Is a 1R 1W 5% resistor good enough?

[jezzbo]

For meassuring the bias via the cathode?
Or should it really be 1%?
So is a difference from 0,05 Ohm (that's 5% max tollerance) close enough for biasing?

I'm asking this because I do have 1 R 1W 5% resistors (carbon film)
But no 1% resistors (at 1W that is).

btw my digital meter can't measure those resistors very accurate (It measures 1,5 Ohm. Colorcode of the resitor is Brown, black, gold, gold)

My meter gives more or less also 1,5 Ohm for my 1/4W 1 Ohm 1% resistors that I do have.

Thanks for helping me to clear this.

[jaysg]

What does the meter read when you short the leads together? On many meters it will be 0.2 to 0.3 ohms.

[Andy Le Blanc]

Yes and no, you have to keep the tolerance in mind if going to be operating
a device very near to it's absolute maximum, design tolerance is up to you.

Tubes drift with age and voltage condition, 20% device, if its a non-critical
application you can relax your chosen tolerances.

If you going to be taking analytical readings on a guitar amp that has god like qualities
for cloning you'll want to have as tight a tolerance as you can find, even .1%,
but your test gear also has to reflect the same tolerance.

I try to keep my design tolerance low on purpose, but I go with designs that
allow that choice.

[jezzbo]

What does the meter read when you short the leads together? On many meters it will be 0.2 to 0.3 ohms.

thanks for your reply.

The meter reads 00,5 when i short the leads.
so I assume that's not really accurate right?

thanks

[tubeswell]

My meter gives more or less also 1,5 Ohm for my 1/4W 1 Ohm 1% resistors that I do have.

Measure the DC resistance on your meter leads first. Then subtract that from your reading on the resistor(s).

FWIW you can stick a bunch of higher-resistance resistors in parallel to make pretty close to 1R. (e.g.; 4 x 4R or 2 x 2R - at least these are theoretically easier to read than 1R). I prefer to use the MF 1% 0.6W resistors for these biasing resistors anyhow - they are accurate (and small) enough.

For my own peace of mind, I prefer to have as-accurate-a-reading as I can on the biasing, even if I choose then to ignore the reading or use this reading to bias the tubes hotter or colder or whatever. YMMV

[Structo]

As mentioned if you can calculate the error in the reading you can use those.
I would use these. 1 ohm, 1 watt, .5% tolerance.

http://www.mouser.com/ProductDetail/Vis ... dP77TQw%3d

[jjman]

Pick up some 0.5% accuracy 1 ohm 2-3watters. No point in looking to match within a few milliamps if the starting point is unknown and possibly 5% or more off. Does Mouser deliver to you?

71-CPF2-D-1

http://www.mouser.com/ProductDetail/Vis ... ILb72jw%3d


71-CPF3-D-1

http://www.mouser.com/ProductDetail/Vis ... yIcVAyk%3d

[jezzbo]

Pick up some 0.5% accuracy 1 ohm 2-3watters. No point in looking to match within a few milliamps if the starting point is unknown and possibly 5% or more off. Does Mouser deliver to you?

71-CPF2-D-1

http://www.mouser.com/ProductDetail/Vis ... ILb72jw%3d


71-CPF3-D-1

http://www.mouser.com/ProductDetail/Vis ... yIcVAyk%3d

Thanks Tom and JJman.

Yeah Mouser delivers to Belgium. but shippingcosts are 20euro! (25usd)
so that makes these resistors very expensive.

so far I couldn't find a supplier for 1R 1W (or more) 1% (or lower) in Europe.

[Structo]

Somebody told me one time that if you use a 1/2 watt or .6 watt resistor on the cathodes that if the power tube shorts hard to ground that it can blow the smaller resistors thus preventing any further damage.
I don't know if this is true or not, just something I have heard before.

[breakfastbuddy]

1 ohm resistors is not a big resistance so 0.6 watt should be big enough .
i dont know how big voltage they are good for , maybe someone with good calculationskills can jump inn here

[Andy Le Blanc]

fender 100w came with a bias/balance, had either 1/4 or 1/8 w 1r parallel
with a diode. The 1r would cook leaving the diode to conduct to ground.

[breakfastbuddy]

well i want some calculation , how big the resistor have to be.
0.6 watt is 600ma , more than you need for connecting to ground . a singel el 34 usally draw around 25watt and with a voltage off 450volt that gives 25/450 = 0.055 or 55ma then 600ma should be more than enough

[tubeswell]

1 ohm resistors is not a big resistance so 0.6 watt should be big enough .
i dont know how big voltage they are good for , maybe someone with good calculationskills can jump inn here

0.6W Metal Film are fine - I use them all the time, and they readily come in 1%. They have a breakdown voltage of 360V, which you never see anywhere near on a cathode anyhow (unless the tube shorts - and then they potentially blow like fuse - as Structo observed).

[David Root]

I don't want those resistors acting as fuses. That's what the HT and/or mains fuses are for. If one blows it is a pita to fix it in a hurry if you have to.

I use 1 ohm 1% Mills wirewound 5W resistors. They are usually within 0.5%. The beauty of these is they are the same size as a conventional 1W metal film, so they fit nicely. 5W sandboxes are a non-starter in this application, especially if you're doing individual bias.

[Tinkerer]

If you really want higher wattage resisters, Weber sells 1 Ohm 2W 1% resisters, but I don't know what there postage cost would be to you. Here is the link (almost half way down the page):
https://taweber.powweb.com/store/resord.htm

Tinkerer