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In Reply to: RE: What happened to all the parts?? posted by vinnie2 on January 27, 2016 at 04:46:28
You say you checked Michael Percy Audio, but Michael does have Mills non-inductive wire wound resistors (as good as anything you can find anywhere, or better) and he does have 50W NI power resistors, last time I checked. So, that's two of your needs at one stop. He also sells a wide variety of potentiometers, perhaps not the ones you need, but Mouser and/or Digikey sell those.
Follow Ups:
The mills are only 12 watt and I want at least 25 watt. He does not have a 820 ohm NI power resistor. The pots he has are the wrong type.
Digikey and mouser do have the resistors and pots, but the resistors are not non-inductive. That's the reason for my last post about dog bones; if I am not going to use non-inductive I can use the dog bones I already have on the bread board. They sound pretty good to me.
Michael Percy also sells Caddock power resistors, which are not wire-wound, I don't think, but which, with a proper heat sink, may take 25W. They're excellent quality.
Might have to give them a try as my old sources seem to be disappering.
Thanks
I also agree with Dr. on paralleling two 12W Mills. If that seems marginal, you could parallel 3 or 4 of them to get the needed power handling and R value.. Even 4 might take up less space than a Caddock power resistor plus heat sink.
If I understand what vinnie is doing with them, one would likely be enough - relatively low voltage and current.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
You are probably right, and they might be worth giving a try. I keep forgetting these are not carrying a ton of current.
Especially for a cathode resistor which is typically of low-ish value.
Most cathode resistors I have used in the past have been for power tubes and I was using 25W or more. I guess it just got to be a habit.
Understood vinnie. Just be aware, neither the 45 or 26 you are running have a lot of cathode V to drop and current is small. The 45 will be the most demanding. As you know, to calc the power (wattage) dissipated:P = I x V (current traveling through the resistor x voltage dropped across the resistor, i.e. cathode voltage)
= 0.0342 x 41 (going by your preferred operating point and assuming you are not direct coupling to the 45)
= 1.4 WattsYour cathode R will be dissipating 1.4W! Ideally, I over-rate my resistor Wattage by 5, but that is very conservative of me and I break this rule occasionally if I know the parts are reliable. Mills are reliable. 1.4W x 5 = 7W. Hence, a resistor with at least 7W dissipation will do it very comfortably. A 5W resistor would also be doing it easy, so long as ventilation is adequate. A 12W Mills will be loafing.
For some context, my next build will use Mills 12W resistors to dissipate 5W in a moderately-well ventilated chassis. I am not worried.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Edits: 01/29/16
Do you still think the mills are the way to go after the posts about them being made in Mexico now and being a lower quality? I was all set to try some until I read those.
I'd try them.
I read someone's opinion saying the Mexico versions were higher quality; another's the opposite. No one is saying in what way, specifically.
More the point, I reckon the Mexico Mills are still a pretty decent product, versus the opposition.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Vinnie,
On Rks, ( and Ra ) you want to use two equal value closely matched Rs, so, use two MRA-12s in parallel. It is always better IF matched well, more DYNAMIC!!!
Jeff
And the MRAs are non-magnetic, unlike the high wattage heatsunk Rs. non-magnetic = better sound, apparently.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Yes, I use MRA-12s a lot. Latest are brown in color, made in Mexico, NOT AS GOOD AS PRIOR USA PRODUCTION, quality wise !!! A shame.
Jeff Medwin
That is a shame. I noticed some in my recent purchase from Michael Percy... only used for the B+ shunt to ground, not cathode thankfully.
Oh well, we can start paralleling high quality 2W or 5W resistors, I guess.
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Well, so much for may plan to try some on the 26 cathodes. I guess it's back to vishay aluminum wirewounds.
Edits: 01/29/16
The parts problem is a sign of the times. If you think this is bad, try restoring or even maintaining an old car. The shipping can easily double your cost, and it doesn't help that companies insist on sending little items the size of a thimble via UPS for $12-15. I fight with suppliers constantly on this issue.
Anyway, about the cathode resistors, I want to point out that hum pickup has a lot to do with the diameter of the windings, not just total inductance. Most 10-turn pots consist of physically large turns of resistance wire. That's a good reason not to use them for this, especially in low-level stages.
I have not had any problems using wirewound resistors for cathodes. If you measure the inductance of a standard resistor, you'll find it's insignificant at these frequencies. The only place it becomes an issue in my opinion is grid stoppers. They need low inductance in order to dampen circuit Q and prevent RF oscillation. That's not the case in the cathode.
I buy most fixed-value power resistors from the folks at the link below. You can also often find these parts on eBay if you have the patience to search values for non-Chinese NOS.
--------------------------
Buy Chinese. Bury freedom.
Thanks for the source. Sad times for all DIYer's in many departments looks like.
I know what you mean. Been doing a lot of parts ordering lately for vintage Alfa Romeo, mostly from the UK.
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