5watt.


If it helps, here's a close up of the area where the resistor burned out.

Click the image to open in full size..



I'm in the UK so need UK dealers....I've looked at WilmslowAudio, Maplins, RadioShack and CPC and none have the same spec resistors. CPC have the closest with a 5watt 22ohm, and Mappers and Wilmslow have 7watters in 18 and 22ohm variety (which seem to be the same thing in which case Wilmslow are seriously overcharging for 1 resistor in comparison).



What would be a suitable replacement availble quickly within the UK?


Thanks for any help.
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Old 3rd July 2004, 06:02 PM   #2
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FWIW, I'd replace the "fried" part and what appears to be its mate with 22 Ohm 7 W. parts. That way, balance is maintained and a little extra heat tolerance is obtained.
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Old 3rd July 2004, 08:53 PM   #3
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It looks like it's the cathode resistor which has been
This usually happens due to tube failure.

These small value cathode resistors are used to check or adjust bias of the output stage. Therefore replacement with the same value, 20 ohm, is mandatory.

However the wattage of the resistor used is onder normal operating conditions way overrated, so replacement with a less wattage type is possible.

Another consideration is that the resistor used is an inductive type. Replacement with a non inductive type would be better.
If you decide to do so, better replace them all.

Also, mount the new resistor above the PCB so that when it blows again the PCB will not be damaged.

I would look for 2 10ohms 2W (available in metaloxide, carbon or film, all non inductive) and connect them in series.

Dick.
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Old 4th July 2004, 12:22 AM   #4
nubbins is offline nubbins  United Kingdom
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Thanks for the help.


The resistor did indeed burn up when a tube started to glow red hot

I have had a search and the best I've found so far at CPC is METAL FILM 2W 5% 10R.


There's no picture but this should do what is necessary?




Quote:
Also, mount the new resistor above the PCB so that when it blows again the PCB will not be damaged.
Sounds like you expect it to blow again....would this be common?
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Old 4th July 2004, 01:24 AM   #5
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Quote:
Originally posted by nubbins
Thanks for the help.


The resistor did indeed burn up when a tube started to glow red hot snip
Sounds like you expect it to blow again....would this be common?
I would test the tube in question. If it is a bias resistor that was blown, it will happen again if the tube's gone south.
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Old 4th July 2004, 01:17 PM   #6
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Quote:
I have had a search and the best I've found so far at CPC is METAL FILM 2W 5% 10R.
Yes, this is perfect.

Quote:
Sounds like you expect it to blow again....would this be common?
Also yes, seen it too many times.
The subjective tube should be discarded.
And it can always happen again.

Dick.
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Old 4th July 2004, 01:34 PM   #7
benny is offline benny  Australia
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Quote:
These small value cathode resistors are used to check or adjust bias of the output stage. Therefore replacement with the same value, 20 ohm, is mandatory.
not true... if it is there to adjust bias, then it needs to be replaced with an equivalent component... but if it is there only to check bias, then feel free to replace with any resistor around this range... all it reqires is 30 secs of mathematics in a simple aplication of ohms law...

this resistor is used to check bias by measuring the voltage drop across it... for example, say it is dropping 2V, then you know that acording to ohms law you have

I = V/R = 2/20 = 100mA

so, if the target bias current is 100mA for example, and you want to replace with a 22ohm resistor, ohms law tells us,

V = R*I = 22*0.1 = 2.2V

so you want this new resistor to be dropping 2.2V if the old one was dropping 2V... of course, this is in a fixed bias amp... if this is a cathode biased amp (which i doubt), then you will need to replace with equivalent value.

simple... eh? if this needs more explanation, just say.

Quote:
However the wattage of the resistor used is onder normal operating conditions way overrated, so replacement with a less wattage type is possible.
but not recomended. like eli recomended, use a higher wattage if possible. resistors are dead cheap anyway.

cheers
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Old 4th July 2004, 02:41 PM   #8
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Quote:
if it is there to adjust bias, then it needs to be replaced with an equivalent component
That's wat recommended.

Quote:
but if it is there only to check bias, then feel free to replace with any resistor around this range...
Ofcourse you can, but you need to recalculate.
Wouldn't it be much easier to stay with the original value ?

Quote:
use a higher wattage if possible
If you like mathematics:
W= I x V = 0.1 x 2 = 0.2 W !! Thats twenty times less then the 4W suggested ( when resistors are put in series you can add the wattage ).
So 0.2Watt is what the resistor is dissipating with 100mA bias current. This I doubt, usually it's around 50mA which brings the dissipation even lower to 0.1 W !!

Therefore there is no need to upgrade in watt.
The resistor will only burn faster when it's stressed by a faulty tube. And it will burn anyway, , no matter if it's 4 or 7 Watt.

Dick
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Old 4th July 2004, 03:19 PM   #9
benny is offline benny  Australia
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Quote:
Ofcourse you can, but you need to recalculate.
Wouldn't it be much easier to stay with the original value ?
well, if he's having trouble with finding a 20ohm resistor, wouldn't 30 secs of (simple) math be easier than the trouble of sourcing a 20ohm resistor???

Quote:
The resistor will only burn faster when it's stressed by a faulty tube. And it will burn anyway, , no matter if it's 4 or 7 Watt.
fair enough... good point.

cheers
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