Capacitor between screen and control grids

[Chuck H]

I remember another thread here about a MB amp with a short lead off one of the preamp filament legs. It was speculated at the time that it was probably there to mitigate hum by introducing inverse phase. I have some designs that place the input preamp tube very close to the input jack to avoid a long, shielded lead for that circuit. Then a little 4.7pf cap from grid to plate. But even the 4.7pf cap is probably more than I need (and just barely audible) so I've considered just a twist or two between the grid and plate leads there. This idea actually comes from Marshall. I noticed that in their JMP amps the input triode grid and plate leads were often placed parallel for a short run and then tacked to the chassis with a blob of goo. Later Marshalls did use an actual capacitor between the grid and plate on the input triode. Then, still later, Bruce Zinky did on some Fender designs. Marshall and Bruce having not come up with Peavey's clever solution for achieving this without an actual capacitor on PCB's where leads can't be placed side by side or twisted intentionally.

[Helmholtz]

Then a little 4.7pf cap from grid to plate.

That will increase the amp's input capacitance from typically 150p to around 400p (Miller effect), lowering PU resonance.
I guess you prefer a low capacitance guitar cable to compensate ?

[Chuck H]

That will increase the amp's input capacitance from typically 150p to around 400p (Miller effect), lowering PU resonance.
I guess you prefer a low capacitance guitar cable to compensate ?

Not really. I like a plain 20' instrument cable. I'll use a 10' if that's all I have on hand but I prefer a 20'. And nothing fancy or expensive in the "low capacitance" variety. Just whatever the guitar store sells off the rack. I think it was you that recently pointed out Jimi's preference for the high capacitance coily cable. And I've noticed that many tone sensitive pro's use a pair of 20 footers coupled together. That may be partly for freedom of movement on stage, but you could use a wireless for that. So I'm thinking it may be for the lowered resonant peak.

The addition of 250pf input capacitance is really only like an additional 8' of cable. No big deal in my opinion. And like I mentioned, I'm considering lowering that capacitance and even eliminating that capacitor in favor of just a couple of pf by a twist in the leads. Not so much for personal tonal reasons but because most players prefer a the brighter tone and removing the cap means there's no component that could short and put HV on the input jack. I know there are capacitor types designed with an OPEN failure mode. And I use those. But I still think about it.

[Helmholtz]

To me a low capacitance cable means below say 400p. I don't like low capacitance cables.

I prefer a guitar cable having around 1nF (+/- 200p).
Gives me the best tone with my guitars (vintage type PUs).

[Leo_Gnardo]

It looks like Lombard street..

Been there! And we've all seen it in movies. Good description - zig zag pattern.

[Chuck H]

Been there! And we've all seen it in movies. Good description - zig zag pattern.

I lived in the area for a long time. And I've driven Lombard Street. If Peavey ACTUALLY wanted to make that track like Lombard they'd also need to make it bumby!

[Enzo]

I always play through a Classic 30 when I am scoring a movie chase scene.