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Old 1st September 2008, 07:41 PM   #1
JandG is offline JandG  United States
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Default Choosing proper g2 grid stoppers for SE UL..?

While searching I found thread that mentioned the importance of G2 grid stoppers for stability while running in UL. The amp I am working with is a 26/6sn7/5881 in SE UL using Hashimoto H-20-7U OT's, I am only using a 100ohm g2 gris stopper @ the moment, but what value should I be using for stability in amp..? The 5881 is running in fixed bias @ plate 260 & g2 @265v & 60mA @ -16.5v .I am running a 1k grid stopper on g1 also.
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Old 1st September 2008, 09:16 PM   #2
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Hi,

there is no general answer to your question, since too many factors involved are unknown or depend on other side factors, like f.e your exact wiring topology.

In general, just use the smallest ohnic value you can get away with without parasitics. If this means just a wire of negligible ohmic resistance, that will be just fine.

The Ia/Ig ratio of a pentode in pentode or UL mode is almost constant over quite a wide range, but leans toward Ig2 when the anode swings downwards to cathode level, especially at very low down swing ("current takeover area").

In turn this means that a static/fixed ohmic resistance at g2 will lead to a non-linear voltage drop across it, if Ig2 is not a linear itself. This necessarily will introduce some nonlinear distortion, because screen voltage swing will not exactly follow anode voltage swing anymore by the desired fixed UL ratio.

Now, considering typical audio power pentodes or BPTs and typical screen stopper resistor values, the typical screen current will not drop much voltage across a 100 ohms or even 270 ohms resistor, so the theory is somewhat moot except for excessively high ohmic values for screenstoppers in excesse of, say, 470 ohms.

IOW, if your design needs screen stoppers in excess of, say, 470 ohms to work stable and without parasitics, donīt use larger screen stopper values but fix the problem for example by reworking your wiring from the OPTs screen taps to the screens of the power tubes.

BTW, the same argumentation and strategy applies when just triode-strapping a pentode or BPT: If you cannot omit screen stoppers altogether, just use the lowest ohmic value that will do. If it turns out to be larger than, say, 470 ohms, fix the _real_ problem instead of upping screen stopper values.

I am somewhat astonished about those stopper resistors showing up and being used everywhere, because in my audio amp building "career" I never ran into a situation where a grid stopper was _required_ for stable and parasitics-free operation. Maybe that is a result of my "wiring style"; I tend to allow some minutes, or even hours for larger projects, for thinking about optimal wiring paths when placing parts on the chassis and before drilling the first hole, not to speak of soldering the first wire.

Regards,

Tom Schlangen
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Old 2nd September 2008, 12:01 AM   #3
JandG is offline JandG  United States
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This is very helpfull to me & thank you for such a great responce. I will stand by this advice, as it only makes sence & will study wiring scheme's & take more time getting layout correct. It is a mess at moment & in seriouse bread board mode, about 5'x5' of amp.......OT's are 2' from signal chassis at moment..allbad * very sloppy, but seems stable, just not the quietest .....a rats nest is basically what I have going on..all will be nice in the real build. Thanks again...
J & G
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Old 2nd September 2008, 01:53 AM   #4
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If it does not oscillate on one of transformer's resonant frequencies avoid stoppers. If it is ringing use Zobels; stoppers are the last resort, and I doubt you will ever need them in the UL amp.
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