AC coupled input jack

[RudeBoy]

Why is this not more common? I've only seen it in couple of designs. While I have never heard of grid-plate short in a 12AX7 but I suppose it could happen.

What's the disadvantage over a direct coupled input?

[defaced]

I too am interested in this. The amp I am cloning uses this and uses a low voltage 220n cap. I guess this would act like a voltage fuse, maybe...?

[booj]

It's not necessary for the most part. In the old days when you could buy "transformerless" amps, I guess they used that option as a safeguard against getting shocked TOO bad if the plug was turned 'round the wrong way. They used other tricks as well. a lot of older amps WITH power transformers commonly used capacative coupling for the input stage, and would use smaller caps for the "bright" input (say .001uf) and larger values for the "normal" inputs (.02 maybe) The grid resistor would then form a low cut - high boost filter.

[Enzo]

I don't think he means the death cap. He means a cap in series with the input signal.

The purpose of an input cap between the jack and the first grid is to prevent any DC on the signal source from getting on that grid. It has nothing to do with protecting the musician from a plate to grid tube short coming out the jack at him.

Your guitar won't likely make any DC, but your wah pedal or OD pedal or who knows what else might.

[Steve Conner]

I used the input capacitor in an unusual amp I built years ago:
http://scopeboy.com/baxeq2.gif

It's needed in this application, because the grid floats at about 50V above ground.

I've also seen it in more conventional designs, and as Enzo says it must be there simply to keep DC from other equipment out of the jack.

Some people seem to think it spoils the "mojo". If you were to overdrive the first stage with a booster pedal, it might charge up and cause blocking distortion, but I can't see any other issues. My design wasn't supposed to be used like this: the circuit shown was used on its own as the clean channel, or as a boost in front of the dirty channel.

[BiBi]

Ah-ha! Thank you Mr.Conner! I have been wondering why I was getting a nasty distortion on a recent build with a boost pedal cranked up before it. The first stage is an EF86 and I had read that it is good to keep voltage off their grid with a coupling capacitor. I never have determined why pentodes wouldn't like it any less than triodes however.

[Steve Conner]

Well, I also have an amp with an EF86 front end, and it doesn't like boost pedals either. It makes a thin, nasty, ratty distortion.

I think the screen bypass capacitor can discharge when the tube is overdriven, and that causes blocking distortion just like a grid capacitor would. Maybe try a smaller screen bypass cap, or leave it out altogether, or connect the tube as a triode.

[daz]

If you were to overdrive the first stage with a booster pedal, it might charge up and cause blocking distortion, but I can't see any other issues

Hmmm....i wonder if thats why my classic 30 didn't like clean boosts. Then i modded the input to traditional specs and the cap was then gone. This has been years, but as i recall i never had issues with pedal boosts after that.