Beginner question tube amp

[Davebassman]

Hi,

Looking at this diagram, Fender says that the resistors are 1/2 watt. However, 1/2 watt resistors usually have a maximum voltage rating of 350V, and in this amp, I see voltages above 400V. Do I need higher wattage resistors, or am I missing something?

Thanks.

[Helmholtz]

Resistor voltage ratings apply to the voltage between its ends.

[Justin Thomas]

I was very relieved whdn I learned what Helmholtz pointed out above; though, dhen I build, I'll frequently use higher wattage resistors anyway, since a modern 3W is about the same size as a vintage 1W anyway. Andthe price difference is negligible since I'm not a factory.

Jusrin

[Chuck H]

What you stated above is that your VOLTAGE is higher than the rating for resistors typically used (or currently available?) in this circuit. And your question was if you needed a higher WATTAGE rating.?. What you need is a higher VOLTAGE rating. If that comes analogous with a higher wattage rated resistor that is fine. Part specs are available. If any part spec is vague and doesn't include the parameter you need a higher (or high-ish) rating on it almost surely means it has a poor spec for that parameter. Component manufacturers do this ALL THE TIME in the hope that you will ignore what they've selected to omit. Look at the spec for the part you want to use. If the part doesn't include that spec it's almost certainly because it's lower than they want to admit. (<period)
This lesson was tought to me years ago by the likes of Enzo and Juan Fahey. And it's been an indispensable piece of knowledge.

[The Dude]

Just reiterating the point Helmholtz made to avoid confusion: Just because there is say 500 volts on a resistor measured to ground doesn't mean the resistor needs to be rated at 500V, unless the other side of it goes to ground. What matters is the voltage across the resistor itself. A resistor doesn't "know" if it's grounded or it's position in the circuit. It only "knows" what the voltage drop is across it's leads. For instance, in the case of node resistors where one side of the resistor is at 500V and the other side is at say 350V, you only need a resistor rated at 150V.

[Chuck H]

Just reiterating the point Helmholtz made to avoid confusion: Just because there is say 500 volts on a resistor measured to ground doesn't mean the resistor needs to be rated at 500V, unless the other side of it goes to ground. What matters is the voltage across the resistor itself. A resistor doesn't "know" if it's grounded or it's position in the circuit. It only "knows" what the voltage drop is across it's leads. For instance, in the case of node resistors where one side of the resistor is at 500V and the other side is at say 350V, you only need a resistor rated at 150V.

Good! Yes! In the face of my post above this is a critical consideration

[g1]

Also, the AC signal voltage as well as DC across the resistor needs to be taken in to account.

[Helmholtz]

For completeness it should be mentioned that there's an additional voltage limit due to the power limit.
It is found from Vmax = sqrt( R * Pmax).
E.g. a 100k/0.5W resistor must not drop more than 224V, even if it's rated for 350V.

[g1]

For completeness it should be mentioned that there's an additional voltage limit due to the power limit.
It is found from Vmax = sqrt( R * Pmax).
E.g. a 100k/0.5W resistor must not drop more than 224V, even if it's rated for 350V.

I'm not arguing with the formula, but how can they then rate it at 350V? What does the 350V rating mean?

[Helmholtz]

I'm not arguing with the formula, but how can they then rate it at 350V? What does the 350V rating mean?

350V would be the type category (based on construction, materials, dimensions). It's the max. allowed instant and continuous voltage for the complete resistor value range.
But of course the average power limit must be observed as well. And that's where the resistor value gets in, as P = V²/R.
The datasheet may specify an additional (higher) peak power limit.