Ye Olde Screed Resistor Thread

Discussion in 'Amp Tech Center' started by Luthier Vandros, Aug 27, 2015.

  1. Luthier Vandros

    Luthier Vandros Tele-Holic

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  2. mRtINY

    mRtINY Friend of Leo's

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    Use 1W flame-proof resistors. They weigh almost nothing and at 5-8mA of screen current, a 2k will only need to be 1/8w (in theory...) or 1/4w for some margin.

    -tINY
     
  3. Fred Mertz

    Fred Mertz Tele-Afflicted

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    The screen grids at idle bias voltage will draw 5 to 8 milliamps depending upon the plate and screen grid supply voltages, and the control grid negative bias voltage. As signal is applied to the control grid, screen grid current increases. At clipping, a 6L6GC screen grid can draw up to 30 milliamps, or so, depending upon the plate and screen grid supply voltages. The power dissipated in the screen grid resistor at clipping would be about 0.42 Watts (.03 x .03 x 470). This figure should be derated to a higher power level to allow for heat according to the resistor specifications.

    I don't care for the cement block resistors either. I like the Vishay Dale RNX series of metal oxide resistors for screen grids. I like the RNX075 for 6L6GC tubes and RNX025 for 6V6GT tubes.

    http://www.vishay.com/docs/31032/rnx.pdf
     
  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    True pentodes like the EL34 and KTxx tubes will draw even more screen grid current during distortion so amps designed for them typically run 1.5k 5 watt screen resistors.
     
  5. BobbyZ

    BobbyZ Doctor of Teleocity

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    Might be worth looking at were the choke is in the 5E7 and all "E" series and earlier tweeds. Compare that to the 5F8A Twin and 5f6A Bassman.

    On the schmatics you'll see the choke is in the circuit earlier on the older amps.
     
  6. Fred Mertz

    Fred Mertz Tele-Afflicted

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    IMO, I believe that those big chokes did a lot to stabilize and limit AC swings under load. I believe they were rated somewhere in the 8 to 10 Henry range at 200 ma. The later chokes used by Fender are only in the 4 to 5 Henry range and are not as effective in limiting AC swings. This change along with higher B+ voltages may be why Fender implemented the screen grid resistors. Something else to consider is that the amps were never designed to be driven to clipping. It's all the fault of that devil music rock and roll. :^)
     
  7. BobbyZ

    BobbyZ Doctor of Teleocity

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    Fred did they use bigger chokes on the older circuits? Seems like there'd be more current going through them that way.
     
  8. Fred Mertz

    Fred Mertz Tele-Afflicted

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    The choke located before the output tubes had to be bigger to accommodate the current drawn by the output tubes. Chokes that are insufficient in their current rating for the circuit that they are used in loose their effectiveness in reducing AC ripple. A 10 Henry choke that is rated at 100 milliamps may only have 7 or 8 Henries of reactance under a 120 milliamp load. The choke can chatter if sufficiently overloaded.
     
  9. jhundt

    jhundt Doctor of Teleocity

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    I read this: if the entire amp circuit is running through the choke, you need a pretty big one. If you feed the power tubes before the choke, you don't need such a big one.
     
  10. Fred Mertz

    Fred Mertz Tele-Afflicted

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    That's correct jhundt. However, there are benefits in implementing the choke prior to the output tubes. I think that cost may have been the deciding factor when Fender changed the circuit.
     
  11. BobbyZ

    BobbyZ Doctor of Teleocity

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    When I built my 5E7 I just ordered a Classic Tone "bundle" with all three trannys. Well the PT is wrong it's either over voltage even with a 5Y3 or using the other taps under voltage with a GZ34.
    If memory serves the CT tranny for a 5F6 would have been about right. (to hit schematic voltage)
    Then there's the choke I'll have to look into that.
     
  12. Wally

    Wally Telefied Ad Free Member

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    These modern PT's built to vintage specs just don't get it done, imho and ime with modern wall voltages. 'Adjusted voltage' PT's are the way to go, imho, if one is looking for vintage voltages in a vintage circuit. And....voltages do things with the sonics, dont' they?
     
  13. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Bobby, is the power transformer the 40-18029 model? If so, I see two different taps for the HT windings (710 VAC or 610 VAC). The 610 volt taps using a GZ34, on paper, look like they would get you around 410 volts DC to the 6L6 plates. The choke is way too small to be implemented prior to the output tubes (4H @ 90 ma.). Mouser carries a Triad 6H. 200 ma. choke that is a lot closer in specs to the original 5E7 choke.
    http://www.mouser.com/ProductDetail/Triad-Magnetics/C-14X/?qs=sGAEpiMZZMv126LJFLh8yzivpguvGNtj7z4vpDIMCZE%3d
     
  14. BobbyZ

    BobbyZ Doctor of Teleocity

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    That's probably the one Fred been awhile since I've thought about it much. But it does have the two taps.
    And I bet that's the choke too.

    When I did it I just saw "transformer bundle for 5e7" and ordered it.

    Stupid of me to think that means the iron is actually for a 5e7. Sometimes I just don't think.

    I'll mess with it more this winter.
     
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