I don't understand what's happening here

[ServiceBob]

A friend brought me a homemade amp he had bought at a flea market so that I could adjust the bias. He had just put in a new pair of 6v6s. He told me that it was a clone of a Silvertone 1482. I looked up the schematic and it is very close to the 1482. Actually, the bias was pretty much perfect so I didn't have to do anything. The 270-ohm resistor was the right choice. But I noticed something I didn't understand. there was a small coil of stranded speaker wire siliconed to the chassis near the power supply side of the board. I couldn't see any purpose for it - it was only attached at one end - to the last node of the power rail - so I unsoldered it and removed it. When I turned on the amp to make sure it was ready to put it back into the cabinet I noticed a "knock" that was obviously the tremolo circuit that had not been there when I first got the amp or after I checked the bias. The only other change I'd made was to remove that coil of wire. So just because I couldn't think of anything else I reconnected it. And shazaam! The amp was quiet as a mouse. The knock was gone! I removed it again and the knock was back. So, I've reattached it and re-siliconed it to the chassis. The amp sounds great. The tremolo is working. But I don't understand what the wire is doing or how it's doing it.
I'm not super experienced. I've built several amps and I know enough to adjust bias in a friend's amp. But I'd like to know what's happening. I've never seen this before or on a schematic. I know someone here will know.
Thanks in advance.

[ServiceBob]

I should mention that the "Knock" was present whether the tremolo was on or off. It was very audible in both channels and was louder when the volume was turned up. It responded to the speed control.

[martin manning]

I guess the oscillator is running all the time, and this coil is functioning as a low value cap to ground, with a little inductance thrown in. Is it hanging directly off the screen power supply node?

[Phil_S]

Unrelated, if you still have the amp, I suggest removing the wire nut on the mains supply. Rework with a proper splice and heat shrink, or better, land it on a terminal strip and make proper connections there. You'll need to drill a hole to mount the terminal strip.

[ServiceBob]

Hi Martin,
No, it is not on the screen supply node, it is on the next node. The node that supplies the preamp anodes.

Where the yellow highlighter is on the schematic.

[R.G.]

Martin's right - it's gotta be a gimmick; that being a quick and dirty small capacitor made with insulated wire. It's easy to test if you have a low-value capacitor with suitable voltage rating. Just sub in the cap from the power supply node to ground. It might work to put it out by the preamp tubes, but it also may be position sensitive and you may have to solder the ground end to chassis where the gimmick was. There may be other equally effective positions.

Phil's also right - do the AC mains up correctly. Given that the builder used a wire nut for the AC, they may well have used the chassis for generalized circuit ground too. That makes the whole amp sensitive to funny noises and such - like tremolo ticking. If it were your amp, you might want to dig through it. Given that it's not yours, no need to engage in a labor-of-love rebuild.

[FUCHSAUDIO]

It's the blue tape. It always is...

[Stevem]

That's some of the worst soldering work I have ever seen!

[ServiceBob]

Thanks for the answers.
When you say "low-value capacitor," do you mean 10pf or ?? And why is this necessary as this node is already bypassed to ground by a 5uF filter capacitor?

And since I still have it, I will redo the mains issue and pretty up some of the soldering.

And the blue tape was me covering the owner's name and social security number (he figured that would help him retrieve it if stolen). He's 75 and from the era of engraving your SSN on your valuables.

[martin manning]

I would say 30-50pF for that coil of wire based on a quick measurement of something similar. Electrolytics have pretty high ESR, especially smaller ones, so this is like the practice of bypassing a filter cap with a film cap to lower the ESR. The value is typically 100 or 1000x larger, though. It'd be an interesting experiment if you can try it.

[Reeltarded]

It's the blue tape. It always is...

win

[ServiceBob]

Curiouser and curiouser.
So, I performed a few experiments, and here are my findings.

First, I redid the mains neutral using a terminal strip and reflowed and added solder to some of the uglier eyelets on the board.

Then I disconnected the coil and bypassed the last filter cap with a 39pf/1000V cap, and the click was back. Just as bad as if the cap was not connected.

Then I attempted to try another capacitor value by using some jumper clips, and what I noticed was that as soon as I clipped the alligator clip on the positive side of the final filter cap (where the coil had been tacked), the knock stopped. This was with no capacitor attached and the jumper wire straight up in the air - not coiled. Then I attached the clip to several of the resistors' leads on that same node all the way down the board, and at each point, the click stops. I brought the other end of the wire closer to the bottoms of the tube sockets, and the knock came back softly and some hum, but as long as the lead was up in the air, the knock was gone. FYI the jumpers are 1 foot long 18 ga. stranded wire, and the coil of speaker wire is about a foot long and looks to be 16 gauge stranded.

At this point, I'm probably going to go with the old adage, "if it ain't broke, don't fix it," and put it back as I found it. But I wish I understood it. I have a lot to learn.

[Stevem]

Just for the fun of it I would spin around all of those non disc caps ( orange drops ) in the trem circuit 180 degrees and then unhook that wire coil.

[martin manning]

So if it isn't the wire's capacitance to ground that eliminates the tick, it must be that it is radiating some energy away, probably related to the the intermittent current flow in the oscillator hitting cut-off. I see the grid lead from the phase inverter tube is running close to leads on the oscillator tube socket. Maybe try moving things around a bit there.

[Masco]

Yup, not a gimmick. More like an antenna.
That would suck if your identity was stolen from the amp made on the cheap, but as Dylan said "if you ain't got nothin', you've got nothin' to lose."