DC voltage on the input of Rocket amps.

[Mark]

One thing I have noticed with the Rocket is the low voltage on the pre amp causes a very small DC voltage to be present on the grid of input of the amp. This doesn't have any affect on tone nor does it affect your guitar till you move your volume pot, then you hear scratchiness through the amp. The problem isn't just present in clones, it was also present in Derek F's real Trainwreck Rocket. I've seen a video of him playing his Rocket through a 4 X 12" cab and the scratchiness occurred, he claimed it was the volume pot, which it might have been but it also could have been the amp.

There are a few ways to get around this, one way is to find a valve which doesn't have the DC present, but I've found the scratchy pot syndrome is worse as the pot gets higher in resistance, for example my Strat doesn't have scratchiness but my Les Paul does, plug the L.P. into another amp and it's fine.

The permanent way to get rid of this issue is to have a capacitor going to the input jack to block the DC voltage but it also means relocating the 1 meg grid leak resistor on the input jack.

Thoughts and findings?

https://tubeamparchive.com/viewtopic.ph ... ratchiness

[Lauri]

With less than 100V on the anode and high value grid leak resistor there will be some grid current that causes small DC voltage on the grid. With your Strat plugged in the grid leak resistance is small enough for there to be no voltage on the grid. You might get lucky and be able to find a tube tube that doesn't draw grid current with such a low plate voltage, but even if you find one it might start drawing grid current later. Grid current in 12ax7 starts flowing when the grid voltage is somewhere around -1V. Warm biasing and a low plate supply voltage isn't a great idea.

[rooster]

This is interesting. I have 3 channels, the TB, the Normal, and the EF86 channel. My first tube DC voltage is around 95. My Tele (with a 250K pot) is slightly scratchy, as you say, into all 3 channels. However, not if I use a pedal in front of the amp, and I typically do. If a higher plate voltage would eliminate the scratchiness I still wouldn't change it to a higher voltage.

There are Magnatones and early Gibson amps that ran a cap up front... Maybe you can experiment with things and come back with something new, Mark?

[Mark]

What I think the fix is, is to take the 1M grid leak off the input jack and have it on the circuit card on the 33K grid stopper and the negative of the cathode cap. Replace the signal wire going to the jack with a cap such as a 0.047uF or 0.1uF, basically a cap that will cover the bass frequencies.

However, not if I use a pedal in front of the amp, and I typically do. If a higher plate voltage would eliminate the scratchiness I still wouldn't change it to a higher voltage.

A higher plate voltage would fix it and some Rocket clone builders have done this, though the higher rail voltage means a brighter amp and more voltage gain (a larger amplified signal).
The reason the effect stops the scratchy pot is the effect would have a cap at the output to block VDC and a similar cap at the input of the effect.

The larger the pot the worse the scratchiness is. I don't hear the scratchiness on my Strat but I do hear it on my Les Paul. Getting back to the Rocket, the only thing I would have to test is how the cap would react when hit hard with an Overdrive pedal, would there be any of the ugly charging effect you see on the grids of push pull output stages.