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I was checking out some of the EL34 tube characteristics from the different manufacturers and ran across that term. In the Phillips Manufacturing tube characteristics they mention that a "common screen grid resistor, non decoupled, of 750 ohm" should be used for G2 for class AB. Does that mean one resistor for both tubes in the PP pair or a single resistor for each tube. I mostly see separate resistors for each tube but the terminology suggests one resistor common to both tubes. Thank you for your input, Dak
Follow Ups:
It seemed from the statement that there was an important reason that shared grid resistor be used. Is it lower distortion or some other reason?
I've never used a single resistor, and I can't recall seeing that in commercial designs. However, many designers use no resistor whatsoever, probably believing (true or not) that screen stability isn't an issue. Regarding one resistor vs. two, maybe there's an advantage to a single resistor in a push-pull output stage. For optimum pentode operation, the screens must be held at a steady voltage, relative to the cathode. Installing a non-decoupled series resistor of any value at the screen degrades this characteristic. However, a single resistor connected to both screens should allow the push-pull voltages created by screen currents to mostly cancel. This probably results in operation closer to true pentode.
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It means one resistor "shared" betw the two g2s of a pair of PP output tubes. Even so, I'd still be inclined to add g2 stopper resistors at each socket terminal using usual values and lead dress.
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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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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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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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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Prolly one resistor for all screens. Seen this in 1950s era and lower end Fender amps.
The schematic you linked does not have a common screen resistor. The screens are simply tied together and bypassed to ground. I don't recall ever seeing a commercial design that used the technique described in the datasheets. However, a single 750 or 1K resistor should be just as effective as two for its intended purpose.
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TK, no they are not. They are tied to PSU past the 10K-ohm dropping resistor. So, the voltage on the screen's voltage is below the anode plates.They tied into the (+) side of the caps (like pi-filter). The caps are grounded on the (-) of each cap. The rail is B+. Not ground.
20-450 = 20/450, in this case. Not (-) ground point.
Edits: 09/26/16
Tweed era. Voltage drop applied past one 10k-ohm dropping resistor onto screens.
Blackface era. Voltage drop applied via 470-ohm resistor to each 6V6GT screen. A screen resistor for each 6V6GT. Past the choke.
The first schematic (the one you originally linked) does not employ screen resistors. The 10K you highlighted is a B+ dropping resistor, not a screen resistor. The 470 Ohm resistors in the second schematic are non-decoupled and serve as screen resistors.
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Buy Chinese. Bury freedom.
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