Rating of pots as variable resistors

[jeff]

I've seen schematics where the 470 ohm 5W cathode resistor is replaced with a 1K 5W pot for fine tuning the bias. Replacing a 5W resistor with a 5W pot seem to make sence at first, but as a varible resistor set to 500 ohms isn't it only now a 2.5W resistor?

Maybe 2.5W's ok in this case but the point is, when set up as a variable resistor a 5W pot is not a 5W resistor where ever it's set, right?

So if you replace a resistor with a pot used as a variable resistor and the pots value is bigger than the resistors value by X then the rating of the pot should also be X times bigger than the original resistors value.

Use a 10W 1K pot to replace a 5W 500 ohm resistor, right?

[rzenc]

Regarding W diss, I would use a higher W to avoid trouble..
However, If it's a power tube cathode resistor it would be a sure shot to trouble if by accident turns the pot to zero resistance... Power tube will redplate and melt. A minimum biasing resistor must be on the circuit for protection.

Rzenc

[kagliostro]

+ 1 for Rzenc

A minimum biasing resistor must be on the circuit for protection.

also

someone told me that he do it with an adequate resistance pot in parallel with the cathode resistor

the most of the current will pass through the resistor that is very lower in resistance in respect to the pot

so a normal pot can be used to have some fine regulation

as many say the fine regulation of the cathode resistor in cathode biased final tubes isn't a must

however may be, if you are looking for a solution with a single resistor for each tube that this can be a way to achieve it

(you must have always a resistor in series to the resistor prevent bias absence and one in series with the pot to prevent a too low value that will be a way for high current to go across it)

Kagliostro

[jjasilli]

Ditto. 

What is the purpose of this circuit?

[jeff]

All good points.

What is the purpose of this circuit?

I think this guy wanted to be able to adjust for 6V6 6L6 EL34 etc. Not the best way to do it. But it got me thinkin'. Just pointing out the rating of a pot set as a resistor VS the rating of the pot not really the circuit.

[PRR]

> a 5W varible resistor set to 500 ohms isn't it only now a 2.5W resistor?

Yeah.

Although tube current will not rise as fast as ohms falls, you sure can toast a marginal reostat this way.

Note that a 12W tube like 6V6 is unlikely to put over 1/10th of that in the bias resistor, 1.2W or so. So if 5W fixed was selected casually, a half a 5W pot might survive.

If your first-guess isn't way-off, taking half of that amount as a fixed resistor reduces the abuse in the variable resistor _and_ avoids accidental instant melt-down.

A string of 50-100-100-100-100-100-500-1 ohms and a jumper-clip will bias-up any of the common tubes in a few tries and give you a constant 1-ohm for current check. Find your optimum and then order the right value.