1st gain stage DC blocking cap

[romberg]

From time to time, I think about the DC blocking cap on the first gain stage. Not the one often connecting the plate to the next gain stage but the lack of one on the grid on many amps. Because the input jack is often connected (through a grid stopper) directly to the grid of the tube, this means that so is the instrument cable and guitar pickups. So, your guitar pickups are directly connected to a wire mesh fractions of an inch away from the better part of 200 volts DC.

What happens if the grid were to short to the plate? With no DC blocking cap on the input, the voltage is applied to the guitar pickups. With luck, I'd guess the coils would short open and probably that would be the end of it. But it does seem kinda bad design to have the possibility of high voltage DC escaping the chassis.

I'm interested in others thoughts on this. My guess is that the above failure condition (grid shorts to plate) is extremely rare (or you would hear about it more). And adding a cap on the input would negatively affect the frequency response of the amp. There may be other details about this I have not considered. I suppose similar logic could be applied to the OT primary shorting to the secondary.

Mike

[pompeiisneaks]

Well first off, the resistor may burn up, if its a standard 68k or lower (32k) it would get over a half a watt, and at 100k its almost .4 watts so that alone means the resistor has a good chance of burning up, but I've not heard of anyone needing much there in the way of protection, maybe that failure mode is so extremely rare that it's not a major concern? I'm definitely very curious as well.

~Phil

[romberg]

The cathode resistor would then be in series with the plate resistor (say 100k). So, that only means a couple of mA. But it would create a voltage divider. So, the full rail potential would not be applied to the instrument cable. I think it would still be a healthy amount .

EDIT: I don't know why I was thinking of the cathode resistor. Of course it would be the grid stopper in series with the plate resistor. So, 130 - 168k or so to ground. So, about .4 watts with a 68k grid stopper, 100k plate resistor and 260v or so on the rail.

[tubeswell]

Put a DC blocking cap in if it bothers you