Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 12AX7 on January 26, 2012, 09:59:23 am

Title: Can i use 1/2 watt?
Post by: 12AX7 on January 26, 2012, 09:59:23 am

cathode resistors on my EL34's that is. I use 1 ohm resistors to check bias across, but all i can find are 1/2 watt at 1% tolerance. I can't see what it would matter, but i saw i think it was ceritone using 2-3 watters in a layout and figuring he knows more theory than me, maybe theres a reason not to use 1/2 watts there.

Title: Re: Can i use 1/2 watt?
Post by: The_Gaz on January 26, 2012, 10:10:24 am

I may be wrong, but I think that they may be over-rated so that they don't open up if there is a tube failure and the current runs away. It may actually save your butt, which is why some companies fuse the cathodes of the power tubes, but it's not reliable (to use a resistor as a fuse). If the amp is properly fused, the fusing of the cathodes is likely not needed, and the lower wattage ones opening up during a tube failure may save nothing and just be an annoyance to replace.

Title: Re: Can i use 1/2 watt?
Post by: sluckey on January 26, 2012, 10:11:09 am

Doug has 1 ohm, 1 watt, 1% tolerance resistors.

You would have to have 500ma flowing thru a 1Ω resistor to dissipate 1/2 watt. You could safely pass 250ma thru that half watter.

 

Title: Re: Can i use 1/2 watt?
Post by: 12AX7 on January 26, 2012, 10:18:47 am

Thanks. I'll do the 1/2w then. Appriciate it.:thumbsup:

Title: Re: Can i use 1/2 watt?
Post by: ckilgore on January 26, 2012, 02:55:17 pm

You would have to have 707ma flowing thru a 1Ω resistor to dissipate 1/2 watt.

Title: Re: Can i use 1/2 watt?
Post by: sluckey on January 26, 2012, 03:13:13 pm

Quote from: ckilgore on January 26, 2012, 02:55:17 pm

You would have to have 707ma flowing thru a 1Ω resistor to dissipate 1/2 watt.

Yep, you're right. Sorry 'bout that.   :embarrassed:

P = I²R

Title: Re: Can i use 1/2 watt?
Post by: stratele52 on January 28, 2012, 05:19:22 am

I use always 1/2 watt resistor

Title: Re: Can i use 1/2 watt?
Post by: G._Hoffman on January 28, 2012, 03:06:57 pm

If there is a short somewhere, you WANT it to fail.  That way, it will burn up and act as a fuse, hopefully keeping your PT intact.

Or at least, that is the rational I've always heard, and it makes some sense to me.


Gabriel

Title: Re: Can i use 1/2 watt?
Post by: stingray_65 on January 30, 2012, 08:26:54 pm

Big resistors are easier for my eyes to see :icon_biggrin:

Those tiny precision 1/4W metal films with 5 color bands make me look for my DMM every time!

The one thing I really think CC resistors are great for.. easy to read! the brown bodies contrast the colors best!

Ray

Title: Re: Can i use 1/2 watt?
Post by: 12AX7 on January 31, 2012, 09:56:59 am

Quote from: drgonzonm on January 30, 2012, 03:23:41 pm

If specs call for 1 watt and all you have are 1/2 watts, then take 4 of those puppies and wire them series/parallel.  Very ugly, but it works most of the time. 

problem is, if you start doing that the tolerance can multiply and theres goes your 1% tolerance. No matter tho, as i found some dale 1%'ers at a local surplus. They're 10 watt if i recall, (the metal chassis mount kind) but thats all i could find.

Title: Re: Can i use 1/2 watt?
Post by: sluckey on January 31, 2012, 10:34:35 am

Why didn't you use the 1/2 watters? Or order some 1 watters from Doug?

Title: Re: Can i use 1/2 watt?
Post by: 12AX7 on January 31, 2012, 10:49:21 am

Don't need anything i could think of besides that, and shipping is $7.15.