Bare Knuckle Pickups Forum
Forum Ringside => Tech => Topic started by: FELINEGUITARS on March 20, 2009, 12:30:18 AM
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Hi
Just bought some 150 ohm resistors from Maplins (my god they are cr@p these days for electronics stuff)
I asked for 1/2 watt resistors as they were for an amp and what they have given me look (to my eyes) to be 1/4 watt resistors. The one I have is small and blue (which threw me as I was kind of expecting fawn coloured ones)
Now I don't want to fit them and have something burn out if they cant handle the power so hence my question:
Is there any way from the colour of the resistor or the colour bands on it to tell if it is a 1/2 watt or 1/4 watt resistor?
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the colour bands don't reveal the power rating unfortunately.
however, i just checked the maplin website, and their small metal film resistors (blue coloured) are claimed to be 0.6w
if in doubt use a 330ohm and 270ohm in parallel
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Thanks G
Am planning on making a mod to a Randall MTS module to reduce the gain a touch as it's too easily overloaded
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This help?
http://www.tpub.com/neets/book1/chapter1/1q.htm
Apparently the only way to tell a 1/4 watt from a 1/2 watt by looking is the size. The 1/2 watt resistor will be bigger.
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power handling has improved over the years though. compared the size of resistor and capacitors of today against ones of 30 years ago with the same rating... alot smaller!
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Just measured a (Maplins) 0.6W blue metal film resistor from my stock. It's just a tiny smidge under 6mm long. if your's is the same length then it will be a 0.6 Watter.
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Thanks for your help
Yes mine are the same size
Just that I have loads of 1/4 watt ones that are the same size, and the 470ohm(0.5watt) one I am replacing is bigger
HEnce the question - many thanks for your help
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Couldn't you use a Multi meter for this?
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The blue ones are metal film resistors, whereas the fawn ones are carbon film. Metal film resistors generally have higher power handling per unit size, hence the confusion.
Most resistors in most amps don't dissipate much power, so you would probably be fine with 1/4 W, especially in a solid state amp which runs at lower voltages than a valve amp.
The main reason I use 1 W resistors in valve amps is the voltage compliance. Most 1/2 resistors are only rated at 350 V, which is less than the B+ in most valve amps. OK, MOST of the resistors won't experience such high voltage under normal use, however the extra cost is small in the scale of things.
Fender use mainly 1/4 resistors (except of plate load resistors which are 1/2 W) in their production amps. We regularly have to replace a blown resistor in the PI of Blues Deville which have a B+ around 520 V. We think that these fail due to the voltage stress as they are dissipating well within their rated capacity.
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Sorry just realized that the Randall module is a valve module!
Unless you are replacing the plate load resistors you should be fine with 1/2 or 1/4 W resistors.
You can also reduce the gain be altering the valve (if there are any) or indeed removing the cathode bypass caps.
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Thankyou everyone for your input - very much appreciated