Why does Merlin recommend these grid stopper/leak values?

Discussion in 'Amp Tech Center' started by EETStudent, Feb 19, 2020.

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  1. EETStudent

    EETStudent Tele-Meister

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    I am reading through Merlin Blencowe's book on designing preamp circuits for guitar and bass. He offers two suggestions for reducing preamp noise and increasing signal-to-noise ratio: keep the grid stoppers as small as possible, and keep grid leak resistors as large as possible. He then describes a general medium-gain topology, and at the grid of the third gain stage he proposes the following arrangement.

    upload_2020-2-19_10-46-57.png
    The 820K grid stopper seems large, and the 220K grid leak seems small. This seems to go against his recommendation of keeping grid stoppers small and grid leaks large.
     
  2. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Designing an amp requires weighing several general principles against one another, because any given part or stage in the amp is likely performing two or more functions governed by different principles. Merlin's medium and high gain topologies all include multiple (3+ I think) gain stages. This requires pretty large and carefully chosen attenuation between stages, or else the signal voltage would be amplified too high to quickly and amp functionality would suffer.

    That 820k:220k voltage divider's main job is to send a lot of the preceding stage's signal amplitude to ground. The compromise is that it goes against the principles you mentioned at the top. You might ask next just what are the consequences of the higher grid stopper and the lower grid leak, in terms of noise or performance. And I think the answer is that it just doesn't matter that much. Perhaps its a negative effect, but a negative effect of negligible size, or at least the negative effect is outweighed by the need to reduce signal without altering the tone too much in order to get the amp's gain structure right. So it makes good sounds across a wide sweep of the controls and across a wide range of instrument (plus signal chain stuff like effects) input signals.
     
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  3. King Fan

    King Fan Friend of Leo's

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    What he said. :)

    Merlin's small grid stoppers are especially pertinent in the input stage, where he notes: "...the input grid stopper adds the most... Johnson noise (hiss) of any resistor in the amp. (A 68k stopper generates at least four times more hiss than a typical 12AX7!)."
     
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  4. EETStudent

    EETStudent Tele-Meister

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    Thanks for the insight.
     
  5. Jon Snell

    Jon Snell Tele-Meister

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    That is not a grid stopper. A grid stopper feeds the grid, directly on the grid, not as a potential difference.
    A grid stopper is an anti-parasitic device to help avid HF take off or noise in valves designed fo high frequency use. That configuration just reduces the amplitude on the grid by about 2/3rds.
     
  6. trobbins

    trobbins TDPRI Member

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    A grid stopper also can have the function nowadays to 'soften' the audible character of a stage that is being over-driven by the previous stage (or input).

    That can mean using quite high value series resistance in line with the grid for latter stages such as a PI stage just prior to an output stage, as the signal can be at quite a high level by then, and some PI stages present very high input impedance for starters. Merlin especially has presented some scope shots of the impact of the overdrive, and how a series grid resistor can alleviate that. In that respect, the 'top' resistor in a resistor divider is effectively in series with the next stage grid when the previous stage is causing grid conduction overdrive - and is often still referred to as providing 'grid stopper' style action. The same applies to the output stage grids, that use a resistor that various manufacturers have found provides an appealing overdrive sound character, as well as providing some defence against the dark arts of parasitic oscillation.

    And as indicated in earlier post, Merlin has pushed the concept that there is often no good reason for having a guitar input stage stopper that is above about 10k, given the almost ubiquitous vintage use of 68k or 47k, with the advantage for modern amps with high gain and the use of metal film resistors that a 10k stopper then doesn't degrade the noise floor of the input stage.
     
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  7. SoK66

    SoK66 Tele-Afflicted

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    Fender has started putting 10k grid stoppers on all of the preamp grids. Check out the 5E7 Bandmaster Reissue, '68 DR Custom, '64 BFDR Handwired, etc. I'm experimenting with them on a noisy BFDR right now and haven't decided if they're worth the trouble.
     
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