what, no grid stopper?

[tubeswell]

oddly when i use smaller or no grid stoppers in the gain stages it does the opposite....more high end.

That's because the bigger they are the more high end they cut. They act in conjunction with the inter-electrode capacitance of the tube, as a hi-frequency/lo-pass filter

[daz]

i know, but what i meant was that on the input the opposite happened...seemed to have LESS high end with a smaller resistor. I'm guessing thats because it didn't actually lose high, but that staticy sound was just removed from whatever high frequency it was riding on, but still the odd thing is why the LARGER value didn't.

[tubeswell]

Hmmm.. did you have a fender 68k-68k/1M type front end?, in which case I'm thinking that swapping one of the 68k for something smaller could dump more highs when you are plugged into the 'Lo' input?? (i.e. the input where you usually have 68k input resistor and 68k grid leak, as opposed to the other input where its 34k input resistance and 1M grid leak)

[daz]

It's just a single input with a 1M ground reference resistor and a 68k grid stopper right on the pin. I changed that from 68k to 10k. I guess whats going on is the change from 68k to 10k is too small to hear a tonal difference, but the fizziness that was lessened were artifacts of the high end and not the high end itself.

[Steve Conner]

With respect to the power tube grid current thing:

All tubes probably have some degree of negative grid current, power tubes just have a lot more of it.

Negative grid current comes from four sources:

1. Leakage from screen or plate to grid across the mica supports. Power tubes are bigger, so they have more surface area. If you look inside a tube, you'll see slits cut into the supports to make the leakage paths longer.

2. Grid emission. If the grid gets contaminated with cathode material, and gets hot enough, it'll emit electrons as if it were a cathode. Power tubes have bigger cathodes, bigger grids, and the grid runs hotter because it receives more heat from the cathode. This is why as Merlin says, higher gm tubes leak more grid current. To increase the gm, you need to space the grid closer to the cathode, so it gets hotter and the risk of contamination is greater. Gold plating it helps.

3. Gas. Ionized gas molecules hitting the grid constitute a negative grid current.

4. Leakage in the tube base. Bakelite is a lousy insulator once it gets above about 150'C.

The last three of these mechanisms increase with temperature, and that's why tubes have thermal runaway.