Blues Jr has me puzzled

[Randall]

1998 green board blues jr comes in for losing power and cutting out, saying it comes back when he moves the speaker plug, and would I replace it with a better metal one. I point out his tube board is looking pretty sorry, and it's been repaired at least once before, so he asks me to replace it. I install an upgrade board, put in a switchcraft jack and fire it up. It doesn't seem as loud as it should, and I fidget with the molded plug, and sure enough, it's bad, so I install a new switchcraft 1/4" plug.

Now here's where it gets weird. It has stopped cutting out, but still doesn't seem loud enough, and it makes quite a loud scratchy sound when I turn the volume pot on my shop guitar, louder than the guitar soiund itself. New cable makes no difference, and guitar is quiet in another amp.

I am stumped. What on earth does changing a speaker plug have to do with this symptom. I am pretty sure it wasn't there when I first powered it up, but not 100%. I inspected my soldering work, looks OK to me. Wiggling the ribbons makes no effect. There is a cliff input jack flown off the main board, but it looks good. I flipped it over and cleaned it. Seems to be a good ground there.

Where do I look for the solution to this odd issue?

https://www.thetubestore.com/lib/the...-Schematic.pdf

[galaxiex]

Got DC on the input jack?

[Enzo]

Did you scope the output for RF? My guts are suggesting you have the plate wires reversed. Those connect to the new tube board, right?

And remember, you didn't just change a speaker plug. You also changed the tube board, and that includes disassembly and other actions, all of which could have knocked something awry. But you reported it didn't sound loud enough BEFORE the new speaker plug was installed. SO we can't likely blame the problem on the plug.

When an amp sounds less loud than expected AND it sounds scratchy, the first thing I check is for oscillation above audio frequencies.

[g1]

You changed the jack as well as the plug? Was the original insulated or was it connected to chassis?

[Enzo]

I looked at the schematic as I posted earlier, and the only chassis connection for the circuit ground was the input jack. The speaker jack was grounded to chassis, but the ground side was not also connected to circuit.

[Randall]

After some more sleuthing, it appears the ribbon traces that serve V1 are inconsistent with the original board. One triode is the same, but the other is not, so that would explain a lot. I have reached out to the seller to see what can be done.

Helps to explain why I have never seen such a symptom before. Thee speaker jack/plug was apparently a red herring. It must have acted this way when I first powered it up, and I didn't notice it, or remember correctly.