Minimum screen wattage?

[daz]

What would i want as the minimum wattage for my 1k screen resistors on a EL34 amp?

[MWJB]

5W is the typical value.

[daz]

I know, but would 3 watt be foolish? i'm just asking because i smoked one by accidentally inserting a tube wrong (key broken off) and i don't like those sand types and don't want to send away for 2 resistors at $1 each then $10 shipping. Plus i would like them by today. I have a gig tonite, and while the screen is still in spec, it's blackened and i don't want to trust it at a gig. Luckily the chassis was out when it happened so i was able to see the smoke on startup before it smoked long.

[MWJB]

I guess it might be fine, I know a builder who uses 4W.

[pdf64]

Is this the cathode bias amp? If so, that bias method would serve to regulate/limit excessive currents, along with reducing the screen-cathode voltage (compared to grid bias), so a lower wattage screen resistor would likely be fine.

[KevinOConnor]

Hi Guys

You have to know the peak or average current to be able to determine a good power rating. It is just Ohm's law in the end, but you need some info about the specific amp.

For no-brainer selection, the universal 1k-5W will assure reliable operation of the tube. if you are going into mass production and every penny counts, then you want to know that, say, in a 10W amp 1k-1W is fine and in the 300W amp it has to be 5W. At 50W, if we use the ubiquitous example, then peak Ig2 is 30mAS, at 30V for a dissipation of 900mW. For clipping, this might double, so a 3W part would be fine.

In a cathode biased amp, it is extremely important to have higher screen-stop values to prevent thermal runaway. Grid alignment can become compromised with the high heat of high bias, and screen heating will reduce screen impedance and cause runaway and melt-own. The high screen-stop limits the extent to which any of this can happen. the tube is not going to protect itself in this situation.

Have fun