Weird rattlesnake noise (not due to vibration) coming out of small 5F1 style tube amp

[Seiche]

Hello everyone,

I have a 10w 6L6 Champ 5F1 clone with a SS rectifier that was built in china and is handwired. I started modifying it with the goal to improve its tone. And now I get this weird rattlesnake noise that is not vibration.

Mods: I reflowed a few solder connections, clipped the bright cap and added a grid stop resistor and a 1k resistor in parallel with the 470R cathode resistor to bring it down to 324R in total. I also changed the coupling caps for TAD mustards with the same value (I thought this would improve the sound).

my voltages:
- cathode voltage across cathode resistor: 21.3V
- plate to cathode voltage: 331.7V
- plate voltage pin 3: 353V
with a cathode resistor measured at 324R this comes out at around 90% plate dissipation when running a tung sol 5881 according to the tube bias calculator.

Now I've found when I dime it the amp makes a weird rattlesnake noise when playing. I have built an attenuator and even when i turn this down all the way, the noise persists at constant volume. It feels like this is an electrical noise.
The noise is coming out of the chassis and is independent of the speaker or cabinet or tubes (i used a different speaker, different tubes, and tapped them with a chopstick).
It depends on the notes played and the strength of digging into the strings.

I made a video showing this noise and I have also attached the schematic.

What could be the source of this?

[Stevem]

The amp may be oscillating now due to your cap swap out.
A simple fix for that might be to just solder them in the other way around one at a time from how they are installed now.

[Seiche]

Huh? The coupling caps? They are a not directional. How would turning them around help?

[Stevem]

They are not directional per say, by the way they are made / rolled up like a fire cracker makes one leads of the cap more shielded then the other and sometimes this swap can cure a oscillation issue

[Seiche]

What would be the cause of this oscillation? Would it sound like that?

[pompeiisneaks]

What would be the cause of this oscillation? Would it sound like that?

If the amp is already sensitive enough to be barely out of oscillations and you add some noise at those caps instead of rejecting it, it may cause the oscillations.

Thus if you try to put the outer foil side of the cap to the other side of the signal path, lower impedance or ground, it can reduce or eliminate added noise.

OTOH I've not ready this thread, did you ensure the negative feedback isn't causing this due to inverted transformer leads? i.e. you can have positive feedback instead of negative.

the test is to swap the ground and anode leads of the OT to see if the squeal goes away. Sometimes they get this reversed when setting up the OT leads, and it can make really loud noises or sometimes just a bit of noise.

~Phil

[Seiche]

So I didn't change anything about the OT and it didn't make this noise in the beginning.

Maybe it's clearer what I mean when watching the video I posted. The noise is not coming from the speaker. It's coming out of the chassis of the amp.

Edit: just read this and now its clearer why and what i should do

[Seiche]

So I looked at the pictures I took before doing the mods and the caps that were there before did have a foil side marking to the lower impedance side.
I switched both caps around (can I at least assume that the foil side would be consistent with regard to the marking text?).

I also lifted the NFB, which helped a lot, but did not get rid of the problem completely. The symptoms are attenuated, but the cause is not.

Could this also be the filter caps that might've gone bad? No idea how long those chinese filter caps last, but the amp could easily be 10-12 years old.

And what is the purpose of this cap at the rectifier?

rectifier cap.JPG

[Phil_S]

The buzz sounds to me like there is too much signal input to the grid on the power tube. Do something to attenuate the signal. I'm not sure what. A voltage divider? Is there a grid leak resistor to ground? Maybe reduce the value so it has an attenuation effect. Someone here with better technical chops than me may have a better suggestion. Also, NFB is going to make this worse.

[Seiche]

I think I identified the mistake. There were several ground loops and current loops (power amp and preamp grounds tied together and weirdly looped).

I read a lot about grounding (aikenamps and valve wizard stuff) and redid and separated the preamp and power amp grounds and now it seems the rattling is gone.