AC15 screen resistors

[wcrankshaw]

I added a Mercury choke to an AC15, and in the process of doing so swapped out the 47uF for a 16uF filter cap, upped the cathode resistor to 130ohm. My power tubes are red plating. Because of the decrease in resistance from adding the choke, should I increase the value of the screen resistors? They're only 100ohm right now. The choke should be around 130ohm I think, and the resistor it replaced was 270ohm. Any help would be great.

[booj]

for AC15's and AC30's I use the equivilent of #1, #2, or #3 Groove Tubes. I'm talking about the rating system, not the tubes. The reason the tubes in that range are considered "early breakup" types is because it takes less input voltage to get max output. Cosequently they need a smaller negative bias voltage to bias properly. Which means in the end, if you stick 'em in there they'll work without overheating.
Maybe someone else can explain it better, but it works.

[KevinOConnor]

Hi Guys

Cathode-biased amps NEED screen resistors of appropriate values for best protection. The high heat within the tube can cause the screen wires to sag a little and they are then bombarded by more of the electrons streaming to the plate. This causes the screen to over-heat, in turn reducing the internal resistance of the tube, and potentially causing thermal runaway.

Cathode biased amps usually run the tube as hot as possible to get as much power out of the amp as possible. This is determined by the voltage environment and the bias resistor Rk. Rk is made as low-resistance as possible to achieve the highest power. However, designers often hedge too much and make Rk too low. This results in the tube being over-dissipated.

Shared Rk for two tubes does not guarantee the tubes will share the current. The problem is worse with four tubes sharing Rk - a design that should never have happened!

Sharing can be encouraged by adding individual screen resistors. This is the same component you need to eliminate thermal runaway, so one component does double-duty. Low-R values are the "fearful" value - the hedge that leaving the pool of lethargic design oversight might be even more dangerous than clinging to wrong values. So, you often see 100R for EL-84 screens, which basically provides no protection to that tube type whatsoever.

EL-84s need much higher Rg2 values for proper protection compared to other tube types due to their construction and high-gain. 1k-1W is the absolute minimum, and 1k5-2k7 are even better. Most "state of the art" hobbyists and designers use 2k2-5W for each EL-84 screen.

The correct Rg2 will likely eliminate the red-plating that is happening now. You can measure Vrk and calculate Pa for the tubes using Ohm's Law and the power equation.

Simplest thing: put in at least 1k-1W flame-proof for each tube; then make Rk a little higher if needed.

Have fun
Kevin O'Connor