Input decoupling capacitor - before V1 grid. Should I?

[Merlinb]

You've never heard of the "Jedi Cap Trick" ?? no, I suppose not.... :[

AFAIK, the Jedi cap trick is the practice of placing a poly' cap in parallel with an electrolytic cap (usually in the PSU). I don't think it has anything to do with this discussion.

[Steve Conner]

These are not the diodes you're looking for...

[mooreamps]

AFAIK, the Jedi cap trick is the practice of placing a poly' cap in parallel with an electrolytic cap (usually in the PSU). I don't think it has anything to do with this discussion.

It was the input cap trick used by EC on some of his gear... Others have called "that" by this name.. How am I supposed to know better by what I read on some of these talk forums ?? :|

Besides, TWNTAWWLF. :}


-g

[Steve Conner]

Well, I spent a happy Sunday afternoon hacking tube circuits, and found something relevant to this thread.

My Ninja Deluxe amp always had an annoying pop when switching channels, and today I was determined to track it down. The amp uses relays for channel switching. Right in front is a relay that connects the active channel's grid to the input jack, and shorts the unused grid to ground.

To cut a long story short, the tube grids had a little DC on them. One channel had 33mV, the other had 5mV. Since the grid leak resistor is 1M ohm, this implies only a few tens of nanoamps of grid current, and I believe it's quite normal: wasn't this how grid leak bias worked? But anyway, when the channel input was shorted, it also shorted this DC voltage, and that was what caused the pop.

Adding an input coupling capacitor to each channel cured it completely, because it took the DC off the relay contacts. So there's one more reason to have input capacitors, over and above the ones we've discussed.

[guitarmike2107]

That interesting about the gird voltage, I had an amp that was hissing like crazy and by changing the valve there was a noticable difference in hiss and the grid voltage was significantly lower too. If you stuck in a higher grid stopper resistor the voltage went up and so did the hiss.

[ThermionicScott]

I have a Strat-copy with a scratchy volume pot that I've been too lazy to clean or replace. Whenever I plug into my 5B3/5B5 build, that scratchiness is much reduced and it's nice. Still should clean it sometime, though...

- Scott

[Chuck H]

I rebuilt an old Traynor Voice Master PA into a single channel Super Reverb once for a friend. He had some old tubes he wanted to use so I put them in. One was a smooth plate Telefunken 12ax7 that I stuck in the V1 position.

When I fired it up all semed fine until I turned down my guitar and still had a small amount of volume.?. So I turned it quickly back and forth, no fix. I figured it for a bad pot until my friend told me that his guitar was doing it also.?. Since there's nothing to speak of between the guitar and V1 grid I replaced V1 and the problem went away. Not sure what could cause this but it seems an input cap would have made a difference, for better or worse.

Since the amp sounded fine except for the wierd guitar volume thing I can't say if there were any bias issues with the Telefunken that may have been complicated by a cap. The replacement didn't sound any different than you might expect changing V1 for a different brand. Broke my heart to throw away his Telefunken smooth plate 12ax7. I don't personally like them for guitar amps but I know how coveted they are.