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Old 29th August 2006, 04:33 PM   #1
Klimon is offline Klimon  Belgium
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Default Wirewound cathode resistor

Hi,

I've got the (bad?) habit of using Dale HG-10 wirewound resistors for the cathodes of my output tubes, that's because I've got some 200-350ohm values in stock and they are overkill (20watts). However, could the fact that these are not non-inductive wirewounds in combination with the low stated values, actually be detrimental for the sound; meaning e. g. a 5watts Kiwame would be better?

Thanks,

Simon
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Old 29th August 2006, 09:11 PM   #2
Sheldon is offline Sheldon  United States
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I don't think the inductance should be a big factor. In Jone's book, he tests a variety of wirewound resistors and found that only low values showed measurable inductance at audio frequencies. He did a sweep from 100-100kHz and measured the phase. A 220R resistor showed a 0.2degree deviation. That's similar in value to your application but still, it ain't much of an effect. If you are bypassing the effect would be even less. But, hey, it's easy to try other types and see if you hear a difference.

Sheldon
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Old 29th August 2006, 09:29 PM   #3
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I think the question of where ordinary "inductive" ww resistors can be used is an important one. I don't know the answer, except I think that resistors used as stoppers should not be ww.
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Old 29th August 2006, 09:33 PM   #4
Tweeker is offline Tweeker  United States
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The wattage of the resistor may increase it and shunt capacitance (non issue here) some. How much it matters? The 225ohm cathode bias resistor was a concern to Jones in the Scrapbox Challenge amplifier*. A "non-inductive" type ww is probably fine.

*Jones, Morgan. Valve Amplifiers 3rd ed. Oxford, UK Elsevier, 2003 pp428
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Old 29th August 2006, 10:03 PM   #5
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If these cathode resistors are bypassed with big capacitors, the caps will swamp out minor resistor variations anyway. If not bypassed, then a WW of any kind will probably be OK. Low inductance would be the supreme choice, but probably not necessary, as the others have said.
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Old 29th August 2006, 10:16 PM   #6
rdf is offline rdf  Canada
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Any possibility of negative effects above the audio band from inductive, low value resistors without bypass? I'm not sure how many Henries one should expect from, say, a standard 10 watt white coffin.
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Old 30th August 2006, 05:08 AM   #7
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I just rebuilt my amp using non-inductive Caddocks and Rifa PEG 124's. I used to have SCR(Solen) and regular "inductive" wirewounds. Low-frequency motorboating went away with this new setup. I don't know the frequency of these oscillations, but I could very well hear them at certain places in my living room. I did read somewhere that "inductive" types could cause oscillations. Is there any truth to this?
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Old 30th August 2006, 09:34 AM   #8
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That's what I would like to know too.
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Old 30th August 2006, 11:24 AM   #9
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Quote:
Originally posted by Brian Beck
If these cathode resistors are bypassed with big capacitors, the caps will swamp out minor resistor variations anyway. If not bypassed, then a WW of any kind will probably be OK. Low inductance would be the supreme choice, but probably not necessary, as the others have said.
Agreed.

Sonics aside, I've tested a WW, non-inductive WW, metal film, metal composition, carbon film and carbon composition on the bench.

Bypassed with a big cap, type was moot. Unbypassed, the non inductive types (all but the regular WW and metal film) had a slight edge above 30KHz, especially on the squarewave response into complex loads.

As to what ones you should use... try some and the one that sounds right to *you* is the right one to use

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Old 30th August 2006, 01:27 PM   #10
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Morgan Jones measured an inductance of about 2uH in a 220 ohm wire wound. Whether this is representative of a “standard 10W white coffin”, I can’t say. But it’s probably in the ballpark. At 20KHz the inductive reactance is 0.25 ohms, a mere 0.1% of the resistance value. At 2MHz it grows to 25 ohms, or roughly 10% of the resistance. At 20MHz, it about equals the resistance. Could it provoke or enable an oscillation? Sure. Can we say for sure? No. At VHF frequencies there are many other strays to be considered too. Motorboating suggests a low frequency instability that would not be affected by this low value of inductance, unless some high frequency oscillation was making its presence known as secondary motorboating.

As always, try it and see.
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