Hello everyone,
What would be the better choice, if it matters for a 10Ω resistor from a cathode in a P-P output to ground?
Thanks
Ray
What would be the better choice, if it matters for a 10Ω resistor from a cathode in a P-P output to ground?
Thanks
Ray
Thank you, I found some 3watt ones on Mouser with either .5% or .1% tolerance that won't break the bank.
3 Watt is nice but 1 would be more then enough (if you do the math) ;-)
1% tolerance is good enough as well.
1% tolerance is good enough as well.
I have managed to make trouble with wirewounds in a cathode return. I would use metal film, carbon film (10 Ohms is not enough to make "sound"), or even Composition (except they cost too much now).
A 3w resistor is well worth using to prevent damage if an output tube arcs or shorts.
Since it is used it to set bias, 2% or better tolerance is worthwhile.
Since it is used it to set bias, 2% or better tolerance is worthwhile.
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Better to use a screen resistor (or a small screen fuse) sized to open in that event.
Having seen many arcing tubes, this works out for the best.
Having seen many arcing tubes, this works out for the best.
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I do not care if a 10 cent resistor blows... especially if it can prevent disaster (screen grid resistors will not be able to help in this case but can be a cause of it depending on the design of the amp).
I have a bag of flameproof 10 Ohm 0.5W resistors, I use them routinely. I do not care what they are made of.
i recently purchased 5 watt 1% tolerance 470 ohm resistors for use as cathode resistor bias in my st70 build, they are not super cheap and not overly expensive either...
the only place i use carbon comps is in the control grids...maybe that will change too.... 😀
the only place i use carbon comps is in the control grids...maybe that will change too.... 😀
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The higher the value of the resistor (in this part in the amp) the more audible it gets.
If however the 2 tubes share the same resistor then most of this will be cancelled out.
If however the 2 tubes share the same resistor then most of this will be cancelled out.
The higher value and the higher voltage drop, the more negative effects. The negative effects will remain.
It's 10R in a several hundred volt circuit. Noise is negligible if even measurable.
This isn't the princess and the pea.
For the record, I've used carbon parts once. I've switched grid stoppers to metal oxide now and for most power applications, but still use metal film in most of the signal circuitry.
Here's a nice little write up about the different types of resistor material.
Resistor materials | Resistor Guide
This isn't the princess and the pea.
For the record, I've used carbon parts once. I've switched grid stoppers to metal oxide now and for most power applications, but still use metal film in most of the signal circuitry.
Here's a nice little write up about the different types of resistor material.
Resistor materials | Resistor Guide
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I had a 2W carbon film 10R cathode monitor resistors suddenly fail and nearly halve in value for no apparent reason - it was like it had suffered some kind of transient or arc. I was actually monitoring both resistors of an output pair on the scope as it happened too, a sudden unbalanced appeared and for a while I thought it was the tubes! There was no overheating or over-dissipation event, just a sudden failure.
Luckily it was just a prototype amplifier. The resistors were on the cathodes of a pair of KT88s running with 600V on the anodes running at 50mA A+G2 current. I changed to some 5W wirewound resistors I had on hand and this didn't happen again. No idea what the failure mechanism was but it perhaps despite the small dissipation here there is some need for robustness in this application.
It did only happen once so it's possible the resistor was just cracked or otherwise damaged somehow.
Luckily it was just a prototype amplifier. The resistors were on the cathodes of a pair of KT88s running with 600V on the anodes running at 50mA A+G2 current. I changed to some 5W wirewound resistors I had on hand and this didn't happen again. No idea what the failure mechanism was but it perhaps despite the small dissipation here there is some need for robustness in this application.
It did only happen once so it's possible the resistor was just cracked or otherwise damaged somehow.
I had the traditional 5W WW resistor open on a transient like that with no sign of overload. The ones I linked will take many times their rating for long enough to smoke, and crack but they still work and measure 10R. 😀