Rocket build: Help sourcing a cathode resistor.....

[Paultergeist]

Greeting!

This should be fairly easy (I hope). Really, my question is in two parts, and it comes down to my need to order a cathode bypass resistor for a (4) x EL84 Rocket-inspired build.....

First, I notice that the layout calls for a 50 Ohm 25-watt wire-wound power resistor. Is the 50 Ohm value usually about spot-on for a quad of EL84s? I ask this because -- in previous builds -- it was the value of the power tubes' cathode resistor which I adjusted to set the bias of the amp. This was not such a big deal with the cheaply-available 10-watt cermet resistors (got a few of those to try), but these 25-watt wire-wound ones are a bit more pricey. If 50 Ohm is usually *the* value for the Rocket, then that is what I'll order, but I thought I would flush out the question a bit more first.

Secondly, my more user-friendly amp supply websites tend to to stop at 10 Ohm resistor ratings. I am having some difficulties navigating the Mouser / Digikey catalogs due to the vast offerings. Can anyone help with a link or source for the 25 watt resistors, such that I can get one ordered?

Thanks!

[Blackburn]

http://www.mouser.com/ProductDetail/Arc ... OiZCFGc%3d

[johnnyreece]

For better or worse, I used a 10W resistor on mine. It hasn't exploded yet. Here's a thread on it. Definitely not for the purist, though!

https://tubeamparchive.com/viewtopic.ph ... ght=rocket

[redshark]

I've been thinking about changing this resistor in my personal rocket to a 60ohm 25W wirewound resistor intead of the 50 ohm.
That would make the amp run slightly cooler!

[Paultergeist]

http://www.mouser.com/ProductDetail/Arc ... OiZCFGc%3d

That's the one! Thank you!

[Paultergeist]

For better or worse, I used a 10W resistor on mine. It hasn't exploded yet.

That might be another way to go: get the amp up-and-running (hopefully soon) with a 10 watt resisitor just to get it started and measure the bias.....then order the 25-watter for the final product....

[Firestorm]

The thing to remember about the Arcols is that the power rating is based on mounting to an appropriate heat sink (which for the 25W units is supposed to be 535 square centimeters by 1mm (!)). If they aren't heat sinked, the 25 watters are only rated for 9W.

[Paultergeist]

The thing to remember about the Arcols is that the power rating is based on mounting to an appropriate heat sink (which for the 25W units is supposed to be 535 square centimeters by 1mm (!)). If they aren't heat sinked, the 25 watters are only rated for 9W.

Interesting........I did not know that.......

Maybe I'll start out with the 10-watt cermet, calculate the current draw/drop in voltage, and see how much dissipation that cathode resistor really needs to be able to handle......

[vibratoking]

...If they aren't heat sinked, the 25 watters are only rated for 9W.

It's even worse than that. Rated for 9W without heat sink at 25°C.

[Paultergeist]

Really interesting stuff......I appreciate all the information.....I have to admit that I have always thought that the gold-colored aluminum body of those wire-wound resistors looked cool, but I would have to de-value the heat-dissipation capacity of one based on (a) not having a sufficient heat-sink and (b) hot temperatures inside the amp chassis.

In comparison, the ceramic (cermet?) [white] resistors seem a lot less fussy. No heat-sink needed, no heat-conductive compound, tolerates pretty high operating temperatures (~ 155 C). I am thinking (2) 10-watters in parallel should be fine.......(plus I probably already own them...)