I was scoping the input of each stage in an amp build,look what I found hitchhiking on my signal

[SoulFetish]

How about if the input stage anode is scoped?
Has a different valve been tried?

Haven't tried another tube yet...

Here is the signal at the plate of V1:



The signal at the input of the Volume pot, while attenuated, looks pretty much the same.
Now, with the bright cap removed, here is the grid of V2:



even worse it seems. Now here's the weird thing;
This is the signal at the grid of V3:






I have some square wave shots. I'll upload those in a little bit.

[Helmholtz]

Time to see the schematic.

[stokes]

"Now, with the bright cap removed, here is the grid of V2". Why did you remove the bright cap and what happens if you put it back? :

[SoulFetish]

Time to see the schematic.

This build is basically a trainwreck front end (preamp). I'll see if I can find a schematic. It's a strange symptom though. I can watch it begin as a clean undistorted sine wave as the amp heats up and only in the final few seconds as the tubes are heated does the top half of the wave form look like it sinks into the other. So weird

[SoulFetish]

"Now, with the bright cap removed, here is the grid of V2". Why did you remove the bright cap and what happens if you put it back? :

I removed it to see if that would help. I put it back and it looks basically the same. Also, changing the tube has no effect

[SoulFetish]

So here is the signal at the plate of v2.



i mean, that looks clean as a whistle. I dunno. Im stumped

[Helmholtz]

To exclude a probing artefact I would scope grid and plate simultaneously using 2 scope channels.

[SoulFetish]

To exclude a probing artefact I would scope grid and plate simultaneously using 2 scope channels.

Okay, i Broke some rules…. I did scope the grid and plate simultaneously.
But, in the interest of trying to solve this, i made a couple of changes first.
the first one was to bring the corner frequency of the input stage down to about 50kHz, using a 2k7 grid stopper and a 1nF cap from plate to cathode. (minus cable/pickup capacitances).
the second one was grounding the shield on the second stage grid wire to the chassis right at the tube socket.

here is the result:

[Helmholtz]

Okay, i Broke some rules…. I did scope the grid and plate simultaneously.

What rules?
The idea behind my proposal was to find out if connecting the probe to the grid changes the signal.
Not sure what your result with changed circuit means.
With a normal gain stage it seems impossible that the plate signal is clean when the grid signal is distorted.

It is no fun to speculate about effects without knowing how the tubes are wired.

How's the square wave response?

[SoulFetish]

What rules?
The idea behind my proposal was to find out if connecting the probe to the grid changes the signal.
Not sure what your result with changed circuit means.
With a normal gain stage it seems impossible that the plate signal is clean when the grid signal is distorted.

It is no fun to speculate about effects without knowing how the tubes are wired.

How's the square wave response?

Forget about the rules comment.
what i mean by results is, I no longer get distortion artifacts at the grid. See violet trace. The yellow trace is the signal at the plate.
previously, the shielded grid wire for the second stage had the shield grounded at the front panel next to the volume pot.

[Greg_L]

previously, the shielded grid wire for the second stage had the shield grounded at the front panel next to the volume pot.

Wow so moving that ground got rid of the little hiccup in the signal?

[SoulFetish]

After more probing, here’s what I observe. If I probe the grid alone, I can get a little of the distortion we saw earlier. If I probe both plate and grid, the distortion goes away (using either channel as the trigger source).
then if I remove the plate probe, the grid remains un distorted.

[Greg_L]

Maybe you're creating a sort of snubber cap when probing the plate and grid.

[SoulFetish]

Maybe you're creating a sort of snubber cap when probing the plate and grid.

maybe. But why wouldn’t it return when the plate probe is pulled?

Helmholtz, trying to drive a gain stage/tone stack/gain stage with a square wave gives interesting results.

[Greg_L]

maybe. But why wouldn’t it return when the plate probe is pulled?

Maybe it takes a moment for that oscillation to return?