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W07VH5 |
In tube amplifiers, carbon comp resistors are still used in certain sections (let's consider grid stoppers) because they are low inductance resistors. Metal film, wire-wound, and other more modern resistors are supposedly quieter in a circuit but their inductance is not desirable in the grid stop. Are there modern alternatives to carbon comp resistors that do not contribute inductance? | ||
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I Deal In Lead |
How about ceramic resistors? https://www.ohmite.com/ox-oy-series/ Ceramic Composition The OX/OY Series of fixed ceramic resistors are ideal for circuitry associated with surges, high peak power or high energy. They offer enhanced performance in high voltage power supplies, R-C snubber circuits, and inrush limiters. The OX/OY resistors can often replace carbon composition resistors which can be difficult to source. The OX/OY series can handle 50 joules and 80 joules of energy for a single impulse respectively with high impulse voltage handling. | |||
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| Member |
In the enclave of perfectionist passive loudspeaker crossover designers no-to -low inductance resistors are widely sought/used. A handy source of small quantities of premium ceramic oxide resistors of limited values and wattage commonly used there are available at Madisound. E.g., here is a sample from one of their suppliers. https://www.madisoundspeakerst...xide-film-resistors/  | |||
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| W07VH5 |
Thanks. Those look pricey but I'll only need a small handful. | |||
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| Member |
Another alternative to ask this question is Eevblog forums. Good folks over there too. | |||
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| Free men do not ask permission to bear arms  |
What are "Grid Stopper resistors"? Define "Quieter"? How much inductive reactance does a "wire wound" resistor" exhibit at audio frequencies? George43. 78 year old electrical design engineer. A gun in the hand is worth more than ten policemen on the phone. The American Revolution was carried out by a group of gun toting religious zealots. | |||
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Seeker of Clarity |
I've forgotten so much more than I remember about all of this stuff. Mark -- Did you go to Penn Tech by any chance?  | |||
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| W07VH5 |
A grid stop in a tube stage can help prevent blocking distortion. It also has an R-C filtering effect with the coupling capacitors. Less buzz. The small amount of current that goes through this resistor doesn't cause much noise but I want to avoid carbon comp resistors because they dwell up in value over time. They're also affected by heat and tubes = hot. Apparently enough that they are specifically not recommended. | |||
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| W07VH5 |
Ah, yes. You've worked on tube amps for classic stereos. I didn't go to Penn Tech. I didn't attend a college and I regret it now. | |||
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| Seeker of Clarity |
A couple to brighten your day.   | |||
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| W07VH5 |
I need to restate this because it was partially wrong. It's not an R-C filter with the coupling capacitor as they are in series. It's an R-C filter with the grid stop resistor and the capacitance of the tube itself. For example a 12ax7 has a capacitance of around 150pF. | |||
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| W07VH5 |
I know you couldn't resist. | |||
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| Member |
FWIW, the audio component supplier of some of Madisound's low inductance oxide resistors, Mundorf, lists the values of some of the more important attributes of their resistors. In particular, they show upper bounds on the inductivity of those they offer, in the standard unit of micro Henrys. This one, a 4.7 ohm/30 watt item, https://www.mundorf.com/audio/...%B130ppmK/?card=4654 for example, asserts that its inductivity is under 0.075 micro Henrys, i.e., 75 billionths of a Henry though admittedly a Henry is a rather significant quantity of inductivity (the preferred precise terminology).  | |||
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| W07VH5 |
At $21 each I'd expect some precision.  This message has been edited. Last edited by: mark123, | |||
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| Member |
Fresh new oats cost $$$. On the other hand, if you are willing to settle for oats that have already been through the horse those come a lot cheaper. | |||
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| Ignored facts still exist |
I don't see how this inductance will matter at audio. . | |||
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| W07VH5 |
Even though you can't hear the oscillation, it still affects the circuit. It can cause issues like squealing, red plating and harsh high end. | |||
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| Free men do not ask permission to bear arms |
Red plating? Harsh high end? Explain please. A gun in the hand is worth more than ten policemen on the phone. The American Revolution was carried out by a group of gun toting religious zealots. | |||
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| W07VH5 |
Ok. Let me know where you stand at the moment. Are you familiar with audio amplification? Vacuum tube circuits, specifically. Are you familiar with Miller capacitance? Edit - going forward it looks like my best choice is to just use carbon comp resistors in the necessary positions. Generally they need too be soldered in very close to the tube socket and just need to be close to the heat. | |||
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| Free men do not ask permission to bear arms |
A gun in the hand is worth more than ten policemen on the phone. The American Revolution was carried out by a group of gun toting religious zealots. | |||
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