5E3 Cathode resistor value

[Urshurak776]

Hi Guys,

I have everything for my 5E3 build except speaker and transformers. I was going to start laying it out and began inventory of components. Somehow I ordered a 270 OHM 5 watt resistor instead of the standard 250. Not sure what I was thinking. Is that a big deal? If so, I’ll just order a 250. I did do a cursory search but sounds confusing and haven’t really had a chance to dig into it yet. Looks like 270 will bias the tubes colder. I guess that’s okay?

Thanks!

 

[FenderLover]

If you don't need to order anything else, I'd just use the 270. No one will know but us, and a little bit cool is just that - a little bit. If you're super Clone-Conscious, you'd better get a 250 on order right away.

 

[Urshurak776]

Thanks @FenderLover !

I need to order transformers and a speaker, so I’ll grab one at that time. Appreciate it.

 

[flyswatter]

Resistors have a tolerance (determined by the band at the end: gold for 5%, silver for 10%; no band = 20%), and if you test them on the ohm meter you'll often find them a little off value anyway. I always try to match values exactly (I test each resistor on the meter before installing it in a build), but using a value that's off by 20 or so ohms rarely makes a huge difference.

 

[jman72]

When I built my 5e3, I initially used a stock 250 ohm cathode resistor, but the voltages were high and my tubes redplated immediately. I switched to a 300 ohm resistor to lower the voltages a bit and it's been running great for the past 4 years. I think the 270 would be a fine in-between value to use.

 

[Old Tele man]

HINT -- a resistance value can always be pulled lower by adding a correctly sized "shunting" resistor in parallel; in your case, that shunting resistor value will be about 3.3K ohms.

R = (Ra*Rb) / (Ra+Rb)

R = (270*3300) / (270 + 3300) = 249.6 ≈ 250 ohms

 

[Jsnwhite619]

My Mojo parts kit came with a 270 ohm. I did the calculator on it one day, and it's not running nearly as high of a bias as it could - 370v B+ - but it is plenty loud enough and dirty enough as is. I've tweaked and played with the front end, but I've never messed with that resistor, so I don't have experience outside of the 270 ohm.

 

[Urshurak776]

Thank you everyone. Appreciate the knowledge.