Input caps.

[utervo]

Very few guitar amps seem to have input caps like in hifi tube amps. I understand in hifi amps it is used to block DC from input source to grid.

Are any of you using input caps to prevent DC to get to guitar in case of a plate to grid short? Has anyone of you ever had (or heard) a plate to grid short and gotten zapped?

[martin manning]

Not me. Note also that the reissue amps currently on the market don't have them either. If there was any danger there I'd bet that the regulatory agencies would have required them.

[Reeltarded]

I use them while I am messing with sasquatch sized plate resistors.



It tickles!

[es345]

Input cap don't hurt. It might prevent DC input to the grid in case of a faulty pedal.

look also here http://music-electronics-forum.com/t31678/

[Structo]

I run a 15pf cap from the grid of the first stage to ground on my 100w D clone.
Not sure I can tell it's there but I figure it should shunt any radio interference.

[ChrisM]

Tom I believe the OP is refering to a large cap in series with the grid (so from input jack to grid).

It's not a bad idea but as mentioned probably not needed.

It is needed however is your amp has power scaling (VVR, VRM, etc.) as when the control is set low you will get some DC on the grid. Don't want that voltage going back to your fancy pickups.

[utervo]

Yes I meant a bigger cap in series with grid. So the signal goes through the cap which will block DC in case B+ goes to grid for some freaky reason. What would be a proper size, 1uf?

[ChrisM]

0.1uF is fine

With the grid leak resistors we usually use it forms a high pass filter well below our audible range.

[xtian]

I just had this problem with my recent "Twenty" build. Brand new JJ 12ax7, and I was hearing a scratchy Volume pot on my guitar. I measured 0.1vdc. Swapped tubes; found a small handful that allowed DC to pass, some that did not. Weird.

[Cliff Schecht]

Old Gibson amps sometimes have this. I've done it on some amps for friends where god only knows what is going to get plugged in. For my personal amps I don't bother usually but it's not a bad idea if many people will use the amp.

[brewdude]

The input cap can be useful when scaling the entire amp's voltages with a VVR.

[printer2]

Old Gibson amps sometimes have this. I've done it on some amps for friends where god only knows what is going to get plugged in. For my personal amps I don't bother usually but it's not a bad idea if many people will use the amp.

If the amp has a grid leak input stage it needs a cap otherwise the input does not get biased properly.

[Cliff Schecht]

Old Gibson amps sometimes have this. I've done it on some amps for friends where god only knows what is going to get plugged in. For my personal amps I don't bother usually but it's not a bad idea if many people will use the amp.

If the amp has a grid leak input stage it needs a cap otherwise the input does not get biased properly.

Ahh yes, this is definitely true and great point. You won't (shouldn't) find a grid-leaked amp without a DC blocking cap. Typically the value is not 0.1uF like stated above though, 0.02uF is what I always see.

[utervo]

So it has to be before (viewing from where signal comes) the grid load resistor, 1M in case of typical 12AX7 input stage, so the grid stoppers are in before the cap?

[Super_Reverb]

So it has to be before (viewing from where signal comes) the grid load resistor, 1M in case of typical 12AX7 input stage, so the grid stoppers are in before the cap?

That's right. For a cathode bias input stage, input jack is connected to first grid through a coupling cap. Typical value 1M resistor is placed from grid to cathode, so that the cathode can "see" the grid. This is part of cathode biasing requirement.

Similarly, when you AC couple from plate of one stage to grid of next gain stage, a grid-cathode resistor is required.


rob