noob question, quick answer

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AndrewT

R.I.P.
Joined 2004
Hi,
the grey red red gold about lower right of pic?
What size are they?

Between 500mW and 1W carbon composition or maybe carbon film. It's just possible that they are 2W but they seem too small.

Could you power down and quickly check running temperature without electrocuting yourself?
 
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rpapps

Member
Joined 2004
Alternatively, what wattage do they need to be?
Measure the voltage across them then use P=V*V/R to determine the power being dissipated. Then just use the the next higher power rating component.
Example: if there's 40 volts across them, then Power = 40*40/8200 = 0.19 Watts.
Half watt resistors would be fine in this case, one watt if you want to be conservative.
Since they are only bleeder resistors, why do you want to change them (assuming they aint broke)?
Cheers
Rob
 
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bigphil

Member
Joined 2005
They are really old, thats why I'm not sure

They are grey red red gold, which makes them 820 ohms, not 8200 correct?

According to my calculations:
voltage across capacitor is 63V
resistors are 820 ohms


then current = 63/820 = 0.076A

then power = 0.076*63 = 4.78Watts, so should be a 5watt resistor...

but they seem too small thats why I'm unsure...

they are only 13mm or so long



that picture is not mine, in my amp the four resistors don't match so I want to change them all.... I know its not a big deal, but I might as well while I'm in there.
 
A

AndrewT

R.I.P.
Joined 2004
Hi,
it seems like 13mm carbon is about 1W.

That 63V is a bit much for an 8k2 600mW metal film.

You could try 15k//18k 500mW or 600mW for an effective 8k18. But a 1W metal film would be much neater looking.
It/they will have much improved long term stabilty and lower temp coefficient than carbon. But will metal film sound as good?
 
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bigphil

Member
Joined 2005
LoL... i've worked with power generation stuff before, I'm not scared of the voltage. I'm not an EE, but I have enough background from ME... they jsut don't teach you all this practical stuff like differences between metal fil and carbon resistors... its a lot more of basic theory. I'm waiting for my new fluke in the mail, but was hoping to make a digikey order before then.

I think the voltage rails are about +-50V on this thing.

I hope you're joking about the difference in sound... I will look at the 1W carbon and 1W metal films, thanks for all the help guys.
 
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bigphil

Member
Joined 2005
These are power supply resistors, not even that.. they are bleeder resistors... far from the signal path. Do you really think they would have that much effect on the sound? If this is true I will stick with direct replacements of what is there, because this amp is supposed to have a signature "tube-like" sound, and I don't want to ruin it. But IMO I don't think these resistors will have much effect on the sound at all, in fact in their position I wouldn't want them to have any effect at all so I would want to go with metal films in that respsect. Can I get some more straight forward advice? If I'm completely wrong don't be scared to tell me, I like learning and learn fast...
 
poobah

poobah

Member
Joined 2005
Big phil,

Measure your voltage, calculate your power, multiply by 2 or 3, get some metal films, and relax.

Carbon comps are junk... they always were. There are a few places were their lack of inductance is valuable, but, certainly not in a bleeder.

Oh, and those are 8200 Ohms gray-red-red = 8-2-[2 zeroes], gold = 5 %

A 1/2 watt would get by 1 Watt much better (assuming 50 Volts)

And don't wire them so close, like they are now. Stand them up a bit with longer leads to keep the heat out of the caps.

As far as that goes... the caps probably stand replacing before the bleeders do.

:cool:
 
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