Squeal/Siren noise Silvertone 1481

[lowell]

EDIT: 1484 not 1481

This amp came in with a good amount of hum and a high pitched squeal/siren noise. I have recapped all electrolytics. Siren sound is still there. The SPEED control for the tremelo affects the siren, and if the tremelo tube is removed the noise goes away completely. The squeal is so high pitched that it is almost out of hearing range. Sometimes the squeal goes "down" in pitch and sometimes it starts as a fizz and rises in pitch. Then the squeal will go away for about 10 seconds. The frequency at which it comes and goes changes and is not consistent. With the phase inverter tube removed the noise is still there. So I'm thinking it's got to be in the PSU or the PA. Being that the tremelo controls the bias this makes sense. I've done some searches for this issue but came up empty. I'm not too good at troubleshooting tremelo circuitry. Any guidance would be great here. Thanks in advance for any help.

Silvertone 1484 Photo by RadioSpares | Photobucket

[nosaj]

Is it possibly a tube issue?

Nosaj

[lowell]

I tried a new tremelo tube and it's still there. I've tried all new tubes actually. Correction to my initial post I don't believe this modulates the power tube bias. However, the bias supply is used to bias V6B's grid.

I'm still at a loss here. Of note as well is that if the DEPTH is turned up half way or more, there is a loud thumpy pulse with it. Again, it happens even with no PI installed. It has to be the trem circuit interracting with the PSU or PA.

[Jazz P Bass]

The only way that I see that the Tremelo/ Strength control can 'modulate' the power supply is through the V6 _photo/resistor neon lamp.

Parallel a good cap across C25B & see if that helps.

Silvertone1484SchematicClean.pdf

[lowell]

C25B? That is the supply for the PI driver tube. Do you mean A or C? Or am I missing something?

[Jazz P Bass]

As blurry as that schematic is.....
The cap that is on V6's plate.
C25C.

[g1]

Here, this is even cleaner (pg.4):

[lowell]

Thanks all. Well, never trust that the circuit is what the schematic says right? Well, I double checked the vibrato circuitry, again. V6 Rk pin 3 only had a 1k, should be 27k. Also, the 15meg R57 from plate to cathode was missing. Corrected those issues. The thumpy pulsing issue is gone now and the tremolo overall sounds much better. However, the siren is still there. I also tried parallel filter caps on the supply nodes and it didn't help. I even hear the siren in standby, although is is far less in volume, barely audible. But still there. Could it be that something is oscillating (like parasitic oscillation) and that this oscillation is being modulated by the tremolo circuit?

[lowell]

Well I unsoldered the point where the resistor in the LDR connects to V1 circuitry. Siren went away. I resoldered it, so that I could then add a high bleed cap to ground, but wasn't necessary. Somehow, from disconnecting/resoldering that joint it fixed it. Odd. Anyway the amp is mostly functional, a decent amount of hum coming from V2, but I don't think it's too obnoxious. Think I'll call it with this.