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Old 15th December 2012, 04:52 PM   #11
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Some probably worthless generalizations-

Inductance of lower values is usually too low to worry about in the audio region.
Non-inductive winding patterns don't work as perfectly as one might like and can degrade things like self-capacitance.
Noise will usually be better than anything but metal foil ($$) parts.
Tempco will be better than any audio project needs.
Use caution if the WW is used for feedback around wide bandwidth parts like opamps. It may be audio, but you might be dealing with a 50 MHz GBW part that isn't aware of that!
The peak power handling ability of many WW resistors is extremely good- see Ohmite and Dale (Vishay) notes on this.
Given a roll of resistance wire, like manganin which is solderable, you can wind perfectly good WW resistors of any value you like and in any of the low inductance patterns you desire.
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Old 15th December 2012, 05:57 PM   #12
DF96 is offline DF96  England
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A bit of inductance does little harm in audio, except in low impedance circuits, provided it is damped by some resistance too which is exactly what happens with a wirewound. Wirewound are usually the lowest noise resistors, because all you get is the unavoidable thermal noise. Other resistors have extra noise mechanisms too.

One proviso: it might be possible for wirewounds to introduce magnetic coupling between circuits so don't put them near each other and parallel unless a bit of coupling would do no harm.
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Old 15th December 2012, 07:24 PM   #13
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Hi Conrad.

SUPERSTAR !
Not worthless... GEMS !
Yep...I got some resistance-wire, & DIY hyper-matched source-resistors are in production...
...make your own resistors...
..." now that's DIY dude ! "
Flat-wound...& more acurrate than Vishay-foil...
...$$$$$$$$$$$$$$$$$$$$$$$ left over, to buy LP's with !

Hi DF96

Nice tip on the possibility of 'magnetic coupling'...
...hadn't thought of that one before.
Next board, designed with that in mind.
Thanks !

Cheers.

Si.
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Old 16th December 2012, 01:48 AM   #14
MGH is offline MGH  United States
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Quote:
Originally Posted by MGH View Post
May be someone can correct me. Is it the grid resistor than can induce oscillations if the inductance is too high?
No one knows the answer?
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Old 16th December 2012, 09:44 AM   #15
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Hi MGH

Is it the 'grid leak' resistor you mean ?

Anyway...
...these are always VERY high value.
It is possible to make this a wire-wound...
...I just scored a pair of 500k WW's on eBay ( 350volt, I think )...
There are lower voltage ratings out there as well.

But probably, you might want to use a 'High Stability' metal-film resistor.
This most likely has a 'spiral cut' anyhow, as well.

I don't think you're at all likely to get oscillations ANYWHERE using decent WW's.

Now trashey NOS carbon-compositions are another story...
...as said, I think before...
...we've measured 'brand new' NOS ones that are simply either WAY high (bad sign of stability), or just plain 'open circuit'.

I don't think you will have 'inductance problems' ANYWHERE with WW's...
...correct or 'over-rated' Voltage spec. is what's important...
...CHECK IT...not all WW's go to say 350v; I would prefer to see 500v or higher rating for a 350v PSU'ed amp.

The only real danger of inductance, I believe, is at very high frequencys, as said before...
...and very-low Ohm values, which aren't in most valve amps anyway.

Try it and see, is the best test.
You might be surprised to hear just how good WW's sound; especialy compared to rotten old carbon resistors.
Never need replacement either.
Well worth the extra 25cents !

Cheers.

Si.
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Old 17th December 2012, 04:46 AM   #16
MGH is offline MGH  United States
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Thanks Space Egg. Please see the figure. I'm not sure what you mean by "grid leak" resistor, but I'm talking about 2.2 kOhm resistor (R6). I want to use noninductive WW resistor in this location.

In fact, I want to use noninductive WW for 100 Ohm screen resistor (R18) and 10 Ohm plate resistor (R12), as well.

Would you say noninductive WW are appropriate for all these locations provided the voltage and wattage requirements are met?
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Old 17th December 2012, 11:20 AM   #17
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Hi MGH

Can you post a decent res. ( readable ! ) schematic of the WHOLE of the circuit you are interested in.

I don't think the 2.2k will have ANY 'worrying' inductance at all.
The smaller ones might...
...but again, perhaps ONLY an issue in a high-frequency RF amp.

I ain't the world's greatest 'expert' by a long-shot...
...but let's have a look at the whole schematic.

Be nice to see what you're making anyhow.

Cheers.

Si.

Also...where did the schematic come from ?
Is it a tested design ?
Is it a 'retro classic' etc. etc.
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Old 17th December 2012, 06:18 PM   #18
MGH is offline MGH  United States
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Quote:
Originally Posted by The Space Egg Corp View Post
Hi MGH

Can you post a decent res. ( readable ! ) schematic of the WHOLE of the circuit you are interested in.

I don't think the 2.2k will have ANY 'worrying' inductance at all.
The smaller ones might...
...but again, perhaps ONLY an issue in a high-frequency RF amp.

I ain't the world's greatest 'expert' by a long-shot...
...but let's have a look at the whole schematic.

Be nice to see what you're making anyhow.

Cheers.

Si.

Also...where did the schematic come from ?
Is it a tested design ?
Is it a 'retro classic' etc. etc.
I would post the whole schematic if I could, but it is a current production commercial amp. So I can't. These resistors are noninductively wound so I don't think there would be any issues at all, based on what you'r telling me.
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Old 17th December 2012, 06:29 PM   #19
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Yeah...
...loads of schematics like that posted on the net all the time...
...but anyhow...
...what amp is it anyway ?

Schematic will be on Google for sure.

Cheers.

Si.
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Old 17th December 2012, 07:38 PM   #20
DF96 is offline DF96  England
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The 2.2k resistor is a grid stopper. It reduces the RF gain of the valve so it can't turn into an oscillator. A non-inductive wirewound might not be best here, because it could have too much capacitance. Just use a carbon film or metal film.
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