Marshall Origin mod troubleshooting

[Ledvedder5150]

I'm hoping someone can help me troubleshoot issues I'm having. I've modded my Marshall Origin 20 to JCM800 spec based on this, https://youtu.be/Q_hiXjOS1Y8. I'm getting very low volume and barely any gain. I've checked all resistor and cap values, tried different tubes, checked ground points and continuity. All my voltages seem within reason. Any thoughts about areas to focus on?

This is the amp with the master and gain dimed. https://youtube.com/shorts/zZCw9pCXTJs?feature=share

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[Reeltarded]

using probe start at PI and touch each connection from there to the input along the signal path. The problem is behind the last strong zzzzt noise.

[Ledvedder5150]

using probe start at PI and touch each connection from there to the input along the signal path. The problem is behind the last strong zzzzt noise.

The last zzzzt noise? Can you elaborate on this?

Do you mean the multimeter, or a chopstick probe?

[Reeltarded]

If you are using a single probe, like an insulated screwdriver, and touch at each point in the signal path starting at the PI and working toward the inputs you will find a place where you didn't hear the expected noise.. you will have to judge that yourself by continuing toward the input a couple more points and determine where the last strongest noise happened.

The problem is between the weaker than expected point and the last strongest point.

This is with the amp ON and OPERATING with the master moderated. Don't touch anything without having an insulated tool to probe.

[Ledvedder5150]

Here's a short video I did of the signal trace from v2 back. Let me know if my assumptions of an issue at v1 are correct? https://youtu.be/Ce-xK5CM_cI

[Ledvedder5150]

I pulled the boards out again today. Traced the signal again. Re-did some solder joints. After putting it all back together, I'm still having the same issue. Ok, here's an interesting thing I found. If I jumper the 10k cathode resistor on v1b with alligator clips, the volume and gain increases. It still doesn't sound right, but I thought it was something interesting. I did the same on the cathode resistors on v1a and v2b, but those both dropped the volume much lower.

[Reeltarded]

Jumper the v1b cathode to what?

You say it is modded to 800 style thing sort of.. Marshall reverses (typically) the A/B side of the tube. Take this into account, at least.

Yes. All the trouble is at v1 including the input jack, wiring to stage 1, weird switchback wiring through low input (if actual 800 style) and the gain control.

You are tapping the connection for v2 at the tube, but let's keep checking every connection in the signal path all the way from there to the input. The first thing going backwards should be the junction of a 470k divider pair. The signal side has a 470p cap over it.. then the actual gain pot.. if it is like an 800.

During this tick tack check the gain control needs to be open or you wouldn't normally hear anything on the input side of the gain control.

There was something about the way it fails that tells me there may be a ground floated at the tail of v1. That divider has a 470k to ground intersecting the 470k signal side with the peaking cap over it. (470p) With the amp off and v2 tube pulled you should measure 470k to ground from the grid on the left side of v2. I read this twice and I hope it's clear.

Because there was a little noise from the 10k side of v1 I think there is a missing ground reference. This is something to check but not to invest hours chasing. It either is or isn't the problem. Let's open the gain control up to about 4 or 5 and keep ticking signal path all the way to the input jacks.

[Reeltarded]

I can't see how everything is wired in the preamp, but if it follows the somewhat hateful confusing 2 input scheme there could be a fault where the low input is closed and grounding the signal side at the second stage. This is also something to know about and take a look at. Sorry missed responding for a couple weeks. I'll make sure to check more often.