Voltage ratings for screen grid resistors?

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Voltage ratings for screen grid resistors?

Postby sitka_spruce on Sat Dec 20, 2014 3:11 am

In order to run EL34s in my Marshall designed for coinbase 5881s I have to either add to the screen grid resistance or replace said resistor. The resistor in place being of unusual design and not so easily accessible I reckon it easier to add a second resistor in series with the stock one. But looking through most ceramic or enamel resistors most at around 5W or so are for a voltage of only 200V. This amp having some 420-430V on the plates it stands to reason the resistor has to take 400V at least not to blow - right?
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Re: Voltage ratings for screen grid resistors?

Postby pdf64 on Sat Dec 20, 2014 7:15 pm

Unless there's a screen grid short, the full node VB+ won't appear across the screen grid resistors, as they would have to draw hundreds of mA!
200V types should be fine.
If you want to get super robust though, various types are available, eg http://uk.farnell.com/welwyn/wmo5s-1kja ... dp/1306249
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Re: Voltage ratings for screen grid resistors?

Postby sitka_spruce on Sat Dec 20, 2014 8:55 pm

pdf64 wrote:Unless there's a screen grid short, the full node VB+ won't appear across the screen grid resistors, as they would have to draw hundreds of mA!
200V types should be fine.
If you want to get super robust though, various types are available, eg http://uk.farnell.com/welwyn/wmo5s-1kja ... dp/1306249

Cheers, mate! As you can tell I'm a tad fresh on these things but trying to learn. Of that reason I haven't decided if I'm to make something similar for the KT88s that are to be run in the other channel or not. Or that Tone Lizard thing as I understand the grid leak resistors are really too large for those. But then at 425 ish Volts I can only expect 35W from each of those anyway, I believe, for which 220k ones are specified. Ah, but that's another matter.

Def have to check those robust ones you suggested.
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