Question on screen grid resistors

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by MikeMcK, Sep 11, 2008.

  1. MikeMcK

    MikeMcK Member

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    From what I've read here and elsewhere, it seems that the size of the screen grid resistors have a significant effect on tone... less edgy, maybe?

    If I want to try 470-ohm screen grid resistors in a 6L6 amp that currently has 1k, I guess I need to double the power rating, right? What else do I need to be careful of?

    Does it make sense to first bias the tubes cold, then swap the resistors, then re-bias on powerup? Or just go for it?

    I plan to measure everything, but I guess I should be prepared to sacrifice the tubes if 470 turns out to be too low... any chance of damaging anything else?

    Thanks in advance.
     
  2. Blue Strat

    Blue Strat Member

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    Power rating shouldn't be affected and could likely go lower but use 2 watters minimum. Lower resistance means lower voltage drop and lower power consumed for the same current. Ohm's Law, check it out.

    Bias shouldn't be affected much if at all. The screen current contribution is small.

    By all means, when changing anything, recheck the bias. It's always a good idea to start at the coldest point rather than the hottest.
     
    Last edited: Sep 11, 2008
  3. donnyjaguar

    donnyjaguar Member

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    What Mike said. The lower the resistor, the greater the efficiency of the amplifier. How much effect it has I'm not sure, but some of the real ball-busting loud amplifiers of yesteryear tied the screen grids together and used only one, or no, screen resistor of lower value.

    If you compare a tetrode output stage (most efficient) to a pentode wired as triode you can see the effect of sagging screen grid voltage during operation as much reduced output power. It has other effects too which are not necessarily negative.
     
  4. mooreamps

    mooreamps Senior Member

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    No need to re-bias. Bias on a gain stage is the function of the delta voltage between the control grid and cathode. If you drop the value of the SCR, you may find a loss of some touch responce in the amp.

    -g
     
  5. scottl

    scottl Member

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    I prefer the 470ohm in a 6L6. The 1K gets a bit too smooshy and smeary in the lows. The amp has better clarity with 470ohm. I use either 3 watt or 5 watt flame proof metal oxides there. You should not have any chance of damaging things as 470ohm is the nominal recommended value.
     
  6. MikeMcK

    MikeMcK Member

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    Hmm, I need to do some more reading. I thought the drop across the screen resistor was fairly constant and that the resistor just restricted current. In that case, halving the resistor would double the power dissipation, no?
    Thanks... from my many posts, you've probably guessed that tubes are relatively new to me.
     
  7. brad347

    brad347 Member

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    The reason to use high power (2w or greater) screen resistors is not because the resistors themselves are dissipating that much power, but because they're located on top of the tube sockets which are really hot. The higher power rating, the better they will handle this high-stress area.
     
  8. donnyjaguar

    donnyjaguar Member

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    I agree with what brad347 says. The bottom of a tube socket in an inverted amplifier is the worst place for any component.
    As for the math, if you replace a 1k with a 470Ω it may draw ever so slightly more current at idle but the 1k will dissipate more power. See my comments about lower value being more efficient (Heat=inefficiency) and also consider the dynamic of the amplifier in normal use.
     

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