1/4 watt resistors... no go?

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1/4 watt resistors... no go?

Post by pompeiisneaks »

I just accidentally ordered quite a few 1/4 watt resistors for a dumble #124 Overdrive Special I'm building (not all, just some). I'm pretty sure in many areas that's not going to be a 'big' deal, like cathode areas, or areas with really high resistance like 10M or 22M etc, as I get how the higher the resistance, the less the wattage it lets through (due to less current allowed), but is there a good way for me to easily know if I'm in a 'ok' zone for that, wihtout needing to calculate the voltage drop, and do that old I = V/R type thing to be 'really sure'?

I know most amps use 1/2 as the standard because that works for almost all use cases except power dropper resistors etc, (all of those are right btw)

Or should I just cut my losses and re-order all in 1/2 watt?

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xtian
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Re: 1/4 watt resistors... no go?

Post by xtian »

As you say, 1/4 watt resistors will work in many places, but they don't look very robust, so your "wow" factor is in the can. :)
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drew
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Re: 1/4 watt resistors... no go?

Post by drew »

If you're talking about Dale milspec resistors, see http://ampgarage.com/forum/viewtopic.php?f=4&t=9882
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Re: 1/4 watt resistors... no go?

Post by pompeiisneaks »

Nah just standard carbon film types. Here's a comparison of 1/2(top) vs 1/4 (bottom):

Front of the data thing, back showing size comparisons
IMG_20171214_190530.jpg
IMG_20171214_190549.jpg
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Re: 1/4 watt resistors... no go?

Post by 10thTx »

I definitely would order the 1/2 watt. Building a Dumble clone is ALOT of work. I'd do it right.

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Re: 1/4 watt resistors... no go?

Post by Stevem »

You do not want to apply more then 300 volts to a 1/4 watter on average!
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Re: 1/4 watt resistors... no go?

Post by pompeiisneaks »

Thanks, I'll look at where the ones are that I screwed up, and if they're in a lower voltage area, think about using them, if not, I'll order the half watters

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Re: 1/4 watt resistors... no go?

Post by R.G. »

I did a design-from-scratch guitar amp back in about 2006. I used entirely 1/4W carbon film unless the specific dissipation was larger than 125mW. Of course, doing the design, I had to know that currents flowed and such, so I did know P = I-squared-R for them, at least where I could not tell by inspection that it wasn't necessary to calculate.

Resistors do have voltage limits. Their datasheets say what these are, and you can find datasheets on the internet easily. Mouser for one makes the datasheet be a simple click to see on the selection pages. Mostly, 1/4W resistors have 200V to 300V limits. Quite a few 1/2W resistors have 300V limits too. You gotta look.

But the decision is generally much easier than laboriously calculating everything, which I detect that you're not wanting to do. :lol:

For the preamp stage, you almost universally use less than a 300V supply, not least because the voltage limits on the 12AX7 are around 300V. So you're generally good on the voltage limit on the preamp simply because there is some voltage across the tube, or it won't amplify, and that voltage comes off the plate resistor.

If you have a 200V supply, and 90V on the plates of your preamp, it's pretty easy to say P=Vsquared/R for a 100K resistor and find out the real dissipation. In fact, let's just do it.

Call the worst case preamp tube plate resistor 150K, and the power supply and tube bias such that you have 150V across the plate resistor. The power is then P = 150 * 150 / 150K = 0.150W. Hmmm. That's only 25mW higher than the rough-and-ready guide of letting resistors dissipate half their rated power. I'd chance that one and use a 150K resistor with 150 V across it.

Actually, you could program a calculator - or just memorize the few keystrokes - or use a spreadsheet, which I often use as a programmable calculator - to relate a resistor's *real* disssipation limit to the DC voltage you'll allow across it.

So for a 1/4W resistor, the biggest DC idle voltage to allow is V = SQRT(P * R), or SQRT(0.125*R) for a 1/4W. The nice thing about spreadsheets is that you can have them automatically make you a table of the allowable DC voltage across **every** EIA preferred resistor value at the same time. That converts a tiresome task on a calculator to a few minutes on a spreadsheet figuring calculations once and having them forever.
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Re: 1/4 watt resistors... no go?

Post by pompeiisneaks »

That's actually outstanding information and I didn't even think of it... duh. Thanks!

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Re: 1/4 watt resistors... no go?

Post by johnnyreece »

R.G. really has a point. I use Excel so often for things I would normally use a calculator for, because it's just better. That, and, many times using conditional formatting is helpful as a visual aid (red = bad, yellow = caution, green = good). I only find myself using a calculator for simple calculations anymore because spreadsheets are just too dang easy.
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Re: 1/4 watt resistors... no go?

Post by dreric »

I'm pretty sure that 1 watt resistors are a big part of the Dumble mojo. 183 is mostly 1/2 metal films.
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Re: 1/4 watt resistors... no go?

Post by pompeiisneaks »

dreric wrote:I'm pretty sure that 1 watt resistors are a big part of the Dumble mojo. 183 is mostly 1/2 metal films.
You're thinking the higher watt resistors change tone? How?

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Re: 1/4 watt resistors... no go?

Post by martin manning »

R.G. wrote: Fri Dec 15, 2017 5:39 pmActually, you could program a calculator - or just memorize the few keystrokes - or use a spreadsheet, which I often use as a programmable calculator - to relate a resistor's *real* disssipation limit to the DC voltage you'll allow across it. ... The nice thing about spreadsheets is that you can have them automatically make you a table of the allowable DC voltage across **every** EIA preferred resistor value at the same time.
Too many numbers to cover all possible combinations of values and wattage ratings. You could make something like like the chart below, though, and a second version for the current limit.
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Re: 1/4 watt resistors... no go?

Post by Stevem »

For the few times I do mods or builds with carbon comp types I always over watt them just for the sake of how it seems to help with noise hiss levels especially as these resistors age!

The heftier leads on 1 watt and up resistors also help with vibration / microphonic issues in loud combo amps !
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Not screaming like the passengers in his car!😊

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Re: 1/4 watt resistors... no go?

Post by M Fowler »

pompeiisneaks wrote: Sat Dec 16, 2017 2:54 am
dreric wrote:I'm pretty sure that 1 watt resistors are a big part of the Dumble mojo. 183 is mostly 1/2 metal films.
You're thinking the higher watt resistors change tone? How?

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Last edited by M Fowler on Sat Dec 16, 2017 11:47 pm, edited 1 time in total.
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