Input wiring idea(will it work)...

[DIY_Guy]

On my inputs on my amps I normally have my grid stopper mounted directly on the tube, with a shielded wire coming off it to the input jack. The wires shield is left disconnected at the grid stopper, and grounded at the input jack.

Pretty standard I suppose.

In my current build I have a cap in between the grid stopper and tube grid, going to ground.

My question is, for layout purposes, can I go ahead and mount the cap to the tube socket, and solder the shield to it there that goes to ground? I'm worried that this will affect the function of the shield as it normally is.

Can someone assure me that this is okay to do? I'd hate to 'test' it and have it cause issues that I may end up trying to chase down elsewhere in the layout.

[Wilder Amplification]

On my inputs on my amps I normally have my grid stopper mounted directly on the tube, with a shielded wire coming off it to the input jack. The wires shield is left disconnected at the grid stopper, and grounded at the input jack.

Pretty standard I suppose.

In my current build I have a cap in between the grid stopper and tube grid, going to ground.

My question is, for layout purposes, can I go ahead and mount the cap to the tube socket, and solder the shield to it there that goes to ground? I'm worried that this will affect the function of the shield as it normally is.

Can someone assure me that this is okay to do? I'd hate to 'test' it and have it cause issues that I may end up trying to chase down elsewhere in the layout.

IMO the cap isn't even needed. What purpose are you thinking that the cap serves?

[DIY_Guy]

I'm using it to cut frequencys above 8k before hitting the input stage. I should add that this amp I'm building is very high gain and I'm thinking this will help tame the high end. It works in my spice model so I'm going to keep it in my build.

I'm going to try it no matter what, so I'd like to stay on the topic in my original post.

I have no problems with putting it on the board, but if I can do what I mentioned and keep it off of the board, I'd be happier.

If not oh well, no big deal. I'm just curious if it's going to cause noise or something.

[Steve Conner]

Putting a capacitor after the grid stopper means that it can't "stop" parasitic oscillations any more.

You're unlikely to have any because the gm of a 12AX7 type tube is so low, but to be pedantic, you should put the cap before the grid stopper resistor, or use two grid stoppers with the cap in between, or wire the cap as a Miller capacitor from plate to grid instead. (it needs to be about 50x smaller in this case)

[DIY_Guy]

Putting a capacitor after the grid stopper means that it can't "stop" parasitic oscillations any more.

You're unlikely to have any because the gm of a 12AX7 type tube is so low, but to be pedantic, you should put the cap before the grid stopper resistor, or use two grid stoppers with the cap in between, or wire the cap as a Miller capacitor from plate to grid instead. (it needs to be about 50x smaller in this case)

I see, care to elaborate for me on why the cap would defeat the resistors function? I'd genuinely like to know, this is all a learning experience for me, books and schematics can only tell me so much before I reach a trial and error point...

[Steve Conner]

Stray capacitance and inductance around the grid pin form a resonant circuit, which is what causes parasitics. The grid stopper resistor damps the circuit and stops it from resonating.

But by putting the capacitor there you create a new resonant circuit that the resistor can't damp, because it's not in it.

If you must put a capacitor straight from the grid pin to ground, connect the cap's other end straight to the cathode pin, to minimize stray inductance. Then hopefully the resonant frequency will be somewhere way up in the UHF band, where the tube doesn't have much gain left.

[DIY_Guy]

Thanks for explaining. Maybe I'll just use a slightly bigger grid stopper instead.

But just for 'design' sake, I'd still like to know if tying into that shield would cause noise.

Edit- bah never mind, I completely see what you are saying now. I'm just going to nix my whole idea, my mind was drawing a blank there for a bit.

[Wilder Amplification]

I'm going to try it no matter what, so I'd like to stay on the topic in my original post.

I was...Steve just beat me to the explanation of such. But that's where I was going with that.

[Steve Conner]

Oh well, it's nice to know someone agrees with me.

When sizing the capacitor, bear in mind the Miller capacitance of a 12AX7 in the usual guitar amp configuration is about 100pF or so, and that already forms a low-pass filter with the grid stopper, and any other resistance you have there.

The usual 1M volume pot is particularly bad in this respect, it has an output impedance of 250k when it's turned halfway up, and that causes a serious loss of high end with the Miller capacitance of the following stage. Hence Fender's use of "bright caps".

[mooreamps]

On my inputs on my amps I normally have my grid stopper mounted directly on the tube, with a shielded wire coming off it to the input jack. The wires shield is left disconnected at the grid stopper, and grounded at the input jack.

Pretty standard I suppose.

In my current build I have a cap in between the grid stopper and tube grid, going to ground.

My question is, for layout purposes, can I go ahead and mount the cap to the tube socket, and solder the shield to it there that goes to ground? I'm worried that this will affect the function of the shield as it normally is.

Can someone assure me that this is okay to do? I'd hate to 'test' it and have it cause issues that I may end up trying to chase down elsewhere in the layout.

Don't worry about it. It will be fine.


-g

[Bruce / Mission Amps]

Thanks for explaining. Maybe I'll just use a slightly bigger grid stopper instead.

But just for 'design' sake, I'd still like to know if tying into that shield would cause noise.

Edit- bah never mind, I completely see what you are saying now. I'm just going to nix my whole idea, my mind was drawing a blank there for a bit.

You could put a snubbing capacitor around the plate load resistor.

Also, this is an old HAM radio trick....
put a small pF, grounded snubbing cap on the center wire of your coax at the input jack and another grounded one on the open end of shield at the tube end.

[mooreamps]

Exactly, Bruce.

Also, he wants to attenuate frequencies above 8 K before hitting the first gain stage. That maybe a little too steep. You want a little bit of "sparkle" in the tone. I would use 470 pF as a start point, and then go from there.


-g

[Joey Voltage]

The usual 1M volume pot is particularly bad in this respect, it has an output impedance of 250k when it's turned halfway up, and that causes a serious loss of high end with the Miller capacitance of the following stage. Hence Fender's use of "bright caps".

Does it? I would assume the worst case scenario regarding the impedence at the output of the typicall 1M gain or volume pot would be closer to 2/3rd to 3/4 of the way up given that it is a log taper most of the time. (although this would be ignoring all other impedances) even then you would probably use that to your advantage if you are playing around with high gain designs..... In which I Personally don't find much use in frequencies or above 5K-6K

[Joey Voltage]

Thanks for explaining. Maybe I'll just use a slightly bigger grid stopper instead

Don't do that., it can make things worse in some cases. others have already suggested other ways to go about this. grab a 390pf-560pf cap and put it either across the plate resistor, or between the cathode and plate pins in the socket.