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Ty_Bower wrote:

notben wrote:Now to track down some 2 or 3 watt carbon comps, not always so easy. Mouser, digikey, angela, parts express and parts connexion don't have any 100ohm. There is a great old electronics store local that may have some. If not, I can just use a small wirewound.

Don't use a wirewound, that's not the right type for this spot. A one watter would almost certainly be OK if that's all you can find. 100 milliamps over 100 ohms is one watt - should the screens draw more than that? Looking back, I'm guessing the parts I used were only one watters as well.

If you really get stuck, send me a PM and I'll dig through my box and see if I've got four more of those 82 ohm parts left.

Ty,

A low value wirewound has barely more inductance than other resistor types since they have very few turns. Is there some other reason you recommend against using them. I do agree that no inductance is best, but I've used w/w without difficulty. Guitar amps use them quite often too. And larger wattage carbon comps are near impossible to find.

Let me know! Thanks.

Jim McShane wrote:A low value wirewound has barely more inductance than other resistor types since they have very few turns. Is there some other reason you recommend against using them. I do agree that no inductance is best, but I've used w/w without difficulty. Guitar amps use them quite often too. And larger wattage carbon comps are near impossible to find.

Hmm. Now that I think about it, I suppose I don't have any good reasons. I generally try to stick to wirewounds for power supply duty and cathode bias resistors. I try to use metal films for long tails and plate loads and any other place where low noise and/or precision is desirable (like in a CCS). I feel good about using carbon composition for grid stoppers and anyplace where there is a risk of flammability, but I guess I'm mostly just repeating what has worked for me in the past.

Honestly, I don't think I can hear a difference between various types of resistors. I mostly go by what I've read, and my amp building experience is limited to less than a dozen. I've never built a guitar amp, nor worked inside one. Though I can't place the link, I must have read somewhere that CC was the way to go for screen duty. I'll agree the high watt CC parts are difficult to find and can be expensive.

From what I've read, there are three characteristics desired in a screen resistor - low inductance, high overload margin, and flameproof. This seems to give three choices for screen stopper resistors:

Carbon composition have the least self inductance, but the highest noise (not usually an issue with small values) and are known to drift with overload. Plus large wattage versions are expensive and hard to find, as previously mentioned.

Wirewounds have excellent overload characteristics, but the highest inductance (although this can be mitigated by advanced winding techniques, this drives the price up). A SWAG might be that they'll be ok if you keep the value low.

Metal Oxides sit somewhere in between. They're made by a film deposition of a metal oxide (tin oxide seems most common) and etched like a metal or carbon film, so they have an inductance somewhere in between carbon composition and wirewound resistors. Unlike metal and carbon film, they have excellent overload characteristics (comparable to wirewounds).