Noise Problem in Fender ’68 Custom Deluxe Reverb

[whoops]

Hello fellow members,
I hope you’re all having a great start to 2006.
I’m helping a friend troubleshoot a noise issue with his (Modern) Fender ’68 Custom Deluxe Reverb.
After the amp warms up, it produces a very strange noise—kind of like frying an egg. An amp tech has already tried to locate and fix the problem, but so far, without success.

What we know:

• The amp is completely silent when cold. The noise appears after 30–40 minutes when the amp has warmed up.
• The noise is independent of volume or tone knob positions and is present even with nothing plugged into the input.

Previous work by an amp tech:

• Replaced the two power tubes with new ones.
• Replaced all plate resistors with 1W resistors.
• Reflowed all PCB solder joints in case the noise was caused by a cold solder joint.
  

Please check out the noise in these videos and let me know what you think, or if you have any idea what might be causing the problem.
If you have any troubleshooting advice for this situation, I would greatly appreciate it.
Thank you so much

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[stratomaster]

Sounds like the tech was in over his head on a basic problem.  I'd avoid them in the future.

Pull the PI to split the preamp from the power amp. If the noise remains then you know it's in the power amp circuitry. If not, then it's somewhere in the preamp.  You can pull V1, then V2, then V4, replacing each (with the same tube, not another) and observe the effect on the noise. The channels are jumped so V4 will not narrow the problem down to the Vibrato channel as in a standard 763 derivative. This exercise is purely to isolate the area where the issue originates. There will be further investigation after reporting back your results.

Additionally, it would be good to investigate the filter capacitors. These amps use capacitors with a known high failure rate. The 68 Customs also have a high noise floor due to decreased negative feedback and increased preamp signal from the lack of a mixing resistor voltage divider into the phase inverter.