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In Reply to: RE: Common screen grid resistor? what does that mean exactly? posted by Steve O on September 26, 2016 at 16:36:17
If owner wants to keep one shared resistor, for all the output screen grids --- I usually will replace with a good wire-wound resistor inline. Like a 5 or 12-watt Mills or 5-watt "sandbox."
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I think you're confusing the screen B+ dropping resistor with the "non-decoupled" screen grid resistor. The latter serves the same purpose as a grid stopper, and it should never be wirewound.
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Buy Chinese. Bury freedom.
Other tubes mention a value for the G2 resistor but so far i have only found the "common grid 2 resistor" being mentioned for Mullard and Phillips EL34. They make it seem like an important point so i was wondering why? And also i don't recall running across that implementation which seems even odder. cheers. Dak
The OP asked about a "common screen grid resistor, non decoupled, of 750 ohm should be used for G2 for class AB" (assuming PP). This resistor cannot serve as a stopper in the traditional sense due to physical limitations of one resistor serving two tubes. A true stopper app would require separate resistors attached at each g2 socket terminal. The referenced resistor is primarily for voltage drop/isolation and is less critical as to physical construction than that of a true stopper app.
"This resistor cannot serve as a stopper in the traditional sense due to physical limitations of one resistor serving two tubes."
Which physical limitations are you referring to?
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Buy Chinese. Bury freedom.
With two tubes and one resistor, the resistor body can be physically close to the g2 socket terminal of one tube or the other, not both simultaneously.
I'm a big fan of short lead lengths myself, but many factors dictate how short they actually need to be. One of those factors is the proclivity of the tubes to oscillate in a certain range of frequencies. If you want to disagree with the datasheet, you're shouldering the burden of proof to demonstrate that the manufacturer made a mistake in this regard, that the tubes are in fact unstable at spurious frequencies aided by the lead lengths required to stretch between two tubes. If you don't have evidence of such performance, it's not reasonable to claim separate resistors will be beneficial.
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Buy Chinese. Bury freedom.
Best to keep lead lengths short. Prolly, one of the reasons Fender went to individual resistors on the sockets --- in the transition, blackface, & silverface era amps.
The 10k-ohm should "see" interactions (AC and others) similar to the 470-ohm resistor in the blackface example. It would have too, No? Only a length and individuality of connecting wires is the difference between the two?
Anyhow... I have put individual ww resistor on each socket --- for screen supply. This also works fine.
What? The resistor is tied to the screen grids, not the control grids. the purpose of that resistor is to drop the voltage applied to the screen grid.
I think I maybe misunderstanding your post, here.
???
the 10k resistor is used to drop voltage. the 470 ohm (or 750 or 1k depending on OPT xformer taps) are used to limit screen current. this is all in datasheets and mullard amp applications book.
Wouldn't the 10k-ohm resistor also limited the current to the screens? How does one resistor in the PSU rail differ from two separate resistors from the rail to the screens?Besides, I would think you don't want this screen resistor to "pop" due to excessive current flow. As this event would take the screen resistor out of the circuit. Grid stopper, yes. Screen resistor, no. Right?
Thanks!
Edits: 09/27/16
"common screen grid resistor, non decoupled, of 750 ohm"
The key phrase here is "non decoupled." Only by being non decoupled can the resistor serve its intended purpose of stabilizing the screens. The resistor dampens the screen circuit by reducing its Q, just as in the case of a grid stopper. It can't do that if the screens are decoupled.
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Buy Chinese. Bury freedom.
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