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5F2-A: add screen resistor (470R) to 6V6, like Hoffman?

Discussion in 'Shock Brother's DIY Amps' started by chas.wahl, Sep 13, 2020.

  1. chas.wahl

    chas.wahl Tele-Meister

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    The well-respected 5F2-A layout by Doug Hoffman adds a 470-ohm screen grid resistor connected to pin 4 of the 6V6.
    [​IMG]
    I've read discussion of these, to keep screen current from exceeding plate current, though I have not seen them on the Fender schematics I'm relatively familiar with for vintage 6V6 models, excepting the Deluxe Reverb AB763. In that case, there's no dropping resistor between the OT B+ tap and the screen grid(s) tap, just a choke.

    I did my diligence, I think, and, with respect to an SE circuit like the 5F2-A, have found only this comment by @tubeswell back in January 2019: "On a SE 6V6 amp that is running these voltages, a 1W 470R screen grid resistor is ample. But you don't even actually need one on the stock 5F2A - because there is a 10k screen supply resistor already in the power rail, which acts to minimise high screen current anyway. YMMV". That seems to me like sensible advice, since the addition of the 470-ohm resistor on Hoffman's layout only increases the resistance between OT tap and screen by 5%. Note that the other Fender schematics that appear to omit such a resistor do have, like the 5F2-A, a dropping resistor (of varying value) between the OT and screen taps.

    So, is there some other reason that Hoffman includes this resistor?? It's not connected directly to the 6V6 pin (as many people suggest when these are provided), so I don't see a real "antenna-defeat" benefit to it, as shown, but perhaps there's some other value to adding a bit of resistance here (such as buffering the screen from its connection to the 8 uF filter cap) that has escaped my attention.
     
    Last edited: Sep 13, 2020
  2. tubegeek

    tubegeek Friend of Leo's

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    I think that's the idea behind it - without the screen resistor, the screen can pull current from the screen cap faster than the supply can refill it, and in some cases this can sound nastier than desirable.
     
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  3. andrewRneumann

    andrewRneumann Tele-Holic

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    That’s a good way to think of it. I like to see the screen supply cap as a exactly like a cathode bypass cap. It’s a short circuit to ground for AC signals. So when you are overdriving the 6V6, the pulses of high screen current that go through that 470R are coming from the supply cap, not through the 10K dropping resistor.
     
  4. boredguy6060

    boredguy6060 Poster Extraordinaire

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    I don’t know the answer to your question, but I will add this.
    Tubeswell is an extremely reliable source of info. You will never go wrong following his advise.
    We’re not related or connected in anyway.
     
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  5. chas.wahl

    chas.wahl Tele-Meister

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    Ah, great; thank you. I guess there's no harm in adding it, provisionally. Can always replace it with a wire.
     
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  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    If I add them on the tube, I place them like point to point wiring or like all of the Fender blackface amps.
    I wouldn't put that resistor on the board.

    Your 6V6 pin1 does not have to be connected to pin8. It can be used to hang the screen grid or grid stopper resistor.
     
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  7. chas.wahl

    chas.wahl Tele-Meister

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    I'm planning to "fly" it from pin 4 directly to the B+ rail on that edge of the board.
     
  8. tubeswell

    tubeswell Friend of Leo's

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    While pentodes can be essentially regarded as a 'constant-current' device, they aren't a perfect constant current device, and the relationship between change in plate current and change in screen current under signal conditions is non-linear (delta Ia changes more than delta Ig2) - see attached RCA data sheet.

    In a 5F2A, where you have a 10k dropper resistor feeding the screen node (and the preamp), the effect of adding 470R as Rg2 won't create much difference in delta Vg2. The main change in Vg2 comes from delta Ig2 going through the 10k*. You have about 2mA Ig2 under idle conditions, and about 10mA Ig2 under signal conditions, so delta Ig2 is about 8mA** maximum. 8mA across 10k is 80V. 8mA across 470R is 3.76V

    * Remember that whilst g2 sources its supply from the smoothing/screen cap node, this is sourced in turn from the 10k dropper resistor from the reservoir cap node.

    ** but that is extreme - most of the time the change in Ig2 won't be 8mA

    We are not talking a HiFi amp here. there's all kinds of other distortion effects going on under overdriven signal conditions in a geetar amp. Ballpark stuff is good enough.

    However, having noted all that YMMV - and the best thing to do may be to try the amp with the 470R Rg2 and without it, and see what you think of the difference. (You could put an appropriately rated shorting switch across the 470R).
     

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    Last edited: Sep 13, 2020
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