In vintage tube amps repairs or clones I've always used 1/2 carbon comps where called for, never gave much thought to the voltage rating as I was doing "like for like". My supply of NOS is finishing, I'm not going to bother with modern carbon comps, the tolerance is sloppy and the leads are too short anyway for repairs or building ptp. I'm looking at the KOA Speer CF carbon films, rating is 350V, is this the max voltage drop across the resistor or the value it'll see on the side feeding it?
Some companies don't give a max rating telling you to use Ohms's law for whatever resistance value you are using but as a general rule when buying batches can I assume 350v rated 1/2W resistors are fine for amps below 500V on the output tube plates, your typical Vox, Fender, Marshall, Dynaco etc?
BTW I do use 1W or 2W in the power string and will use 1W for cathode followers or on the tail of a phase inverter, and yes I should just use 1W carbon film all around but I want to stock up on 1/2s as I also play with ss and I want to try the KOAs which have a good reputation, Mouser, Digikey etc don't offer 1W KOA carbon films in many values.
Some companies don't give a max rating telling you to use Ohms's law for whatever resistance value you are using but as a general rule when buying batches can I assume 350v rated 1/2W resistors are fine for amps below 500V on the output tube plates, your typical Vox, Fender, Marshall, Dynaco etc?
BTW I do use 1W or 2W in the power string and will use 1W for cathode followers or on the tail of a phase inverter, and yes I should just use 1W carbon film all around but I want to stock up on 1/2s as I also play with ss and I want to try the KOAs which have a good reputation, Mouser, Digikey etc don't offer 1W KOA carbon films in many values.
across the value.
On the high quality resistor on datasheet you can find the limit; in every case it is better to use a bigger wattage (for the same family)
See if you can find a voltage rating for a 100R 1W resistor that says 350v or 250v or 100v or 50v.... and you'll have your answer if it's the insulation value or the voltage/power rating.
Resistors have a voltage rating across their terminals, which can sometimes be less than the voltage that would exceed their power rating. Maybe surprisingly, some old-fashioned types can have higher voltage ratings than modern film types. I see failures in metal film resistors of good brand and high values at low operating temperatures but high (100 - 200 VDC) voltages even in prestige brand products. Symptoms are intermittent noise and rarely failure.
For replacements, just be extra conservative. Parts are cheap; your time and reputation are valuable.
All good fortune,
Chris
For replacements, just be extra conservative. Parts are cheap; your time and reputation are valuable.
All good fortune,
Chris
for guitar amps, the optimal resistor types nowadays seem to be:
Power Supply: Metal Oxide - High heat coefficient: behavior stays consistent over a wide temp range. 1 or 2 watt
Signal Path, Grid Leaks: Carbon Film - low noise at low voltage 1/2w
Plate Load: Carbon Comp for best sound; Carbon Film, budget choice. 1/2w may be OK, switch to 1w if noisy.
K bypass: Metal Film for Preamp Tubes (1/2w), Wire-wound (Non-Inductive if availaible) for Output Tubes (5w+)
I'm just regurgitating, any EE types welcome to chip in.
Power Supply: Metal Oxide - High heat coefficient: behavior stays consistent over a wide temp range. 1 or 2 watt
Signal Path, Grid Leaks: Carbon Film - low noise at low voltage 1/2w
Plate Load: Carbon Comp for best sound; Carbon Film, budget choice. 1/2w may be OK, switch to 1w if noisy.
K bypass: Metal Film for Preamp Tubes (1/2w), Wire-wound (Non-Inductive if availaible) for Output Tubes (5w+)
I'm just regurgitating, any EE types welcome to chip in.
The rating also covers the voltage seen between the resistor and anything physically near it. A 350 volt resistor that is touching the chassis should not have more than 350 volts on either end. "Gotta keep em separated" in a PTP build.
This is especially important when stuffing a PC board that has a ground layer under some or all of the parts. Somewhere I have a board with a chunk of that ground plating gone from a fireball that erupted from one end of a resistor to ground. It was a 2 watt feedback resistor connected to the plate of an output tube. Remember that the plate of an output tube can see twice the B+ voltage in normal operation. I have measured 2400 volts on the plate of the output tubes in a guitar amp that was cranked well into clipping while driving a guitar speaker at or near resonance.
This is especially important when stuffing a PC board that has a ground layer under some or all of the parts. Somewhere I have a board with a chunk of that ground plating gone from a fireball that erupted from one end of a resistor to ground. It was a 2 watt feedback resistor connected to the plate of an output tube. Remember that the plate of an output tube can see twice the B+ voltage in normal operation. I have measured 2400 volts on the plate of the output tubes in a guitar amp that was cranked well into clipping while driving a guitar speaker at or near resonance.