Hi folks. Really apologize if this one's been covered ad infinatum, but my poor search skills didn't uncover the answer to my question. I'm building a Morrison Micro 45 and need a "ballpark" value on the 45's cathode resistor. I'm thinking it's in the 4K to 6K range, but power resistors in that range seem to be limited in value, and 25 watts seems to be the go-to. I'll fire it up with a variac to preserve my precious stash, but if the voice of experience would weigh in, that'd be great. I've got all the other parameters covered, and this is the missing piece.
You don't like the datasheet numbers?
Dissipation can be figured from I^2*R. The "max" condition figures 2.2 Watts so a 5 Watt part will last for years.
PS: It's a 10Watt tube. Cathode resistor heat will be smaller, by at least Mu. Even at the '45's low Mu of 3.5, that suggests under 3 Watts in the resistor. It will typically be even less than that. The only reasons for a 20-Watt are for looks and so it runs cool enough to not burn skin much. (Though at 56V it is not totally touch-safe.)
If the anode voltage of the paralleled 6sl7 is about 130v and -vg of the 45 is 56v then Rk becomes about 5k2 20 watts. But first check Va of the 6sl7 because that determines the value of Rk.
Thanks. No, I get the dissipation calc, and I appreciate your response. The Morrison Micro is DC-coupled, 6SL7 plate to DHT grid. So the 45 cathode resistor will need to drop a lot of voltage with, say, 36mA across it. So your point about wattage is well-taken. Some other DIYers on another forum pegged the value at about 5K ohms and suggested a Mills MRA-12. A 10 watt trim pot in the 7.5K ohm range would be preferable, but again, tough to find. Thanks again.
OK, I did not see your schematic. (Google returned a lot of guns.)
While the designer can lean this way or that, a reasonable first-pencil has half the B+ on the driver and half on the output tube. Then the cathode resistor and power tube have equal dissipation. Resistor life at rated power may be short, we derate maybe 2:1. So a 10W or 15W tube sure leads to a 25W resistor, as your friends said.
I generally derate resistors 3:1, trying to keep the temperature low enough to avoid overheating nearby components.
Lately I've done direct-coupled designs with a trimpot for the driver's cathode resistor, so its plate voltage can be adjusted for tube aging and/or varying powerline voltages. A bias servo would do the job, but is more complex.
The Lipman amp just takes the driver load resistor from the output tube's cathode so it's self-adjusting; the Loftin-White used an array of resistors for similar effects (among other purposes).
Lately I've done direct-coupled designs with a trimpot for the driver's cathode resistor, so its plate voltage can be adjusted for tube aging and/or varying powerline voltages. A bias servo would do the job, but is more complex.
The Lipman amp just takes the driver load resistor from the output tube's cathode so it's self-adjusting; the Loftin-White used an array of resistors for similar effects (among other purposes).
+1
3:1 .. 4:1
BTW some wirewound resistors -for example Mills- prone to overheating (burning, and sometimes elongating) at fraction of declared power.
MRA12 isn't exaggeration for 3W power, even if it's cooling with flowing air.
In these positions I usually use high wattage Welwyn resistors (for example W24).