Reverb pans...WTF

[Steve A.]

I often use small value caps (like 4.7pf) on the input grid.

That would be between the grid and plate? I think you mentioned this in 2011 and I ordered a bunch of low pF caps.
So would two 10pF ceramic caps in series be an acceptable replacement for 5pF?

Thanks!

Steve Ahola

P.S. If 6" of shielded cable produces 10pF why not just use 3" of it? So does 6" of shielded cable connected ground have the same problem or are you referring strictly to plate to grid capacitance? You could have 3" of shielded cable connected to ground at the input jack and 3" of shielded cable connected to the plate...

[Enzo]

Look at the layout drawing in the PV Classic 30 schematic. Look at the copper trace art next to V1. Those "meshed comb" looking traces in the corner are a small value cap for this exact purpose.

GFuys have been making "gimmick" caps for longer than I have lived. That is two pieces of wire soldered to the two points you want capped, twist them together, and cut it to length for the desired picofarad or two you want.

Google gimmick capacitor, the pictures you will find explain it better than I can describe.

And sure, that would include short sections of coax if you like. It can just hang out in the air.

[Chuck H]

That would be between the grid and plate? I think you mentioned this in 2011 and I ordered a bunch of low pF caps.
So would two 10pF ceramic caps in series be an acceptable replacement for 5pF?

Thanks!

Steve Ahola

P.S. If 6" of shielded cable produces 10pF why not just use 3" of it? So does 6" of shielded cable connected ground have the same problem or are you referring strictly to plate to grid capacitance? You could have 3" of shielded cable connected to ground at the input jack and 3" of shielded cable connected to the plate...

Yes, that would be a plate to grid cap. And no, it's not the same as a grid to ground cap of the same value because the effect of a cap between the plate and grid is increased due to the plates amplification of the signal. A cap to ground only recognizes the static grid signal. But the signal from the plate to the grid is much greater, and increases with amplification. So there is a dynamic there as well.

And you could certainly use a shorter amount of shield. But that would leave a length of input lead unshielded (and therefor susceptible to noise). My typical MO is to locate the input grid VERY close to the input jack. So close that there is only room for the grid resistor with maybe an inch or less of leeway. That combined with the small grid to plate cap has proved to be a very stable arrangement. But I'm not above using a grounded shield AND a plate to grid cap when the layout demands. 10pf to 20pf capacitance to ground at the input stage (as with a grounded input shield) doesn't really amount to much. You probably wouldn't even hear it switched in and out.