help with resistor voltage rating

[mikepukmel]

Hi All
I had read posts where people said to make sure the resistors have adequate voltage rating, e.g. 500v for tube amps. I started shopping again today, since my chassis arrived, but I can't find 500 or 600 v rated metal film.

These:

http://www.mouser.com/Xicon/Passive-...h7zb?P=1z0zls8

and every other 1/2 watt metal film I can find, those with ratings are 350v working, 700v pulse. Most of the geeky tube amp parts sites don't even list a voltage rating, just wattage.

Throughout most of the preamp, up to the power tubes, looks like 350v is fine, but behind the 6V6's (this is a Deluxe black face clone), definitely over 400v, maybe 450 depending on load.

Anyone know of a source for higher voltage rating resistors for the power supply and behind the power tubes?

Thanks!!!
Mike

[catalin gramada]

Resistors should be voltage rated in respect with voltage droped over resistor,so you don.t need a 600v resistor if it drops 100-200V. Good quality resistors are need it for plates, specially under big currents. Poor quality resistors will do annoing crack noise under big currents. Power rating is important to not drift under thermal derive, so double is better

[mikepukmel]

Resistors should be voltage rated in respect with voltage droped over resistor,so you don.t need a 600v resistor if it drops 100-200V. Good quality resistors are need it for plates, specially under big currents. Poor quality resistors will do annoing crack noise under big currents. Power rating is important to not drift under thermal derive, so double is better

Ahh OK thanks! So, most resistors as far as I can tell, only drop a few hundred volts max, most much less. I had looked at a few amp build photographs, and most were 1/2 watt, but a few 1 watt that didn't say so on the schematic. Reading some more parts lists on the Mouser web site, it looks like the voltage and power ratings go together, is this always true? I mean, you can't get 700v rated 1/2 watt. The 700v resistors are all 1 watt. But only need a few of these, those around the power supply and powering the power tubes.
Thanks for the tips, Catalin!

[mikepukmel]

Resistors should be voltage rated in respect with voltage droped over resistor,so you don.t need a 600v resistor if it drops 100-200V. Good quality resistors are need it for plates, specially under big currents. Poor quality resistors will do annoing crack noise under big currents. Power rating is important to not drift under thermal derive, so double is better

Catalin, there's a Q&A on AikenAmps that says they use 2w metal film, this OK for the plates?

[catalin gramada]

Ahh OK thanks! Reading some more parts lists on the Mouser web site, it looks like the voltage and power ratings go together, is this always true?

No. it is not. You can find 600-800v rated resistors in 1W format. But usually power resistors are high voltage rated. Better to check first

[catalin gramada]

Catalin, there's a Q&A on AikenAmps that says they use 2w metal film, this OK for the plates?

It.s a little bit hilarious to ask a newbie diyer to confirm what Mr. Aiken stated . But Yes,Best

[J M Fahey]

350v working, 700v pulse

Then why worry?
The preamp supply will hardly have more than 350V , with fully saturated tube you might lose around 40V so *peak* might be around 310V tops , and idle ("working") will usually be between 100V and 160V , so ALL well within rating.
And we are talking 1/2W resistors here, so 1W and even more 2W will be beyond the call of Duty he he .

There is another point: official ratings must apply to the harshest conditions and for good measure amply surpass that, double so in trigger happy lawyers USA.

I think they mean those resistors will safely stand intermittent but often applied 700V DC if used as, say, power supply bleeders, or 700V peak mening a 500V RMS transformer winding .... and under a wide range oftemperature and humidity (which are rated in the full datasheet).

But inside a preamp, job is way milder
Even if the voltages are there, resistors are in series with a tube which limits how much current will pass, usually in the order of 1 mA.
And if tube shorts, only then will they get the, say, 350V rail voltage.
Which they will usually stand with no big trouble.

[SoulFetish]

Juan covered some good ground here. Personally, I use 2W 500V+ resistors throughout. It's not required in all applications but it does give me flexibility in build/repairs. And protects the components against shorts and surges. So I sleep easy. But two main reasons are, they tend to.have heavier guage leads which are more mechanically robust and, I think, nicer to use.
Plus, they're so friggin cheap, I can buy a bunch and be done with it! Dude, I just bought some carbon film and metal film 500V. Go to the parametric search and specify the voltage/power/material you want and they'll show up. If, by some Bizzarre chance, mouser doesn't have what you're looking for...
Newark.com
Alliedelec.com
Digikey.com
Octopart.com

[mozz]

If you are shopping Mouser as i usually do, buy 2 or 3 watt for the power supply nodes, metal oxide. The 1 watt carbon or metal film are good for plate resistors, 1/2w for generally everything else.

[mikepukmel]

It.s a little bit hilarious to ask a newbie diyer to confirm what Mr. Aiken stated . But Yes,Best

so many amp sites and blogs. These old Fenders used 1/4 watt in places that the new amp builders say "NEVER DO THAT!". Ive had 30-40 year old amplifiers with the original resistors, that work fine. So, Im not sure if the new amp builders are being way too careful, or newer components are just not as good or ...

[mikepukmel]

If you are shopping Mouser as i usually do, buy 2 or 3 watt for the power supply nodes, metal oxide. The 1 watt carbon or metal film are good for plate resistors, 1/2w for generally everything else.

Thanks Mozz.