Volume/Gain pots - Lowest reasonable wattages and Fake Law Log?

[SamRE]

I have a build idea in mind (long-term, still need to finish rebuilding my Eminar) for a cathode-biased amp w/ a Supro/Watkins 18W style preamp and a Bad Cat Hotcat power amp w/ some 6CA7s I have on hand.

I am hoping use some slide pots for the Volume, Tone and Trem depth controls for aesthetic purposes, but i'm concerned about 1/5W and 1/8W pots being too low wattage for a valve amp?

The highest wattage ones that are reasonably available are the 45mm Bourns 1Meg PTA Linear sliders at a 1/4W, but I'm after 500k Logs which are only 1/8W.

Can I reasonably get away with an 1/8W pot, or do I need to look into how I can fake a 500k log w/ a 1meg lin?

[pdf64]

P=VI=(VxV)/R
So for P= 0.125W on a 500k pot, VxV=0.125x500000=62500
So V=250V
The signal V AC level can't feasibly reach even half that, about a third seems more likely.
So a 1/8 or 1/10W pot should be ok in that regard.

[Snicksound]

There's not supposed to be any DC on those pots, power is a non issue.

The reason we tend to like bigger higher power pots is they are typically lower noise

[Stevem]

A big yup to the above.

It helps noise / hiss just like using a metal film 2 watt plate load resistor in place of the normal 1/2 watt.

Also how are you planing to cut out a chassis for slide type faders?

[SamRE]

A big yup to the above.

It helps noise / hiss just like using a metal film 2 watt plate load resistor in place of the normal 1/2 watt.

Also how are you planing to cut out a chassis for slide type faders?

Yeah that was part of the concern, I felt there might be a bit much noise for a 1/8W pot.
I have a router with an Alu cutting bit and could mark out the path with the PCB faceplate.

[SamRE]

There's not supposed to be any DC on those pots, power is a non issue.

The reason we tend to like bigger higher power pots is they are typically lower noise

1/8W gonna be a fair bit hissy ya reckon?

[nuke]

1/8W gonna be a fair bit hissy ya reckon?

No discernible difference in hiss.

Hiss is thermal noise, a product of the resistance value and the temperature around the resistor. The size and material are not a factor, only the resistance and the ambient operating temperature.

Current noise is related to the resistor's material and construction and occurs when a current flows through the resistor. Larger wattage carbon comp resistors used as plate loads, or carbon film or metal film resistors, reduce total amplifier hiss from reduced current noise.

With pots and sliders, sometimes bigger construction means the wipers are larger and have less mechanically-induced noise when turned (or slid).

[martin manning]

There's not supposed to be any DC on those pots, power is a non issue.

For a pot fed directly from the plate of a gain stage through a coupling cap, the voltage could be around 100V RMS. So for a 500k-1Meg pot, dissipation is 10 - 20 mW... 1/100 - 1/50 W

[Snicksound]

There's not supposed to be any DC on those pots, power is a non issue.

The reason we tend to like bigger higher power pots is they are typically lower noise

1/8W gonna be a fair bit hissy ya reckon?

Doesn't sound like you're building something high gain, I doubt it will be an issue.

Even then, I've had an SLO circuit on my bench, using 1/5W pots, even modified for higher gain. Wasn't an issue.

[SamRE]

There's not supposed to be any DC on those pots, power is a non issue.

The reason we tend to like bigger higher power pots is they are typically lower noise

1/8W gonna be a fair bit hissy ya reckon?

Doesn't sound like you're building something high gain, I doubt it will be an issue.

Even then, I've had an SLO circuit on my bench, using 1/5W pots, even modified for higher gain. Wasn't an issue.

Perfect, thanks.

[SamRE]

1/8W gonna be a fair bit hissy ya reckon?

No discernible difference in hiss.

Hiss is thermal noise, a product of the resistance value and the temperature around the resistor. The size and material are not a factor, only the resistance and the ambient operating temperature.

Current noise is related to the resistor's material and construction and occurs when a current flows through the resistor. Larger wattage carbon comp resistors used as plate loads, or carbon film or metal film resistors, reduce total amplifier hiss from reduced current noise.

With pots and sliders, sometimes bigger construction means the wipers are larger and have less mechanically-induced noise when turned (or slid).

Thank you, I should know this but the sanity check really helps.