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Thread: Using large value screen resistors w/ bypass caps

  1. #1
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    Using large value screen resistors w/ bypass caps

    Has anyone done this? what are the results?

    I got the idea from an arcticle in Vacuum Tube Valley about EL34's, the idea being to decrease the chances of tube failure in amps using modern EL34's.

    I contacted the author and he more or less said to use 10k or so for the screen resistor and then an electrolytic cap to ground, 20uf or so.

    Has anyone tried anything like this? If so, how does it alter the sound of an amp?

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    Isn't that one of the Mesa "Patents" for simulating tube amp sag? The idea being that fro quick transients there is no sag but for long term loud notes the cap can't charge up all the way, lowering the screen voltage & simulating "sag".

    I always thought it sounded like an interesting idea. I'd love to know if anybody has tried it or how it sounds.

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    I found even 1.5k shared screen R (in an old PA that ran 2x 6ca7 at 550v with a 470-ohm screen R shared) seriously reduced both output power and dynamics. It may be that the cap has to be scaled properly with the change in reisistance. It also depends on the B+, however, and the gain of the screen grid. Different tubes (say 6ca7 fatboys vs. el34) may have different screen grid gains. If your B+ is 800v, you would need a much larger screen resistor to end up at the same place as my 550v started at. It's worth a try, since the new tubes have bad screen winding or materials, it seems.

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    I may give this a try in my single ended 807 amp. There is plenty of room inside that one to play around.

    Wiring the tube up this way makes a little sense in that any time you see a pentode preamp circuit, there is a cap from the screen to ground, though usually quite small, .1uf or so.

    example:
    http://www.notbreathing.com/img/SIMPLEPREAMPS.pdf

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    In pentode power stages, at least for fidelity, you try to keep the screen volts constant. By using a larger bypass cap on the screens you come closer to doing so, a smaller one would have an effect on both frequency response and characteristics. With no bypass cap, you essentially have "triode mode". Some bass amps or hifi amps will use a dedicated screen supply to run the power tubes, instead of the whole cap/resistor from the OPT CT.

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    Quote Originally Posted by Chrome Dinette View Post
    Has anyone done this? what are the results?
    I don't say I've tried it. It's how I build all of my power amp stages. I do calculate different resistor values for different tubes ; including the ef-86. The effect is to put a clamp on screen grid current. Now ; in a perfect tube ; where the windings of the screen grid align with the windings of the control grid, there would be zero screen current. In the real world, and in the world of hauling tube amps in the back of cars, trucks, vans, ect, those screen windings can get bumped out of alignment. Thus a current clamp helps keep screens from burning up.

    Quote Originally Posted by Chrome Dinette View Post
    Has anyone tried anything like this? If so, how does it alter the sound of an amp?


    To my ear, it tightens up the tone, and gives the amp a little bit more touch response. I'm not so sure about the sag. Sag and/or attack, is really more of a function of how the B+ drops after the note is strummed.


    -g
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  7. #7
    Senior Member Enzo's Avatar
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    Gary, are you verifying that you use 10k screen resistors?
    Education is what you're left with after you have forgotten what you have learned.

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    Senior Member Old Tele man's Avatar
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    ...it's an RC-time constant function...R of the screen-grid resistor and C of the screen-grid voltage filter capacitor.

    ...hit the tube with a really big signal and eventually, the RC-time constant will become swamped...likewise, hit the tube with a fast repetitive series of signals and (again) eventually the RC-time constant will become swamped...in both cases, the screen-grid voltage "sags" which reduces the overall "effective" tube gain...until the RC-time constant voltage becomes restored.

    ...that's why screen-grid caps are typically VERY big in value.
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    and , what values of R and C are you using in your calculations ?


    -g

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    Senior Member Old Tele man's Avatar
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    Quote Originally Posted by mooreamps View Post
    and , what values of R and C are you using in your calculations ?


    -g
    ...you asking me? If you are, except for custom designs, I just use either the "standard" Fender (470 ohm) or Marshall (1K ohm) values...which basically amounts to almost "...no screen-grid resistance..." in the big picture.

    ...however, for custom designs, I use the "old" power equation that R. F. Shea shows in AMPLIFIER HANDBOOK, 1966, McGraw-Hill, on page 9-20 of the 'Electron Tubes' chapter:

    Rg2 = (Ecc2)^2 / 4*Pg2

    where:
    Rg2 = screen-dropping resistor (Rs, ohms)
    Ecc2 = screen supply voltage (Vs, volts DC)
    Pg2 = maximum screen dissipation rating (Psd, watts)

    ...however, Shea also states: "...some audio output tubes have two maximum screen dissipation ratings, one for continuous operation and the other for peaks of speech and music; the peak rating is usually twice the continuous rating." [emphasis is mine]

    ...and, I use F. E. Termans' approximations (ELECTRONIC AND RADIO ENGINEERING, 4th Ed., 1955, McGraw-Hill) found on page 203:

    mu.screen ~ mu.triode

    gm.screen ~ (Ic2/Ib)*gm.pentode or beam

    r.screen ~ ((Ib+Ic2)/Ic2)*rp.triode

    where:
    "~" = approximately
    Ic2 = screent current
    Ib = plate current

    ...and, thus: r.screen ~ mu.screen / gm.screen

    ...then, equation-wise, I 'aim' for a screen capacitor (C.screen) and the total screen resistance (r.screen || Rg2) to yield an Xc value slightly LOWER than 1/10th of the r.screen value estimated above, ie: Xc < (r.screen || Rg2)/10.

