Weird note decay - Express build

[Reeltarded]

If you tap the input wire with a stick is the wire hyper microphonic? Try this with nothing plugged into the input and with a guitar plugged in and guitar volume @ zero.

[Reeltarded]

I am too ultra focused on the input stage. Chopstick every stage grid until you find something that seems related in frequency to the frittering note tails. Really examine shielding on wires and the dress as bepone said. Curl around seperate and snug flat against the chassis.

Swap all preamp tubes with any other position. Swap them in order 1-2 2-3 3-1 if the response changes we can maybe find the problem if it follows a weak of very strong tube..

[Ledvedder5150]

I've already started dismantling it, so I can't test anything else

Yesterday, I tried tracing it with a signal generator and my oscilloscope. With just a resistive load in the output, I could hear the signal noise from one of the tone capacitors. I think it was the 500pf treble cap. To make sure I wasn't going crazy, I changed the signal generator to different frequencies, and the sound followed suit.

I used a layout from Modulus to build it. I'm not sure if Michael would appreciate me sharing it here.

[Ledvedder5150]

What type of caps are recommended for the treble cap and the bright caps? Right now I'm using mica. Would ceramic be better?

[Littlewyan]

It’s normal to hear some sound without a speaker. I think it comes from the OT.

Use Silver Mica, don’t use Ceramic.

Are you rebuilding it as an Express again?

[Ledvedder5150]

It’s normal to hear some sound without a speaker. I think it comes from the OT.

Use Silver Mica, don’t use Ceramic.

Are you rebuilding it as an Express again?

Yeah, I think so. I liked the Komet 60 sound, but it's not all that different from a few of my Marshall type amps.

[Ledvedder5150]

I found a pic I took before I started ripping it apart. Ignore my attempt at the split load switch, and the few component additions. That was my attempt at converting the preamp to a Komet 60. That odd noise was happening before and after those changes.

20231202_194727.jpg

[Reeltarded]

Silver micas have a reputation for one in many being shit. Possible.

FLY the switch wires ALL THE WAY to the deck of the headbox. That situation is not great, there are also some safety reasons to use the Farady coil instead of shield there depending on wire prep and voltage capacity.

[Ledvedder5150]

Silver micas have a reputation for one in many being shit. Possible.

FLY the switch wires ALL THE WAY to the deck of the headbox. That situation is not great, there are also some safety reasons to use the Farady coil instead of shield there depending on wire prep and voltage capacity.

I'm probably gonna remove the switch anyway. Although I like what it does for my Liverpool, I don't care for it much in this one.

[Reeltarded]

It is rarely something worth doing. That variable cathode tail I described is hundreds of times more useful for a similar but better action with infinite degrees.

[Ledvedder5150]

It is rarely something worth doing. That variable cathode tail I described is hundreds of times more useful for a similar but better action with infinite degrees.

Yeah, can you explain that variable cathode tail thing?

[Reeltarded]

So, you simply add 25k pot on cathode tail to ground. It is that simple and you dial in how much fire overall.

[Littlewyan]

Only things jumping out in that pic are:
- Your grounds, looks like you’re using the jack sockets as ground points.
- Spare secondary is a bit close to the split wiring.
- Maybe what appears to be your OT primary is very long.

I haven’t been able to listen to the recording yet (soundcloud is forcing me to login and I don’t have an account).

Are you using an attenuator by the way?

[Littlewyan]

Ah, found a way to listen. Yes sounds like instability or maybe a bad connection somewhere. Will have to see once you’ve finished rebuilding.

[Ledvedder5150]

So, you simply add 25k pot on cathode tail to ground. It is that simple and you dial in how much fire overall.

Maybe I'm just dumb but I'm not sure what you mean by cathode tail.