Screen resistor wattage rating on JMI-era AC15

[fazeka]

Hi all,

Working on my AC15 clone, have a question on the 100-ohm screen resistors.

Referencing the available schematics, I didn't see that these were anything other than 1/2 watt types. I used 1/2 watt here.

Looking at some other builds online, I see that peeps are using 3 watt resistors here...

I haven't powered up the amp yet. Will I burn up those resistors on power up? Or otherwise damage the transformers or amp?

Cheers,
Chris

[pompeiisneaks]

I've always seen at least 3 watt if not 5 watt for screen resistors. In the amp that I and Sluckey built (His is the ac15 and mine was an ac30 using the same basic amp but with 4x power tubes instead of 2x) it used 3W.

The screens can drop a decent amount of current for a 100 ohm resistor in use. normally it's pretty low, as often you'll read them at almost identical voltages as the B+ coming into them. I would love for someone to help me with the math though, because I thought only about 5mA were the norm for Screens and that shows to me as maybe .25W of current?

~Phil

[JD0x0]

FWIW the half watters in Fender blues Jr's routinely blow. Probably why you see the higher rating (because IMO they're needed)
I would personally increase the 100 ohms to a higher resistance for new production tubes, too.

[pdf64]

Power P = current squared (i2) x resistance (R)
So P/R = i2
For a 100ohm resistor to dissipate half watt
0.5/100 = 0.0005 = i2
Hence i = 0.0707
The limiting value of cathode current is 65mA *, so there’s no way the screen grid of an EL84 could draw anywhere near 71mA unless it had shorted to another electrode, hence a 100 screen grid resistor would be fine to be 0.5W.
Preferably an MO type with a flame retardant coating, ideally a fusible type.
A carbon technology resistor type would be a very poor choice.
* https://tubedata.altanatubes.com.br/she ... e/EL84.pdf

Alternatively, use beefier resistors, retain the stock ‘unfused HT’ arrangement, and watch in dismay when a valve shorts and takes out the OT / choke / PT.
That’s a lesson I learned the hard way, but you’re welcome to follow in my footsteps