    ...too small of a C.screen value and "sag" will occur too soon (squishy dynamics), while too large of a C.screen value and "sag" will only occur during (basically) continuous over-driven tones.
    Last edited by Old Tele man; 04-14-2009 at 03:15 PM. Reason: corrected "+" to "||" in equations
    ...and the Devil said: "...yes, but it's a DRY heat!"

  11. #11
    Noodle of Reality Steve Conner's Avatar
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    Only beam tetrodes have aligned grid wires that can get bumped out of alignment. True pentodes don't have the grids aligned to start with, so they always draw lots of screen current.

    If you look through the holes in the plate of an EL34, you can often see the screen wires glowing red in patches, even in normal operation. But you never see this with beam tubes unless they're faulty.

    Using a large screen resistor with a bypass capacitor will indeed make the power stage squish like a compressor. That's effectively what you've built, a ghetto vari-mu compressor, since the gain of the power tubes depends on the screen voltage, and you've made the screen voltage depend on the average screen current, which depends on the envelope of the audio signal.

    The large screen resistor will also cripple the sustained output power.

    The above comments apply mostly to class-AB amps. Since the sum of screen currents is theoretically constant in true class-A, the screen voltage shouldn't sag.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Senior Member Old Tele man's Avatar
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    Quote Originally Posted by Steve Conner View Post
    ...and you've made the screen voltage depend on the average screen current, which depends on the envelope of the audio signal.
    ...exactly, but with Class-AB push-pull operation, the average current is only effected by 1/4th of the signal peak (rms-value) change, so it's often not as big a change as many assume, ie:

    Is.avg ~ [ Is.q(dc)/2 + Is.max(rms)/4 ]

    ...which is why having a 'high' idle current (Is.q) often 'over shadows' the signal peak changes (envelope)...and, yes, this method only applys to push-pull Class-AB operations.

    Summary: the "stiffer" the screen-grid voltage (Vs), the more constant (and higher) the 'effective' tube gain (AV):

    AV :: Vs/mu1 ...where "::" means "...proportional to..."

    ...and "stiffest" occurs when there's no resistance in the screen-grid circuit to "drop" voltage as screen-grid current ebbs-and-flows as it follows the signal envelope. This is because the changing screen current creates a negative-feedback back path within the tube...and any screen resistance allows the changing screen current to "modulate" the screen-grid voltage seen by the tube--the more resistance, the greater the screen-grid voltage modulation.

    ...adding a large capacitor to the screen-grid voltage "node" simply attempts to "stiffen" the voltage seen by the screen-grid.
    Last edited by Old Tele man; 04-14-2009 at 05:10 PM.
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    Quote Originally Posted by Enzo View Post
    Gary, are you verifying that you use 10k screen resistors?
    I am verifying that I do use screen resistors. The value I use depends on the tube in question. I use despite the fact that when I first opened the subject on one of these internet forums, I was fairly burned at the stake for doing so, and the resulting backlash was the basis for getting me kicked off "other" forums which names I am not allowed to say. So, if others in the meantime are catching on to this, then that's nice.

    -g
    Last edited by mooreamps; 06-04-2009 at 07:48 AM. Reason: syntax

  14. #14
    Senior Member Enzo's Avatar
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    Not criticizing you in any way, gary. The original post suggested 10k resistors, and your earlier post left me unclear if you used such a high value at all. Seems awfully high to me anyway.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    Not criticizing you in any way, gary. The original post suggested 10k resistors, and your earlier post left me unclear if you used such a high value at all. Seems awfully high to me anyway.
    I do use this value, but only for the EF-86. Since the value has such a direct impact on the "tightness" and "touch response" , I select certain values for specific power tubes, and yes much lower than 10 K.

    -g

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    In a Tweed Twin 5E8A, there is no screen resistor. Does it make sense to add screen resistors of let's say 1K- 3 watt? I'm not thinking about adding the capacitor in parallel like you folks are discussing.

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    Quote Originally Posted by Diablo View Post
    In a Tweed Twin 5E8A, there is no screen resistor. Does it make sense to add screen resistors of let's say 1K- 3 watt? I'm not thinking about adding the capacitor in parallel like you folks are discussing.
    yes, I see that. For this tube, I use a 1 K 3 watt resistor.


    -g
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  18. #18
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    Thanks Gary, I'll do it too.

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