Weird note decay - Express build

[Ledvedder5150]

Hi everyone, it's been a while. I finished up an Express build tonight. When testing it, I noticed this strange noise while notes are decaying. Any thoughts what to check?

https://on.soundcloud.com/WX9rH

[pdf64]

My guess is that’s it’s unstable.
It’s to get the scope out, verify that, and identify what stages are included in the pfb loop.

[Ledvedder5150]

My guess is that’s it’s unstable.
It’s to get the scope out, verify that, and identify what stages are included in the pfb loop.

Can you explain this in a bit more detail? I have a scope, so I'd like to get to the bottom of this.

[bepone]

first check that all points are soldered.. sound to me like one pin is not soldered

[Ledvedder5150]

first check that all points are soldered.. sound to me like one pin is not soldered

I've chopsticked every component and connection. No issues there.

[bepone]

to me sounds that you have some error, try to replace all the tubes (for test, take from other amp)

[Ledvedder5150]

I realized that I had only biased it to 35ma. Raising it to 42ma has made it sound much better. Those odd decaying artifacts are gone.

[RockinRocket]

Sounds weird to me but I don't think the bias is the issue.

[Ledvedder5150]

I also swapped in a 5751 in v1, and a 12at7 in the PI. So it could've also been preamp tubes.

[Ledvedder5150]

I'm still getting this odd decay noise. I've reconfigured the preamp to K60 spec. The noise seems more noticeable when I'm on the 500pf bright cap. How can I test it with my oscilloscope?

[Reeltarded]

If that, then the input is dirty. RF is amplified at input stage. You have to stop it from entering. It is either shield not good or stopper too small or jack issue or that side of V1 tube is a whiney assed baby.

Tip tap a stick on the chassis near V1 and if it changes it is mechanical, if not then electrical.

[Ledvedder5150]

I think I'm gonna tear it apart and start over. For the life of me, I can't find what's causing this issue.

[bepone]

some grid wire close to the anode? nfb wire over PI? check lead dress better?

[bepone]

there is no any picture of this build?

[Reeltarded]

Ot can even be anything futzy with V1 socket. It could be a tenuous cathode tail ground. It could be input jack.

Frittering RF can be dirty pots on your guitar or even the cable. Everything leading up to the next stage.

Have you checked the grid blocking resistor value and that it isn't somehow damaged?

Does the frittering happen MORE when you moderate your guitar volume, but it disappears at fully opened or fully closed?

Check the congrol bus ground. Are you using isolated pots or are they grounding on the chassis but also on a shared bus and how is that bus wire mounted to theinput grounding point?

Is your presence a part of your control bus ground?