Andy Fuchs and John Suhr got screwed by these tiny resistors, they open up in plate load duty.
Andy also posted a few years ago on TGP about his problems with Koa-Speer 2W carbon films failing with only a 400 mW load, so he must really be unlucky when it comes to resistors...
I use the Panasonic or Mouser Metalfilms now. Metal Oxide for power supply voltage dividers.
I liked the sound of the KOA and these too (scottL suggested them), but they did have a breakdown problem in plate load locations. Not overstressed wattage wise, but apparently voltage wise they didn't like high voltages.
When you consider cold start is virtually NO voltage across the part, and at current they have 100 to 150 V or so volts on them, it doesn't seem they should have died. I did see arcing to ground planes with some parts, which we fixed by raising the part or jacketing in heat shrink...
Nice to know I wasn't the only guy who got burned..
Some are outstanding, most come cheap thanks to military surplus.
Well, this is the least of my worries. I'm taking a big leap during the weekend, upgrading from my old trusty Windows 2000 to W7.
I still have got a couple of boxes running W2K, others - including the laptop I'm using - run well tuned versions of XP. No Vista, no W7 anywhere. Oh and a lone XP/W98 measuring station using ISA cards. Perfect for the job. Why should I upgrade?
I hand layout the boards, no auto layout or routing for that reason.
I route manually all important tracks, then use Eagle's autorouter with a very strict set of rules, then fine-tune manually the final routing. Time consuming but very well worth it.
The arcing had occurred on prototypes without solder masking. On production this issue does not exist.
Creepage distances are a problem when you mix HV and PCBs. Add residue, dirt, smoke, debris, Jack Daniels and beer in the equation and that can become a BIG PROBLEM.
FYL wrote:
Creepage distances are a problem when you mix HV and PCBs. Add residue, dirt, smoke, debris, Jack Daniels and beer in the equation and that can become a BIG PROBLEM.
They don't carry all values in a certain type of resistor so you have to hunt around for them.
Be sure to read the data sheet before committing.
If it says the outside diameter is 2.5mm or so, that is a tiny resistor.
I like the standard one watt size for tube amp building because the leads are thicker and they are just easier to handle.
But these 1/2 watt KOA Speer resistors are pretty nice.
But, they don't carry every value in the Speer's so you have to look at other suppliers.
Here is the list of Speer 1 watt resistors. Like I said the 1/2 watt are also a decent size.
Yeah, I don't understand how they decide what values to carry.
Most probably tube amp builders aren't high on their list.
Don't forget Digikey and Newark as sources as well.
For example, Digikey carries the locking PEC pots I like to use for bias for $8.37 each.
They also carry some resistors that Mouser does not.
This is a PR03 Vishay/ BC 3 watt resistor. The outside diameter is .205" so just shy of 1/4 inch. They are 5% tolerance but the ones I measured are right on the money.
I agree that Mouser etc. are aggravating because you can't get all the values you need for Dumble type circuits (510R, 910R, 9K1, 91K and 110K spring to mind).
In 1/2 and 1W metal films I've standardized on PRP and Dale RN65s. Carbon Film, I use Takman CFs, carbon comp A-B NOS or Ohmite current production. The Ohmites and Dales I get from Mouser, the PRP and Takman from Parts Connexion (much cheaper than Angela for the Takmans). A-Bs wherever I can!
Takmans are a bit pricey ($0.75) but they are spot on usually value-wise, like metal films. Takman also does a metal film, but I haven't tried those.
2W I have been using Kiwame from Parts Connexion, not super audio grade but I don't have to have that.
So they are. FYL unmasked them 3 years ago apparently. KOA SPR-2. I didn't buy a bunch of them from Parts Connexion, will use Mouser from now on for these. Thsnx for the tip.
The other resistors I like are the 5W Mills wirewounds but I had one open up in PS dropping duty, a 1K, feeding the PI in my 50W '70s ODS at around 425V, so I don't use them in PS dropping applications any more, replaced it with a 5W sandbox, no further problems.
