correct value cathode resistor for an 18 watt rocket

[wsaraceni]

im an idiot. ordered a 50 ohm. do i need a 100 ohm or 25 ohm?

[rock_mumbles]

That will depend on the voltages you are running ... with the actual plate voltage over 310V in a modern circuit you'd typically start with a 150 ohm resistor.
You may need to experiment with resistors to bias the tubes properly. In a cathode biased 2xEL84 Liverpool fishy built he used a 150 ohm cathode resistor. He was using a 550V @ 100mA center tapped power transformer.

It will also depend on the tubes you use, JJ's are pretty picky about being ran at or below maximum dissipation or they fail. Soviet NOS tubes; 6P14P, and variants can be ran harder and not fail because they are rated for 14 watt dissipation.

[wsaraceni]

thanks. i guess i'll order some new resistors. the transformer is a 290-0-290

[rock_mumbles]

thanks. i guess i'll order some new resistors. the transformer is a 290-0-290

You'll need at least a 150 ohm maybe even a 170 ohm ??? you'll have to measure voltages and calculate the dissipation once built.

[wsaraceni]

ok. i'll order a few in both of those values. those nice shiny metal resistors with the heatsink on them arent cheap. i'll order a few different values to have around

[wsaraceni]

luckily i have a 150R 5w or two. so i'll start with that. this is what the 18 watt lite iib calls for with the same transformers so hopefully that will work. i'll start there. i have jj tubes to test, and if it sounds really nice. a set of mullard el84s to go in there. both ez81 rectified

[wsaraceni]

what's the minimum wattage you should be able to use there?

[sliberty]

I have used 130 ohm cathode resistors on several AC15 builds. Remember, you want these EL84s really cooking - maybe 90-95%.

[rock_mumbles]

I think most of the time in a Vox/Matchless amp the tubes are ran over 100% dissipation ...

A 5watt resistor is fine for the EL84 cathode.

I'd plan ahead and be prepared to swap out the cathode resistor to get the bias where it needs to be after you've got it together and ran it for a while.

[Markusv]

If you can run a liverpool with a 50 ohm resistor- why can't you go 100 or 120 in an 18 watt?

[fishy]

If you can run a liverpool with a 50 ohm resistor- why can't you go 100 or 120 in an 18 watt?

I think it's the Rocket which uses the 50ohm. A Liverpool is biased a little cooler. IIRC, in this build, the PT is higher than a Rocket or Liverpool usually are so the resistor might need to be adjusted accordingly.

[M Fowler]

You can go 120R.

I usually start with 130R/10w on mine.

Mark

[wsaraceni]

i'll take voltage readings tonight. anyone have a link for a good chart that i can fill in and make sure i don't miss any specific voltages?

[ampgeek]

Have a look in the Trainwreck Files section. IIRC each model has a voltage recording chart (Excel format?) that ought to get you very, very close.

Cheers,
Dave O.

[ampgeek]

https://tubeamparchive.com/viewtopic.php?t=5684&start=0

Four posts down from the top for the voltage chart.

Dave O.