wow the temps are cool and I can work on stuff! Q: grounding the grid

[N3DRB The Derb]

Been in garage for 3 hours. I got some 3/4" wide thru the wall tubes to put the W7FG wires through. working on putting some HV ceramic feed-thrus into maul #1 ( I just don't trust the dried up rubber grommets with 1700 volts on them) and adding the Richard Measures mods. Got my copper bus bars and am installing them. Matchbox is getting the wiring error fixed ( I didnt put a ground in the right place ) and the handles & nameplates put on.

I doubt I'll finish all this today, but it sho feels good to be able to work again with no heat issues and I dont have to go anywhere today.

heres the Q: can someone go through why some 'grounded grid' amps actually have grounded grids, and others 'ground' the grid through small by pass caps? I am fuzzy on the reason(s)? to take either one approach or the other. The more sources I look at the more confusing it gets. there seems to be no rhyme or reason why one would take one approach vs. the other.

Is it more of a matter of gain and internal shielding ( or lack of ) of the tubes in use that determines what's the best?

[WQ9E]

I am not sure if there are additional reasons but some of the amps (for example the Heath SB-220) use a capacitor and choke combo to provide a small amount of negative feedback to improve the IMD performance of the amplifier.  Others (often those using tetrodes) keep the control grid above DC ground so grid bias can be applied.  In some of his early GG amps Bill Orr kept the grids above ground to allow for the measurement of individual tube grid current in parallel tube amps.  I imagine there are other reasons and someone will probably contribute those.

Rodger WQ9E

[Ed - N3LHB]

The 'grounded grid' amps in which grids are directly grounded are those requiring no grid bias. When bias is required, then the grid is bypassed to ground with a cap.... Both techniques ground the grid rf-wise one way or another.

[Steve - WB3HUZ]

Zero-bias tubes have the grids bypassed too. The cap is for AC grounding and the direct connection to the chassis is for DC grounding.

[N3DRB The Derb]

ahhhh ok. That makes sense. since rf is a ac source you cant ground a grid effectively at SOME frequency - it will resonate in he vhf range somewheres. the idea is make sure the input circuit does not resonate close to whatever frequencies the output circuit is, becuase then you have a TPTG oscillator which is very bad bad bad.

you can somewhat tune the VHF input circuit resonances by changing cap values and or lead lengths of the caps right?

[Steve - WB3HUZ]

Sure. I wouldn't call it tuning but just make sure any resonance occurs well above the operating range of the tube and you're good to go.

[N3DRB The Derb]

ho-k, thanks man. All I was getting looking on the web was examples of "it should be done this way" without any real explanations as to why.  Maul #1 is getting real close to firing up again.