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Screen grid at B+ (low voltage!)

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  • Screen grid at B+ (low voltage!)

    I've recently got a hold of some nifty pentode tubes (1LG5, 1LC5, lots of variants) that were designed to be used in portable radios and were actually intended to run at 45v plate/screen (Heater current is 50mA at 1.4v). Pretty much a perfect fit for my pre-amp powered off a laptop supply, and gives me an opportunity to fiddle with pentodes.

    Usually, I see that resistors are included to drop the voltage down to acceptable levels, and/or to make sure the screen doesn't melt... In my case, neither is of concern, so I'm assuming I just connect B+ straight to screen (I wouldn't need a bypass cap, because no screen resistor is present, right?).

    With B+ straight to the screen, it would seem no screen compression is going to be present, but say if I wanted to include this, what would be the best option for incorporating it? Are there any fancy screen voltage modulating circuits out there, or should I just drop the screen voltage with a resistor (not sure it would like this...) to say, 30 volts. Another option is to simply voltage double the B+ supply, then drop the screen voltage with a resistor down to 45v.

  • #2
    Before you do anything, reasearch the tubes that you have.
    Link: Franks Electron Tube Data: electron Tube Data sheets - Search
    The 1LG5 & the 1LC5 are two totally different pentodes.
    The G5 is a remote- cutoff pentode.
    This is a good tube for "compression"
    The C5 is a standard sharp- cutoff pentode.

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    • #3
      Actually both appear to be sharp-cutoff pentodes according to the Tung-Sol datasheets (the RCA one begs to differ), and the Sylvania datasheet lists it as a semi-remote cutoff to make things even more confusing! The grid curves for both tubes at 45V screen is remarkably similar except that, the grid curves are much more bunched up for the 1LG5 at the bottom, going to -7.5v instead of -2.5v (probably leading to the semi-remote-cutoff moniker). I imagine it would make a fine compressor.

      Might mix and match and see what happens...

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      • #4
        Originally posted by exclamationmark View Post
        With B+ straight to the screen, it would seem no screen compression is going to be present, but say if I wanted to include this, what would be the best option for incorporating it? ... should I just drop the screen voltage with a resistor (not sure it would like this...) to say, 30 volts.
        An unbypassed Rg2 is the simplest way I can think of to get screen compression. Why not try it and report back?
        Building a better world (one tube amp at a time)

        "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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