Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: whoops on January 04, 2026, 09:41:53 pm
-
Hello fellow members,
I hope you’re all having a great start to 2026.
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
i=f4ScwMIBzXzasPnr
-
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.
-
I was said that when removing the preamp tubes there was no noise, but I can't be certain on how it was tested because the noise might be an intermitent fault and a bit random when it appears, although when it happens the amp is warm.
As for the filter capacitors, those can be changed no problem, but I don't see the filter caps being the source of this specific noise.
Also although these amps have an higher nose level compared to the standard Deluxe Reverb circuit, like you've said, this specific noise in the video it's not related to that, they're different things, this noise is a Fault.
I will try your suggestions and thank you so much for your reply and for being helpful