1K 25w resistor... Aluminum Chassis mount ok?

[johndandry]

Building a Express style amp. For the sake of space, can I use one of those 1k 25w Aluminum wirewound chassis mount resistors instead of the ceramic? Not really going for an exact clone. Thanks

[martin manning]

Should be fine.

[Littlewyan]

As long as it's mounted to the chassis or a decent size heatsink it should be fine. 25W may seem excessive, but it gets pretty hot when playing through the amp. Unfortunately I've lost my notes as I did work this all out at one point. It might even be in an old thread on this site.

https://static.rapidonline.com/pdf/62-8128.pdf

The datasheet will specify the correct heatsink size.

[pompeiisneaks]

I thought I'd heard that if mounted directly to the chassis, that's generally enough heat sinking for most things, since the chassis is huge. Is that not correct? Of course you may want to check how hot the chassis gets and make sure it's not accessible by users directly when in use so they don't burn themselves ;D

~Phil

[sluckey]

Building a Express style amp. For the sake of space, can I use one of those 1k 25w Aluminum wirewound chassis mount resistors instead of the ceramic?

Sure. That resistor is only dissipating .225 watts, so you could even use a 1/2W resistor and save even more space. Or feel comfortable and use a 1K/3W resistor. That 25W spec is waaayyy overkill.

[Roe]

Building a Express style amp. For the sake of space, can I use one of those 1k 25w Aluminum wirewound chassis mount resistors instead of the ceramic?

Sure. That resistor is only dissipating .225 watts, so you could even use a 1/2W resistor and save even more space. Or feel comfortable and use a 1K/3W resistor. That 25W spec is waaayyy overkill.

perhaps at idle yes but not at full power

[Littlewyan]

At full power that resistor dissipates at least 10-15W (from memory). The 25W Cement resistors get quite hot, so it's not an overrated part

[sluckey]

Yes, I just quickly used the numbers from the schematic, overlooking the full power factor.

Most engineers would use a power resistor that's rated twice the power that will actually be dissipated. This is a safety factor. With that in mind, a 1K/25W (derated to 12.5W for safety) could easily handle 112mA. That's a lot of screen current!

[martin manning]

That's a lot of screen current!

Express is running the load line well below the Vg1 = 0 knee.

[katopan]

Fully cranked I've measured 74V across the 1K resistance which equates to 5.5W. But it's mounted inside a chassis and with the 2:1 safety factor that's been mentioned a 10W is marginal. Also if you look at the temperature rise info for those resistors, a 10W will get really hot hanging in free space where the 25W is OK (still hot to touch but comfortable for the component).

I actually used two 5W resistors in series mounted in a aluminium channel I bent up out of 1mm sheet screwed against the chassis. It's a cheap way to do it without buying one of those chassis mount heatsink resistors. Runs only mildly warm to the touch after 2hrs of cranked bench testing. You could do a single 10W strapped to the chassis and it'd also be good.

[Littlewyan]

When you measured that were you running EL34s or 6V6s?

[katopan]

When you measured that were you running EL34s or 6V6s?

Good question! Sorry didn't think to clarify - definitely EL34s. I don't think much of an Express with 6V6s in comparison to EL34s.

My amp is the Express circuit true to the schematic but with a very different layout.

[Littlewyan]

I'm the same, I tried 6V6s, but was not a fan at all.

Interesting you measured only a 74V drop, however I noticed that screen current between different branded EL34s seems to vary a bit! I'm sure I measured 10W dissipation when I had my chassis out and I think that was with TAD EL34Bs. JJ EL34 IIs draw even more screen current I think from memory!

[Roe]

For max screen current, try RTF/Siemens e34s