Quick Express question

[labb]

Resistor R25 in the PS. What is the rationale and math that it is 25 watt?

Thanks.

[geetarpicker]

You might do some searching here as I seem to recall a long thread on this some time back. That said, the one in my original get's pretty darn hot when you crank the amp for a while. Hot enough that when I built my pair of clones I made sure the wires running underneath the resistor did not touch it, just to make sure nothing got melted!

[paulster]

Either reliability because it still runs really hot when the amp is cranked, or because that was what Ken had on hand that particular day. Or both.

[Cliff Schecht]

Even if that resistor only dissipates 5W (didn't calculate this) in a quiescent state, the highest peak power required is usually much higher than this. I think the 25W is just to guarantee that the resistor doesn't ever overstress. Sand cast ceramic resistors do heat up quite a bit and can only take so much heat before they crack. The fact that the resistor heats and cools often (i.e. growing and shrinking) means that overrating it would prevent it from heating up and cooling down so much that it actually cracks itself.

[gearhead]

Search feature is kinda funny; need to include "and" between words to get a google-ish search.

For this one, try "25w and resistor and current" to get a readable set of posts.

[labb]

Thanks gearhead..now I see why I could not find anything using "search" function...

[bigbeck]

Resistor R25 in the PS. What is the rationale and math that it is 25 watt?

Thanks.

I used a 10 watt in mine. It does get rather warm but that's normal in a power supply. I measured R25 and it dissipates 3.6 watts at idle and 4 watts at full volume while slamming bar chords. Based on my figures a 6 watt resistor would be "safe" in that amp. Since all I had were 5 & 10 watt resistors..................... I just used what I had on hand. Maybe all that Ken had on hand were 5 & 25 watt resistors? 5 watt would work but that's just cutting it too close for me.

My B+ is 390V. If yours has a much higher B+ then R25 will dissipate more power. You would really need to measure it while the amp is operating.

Just use ohms law to find current (amps) then power (watts). You first need to measure the voltage drop across the resistor.

[labb]

"Just use ohms law to find current (amps) then power (watts). You first need to measure the voltage drop across the resistor."

The Schematic shows 15 Volt drop across a 1K resistor..That gives me 15/1000= .015 amps. P=VI=15x.015=0.225 watts..What am I missing?

[bigbeck]

You did that correctly. Many people,(myself included) like to "oversize" the resistor by 100%.

My R25 is dropping 16 volts so I'm dissipating 2.6 watts at idle,not 3.6 watts as I stated earlier. It was a typo .

Remember,different tubes draw different amounts of current. And the current increases at full volume - but not by much.