Hmm..June must be Vibrolux Reverb month here at Sunnyside!
This one: a 1969 Silverface. The owner brought it in due to an annoying “ticking” sound due to the vibrato (actually tremelo) circuit. Most Blackface and Silverface Fenders with the LDR (light dependent resistor) tremelo circuit have this problem to some degree.
Ever held an analog (battery powered) wristwatch near your guitar pickups (with guitar plugged into amp, and amp turned on)? Same thing happens more or less. The clock signal is feeding into the analog signal chain.
The tremelo (inthis type of Fender amplifier) uses an oscillator, in conjuction with the LDR, to create the pulsing volume effect. The problem is that the oscillator and LDR signals can couple into the rest of the circuitry. I have seen this so many times. But there are remedies….
This one is a whole lot quieter now!
A 1965 Vibrolux Reverb: My friend Tim has brought in this fine amp to be worked-on. As far as I can tell it has never been serviced, all the parts inside the amp are original (The tubes might just be original as well)! But the old electrolytic capacitors are failing and need to be replaced. See photos below of the big brown capacitors underneath the amp. Also, all the electrolytic preamp cathode-bypass inside the chassis will be replaced. The real showstopper was that the electrolytic bias capacitor was bad, causing all sorts of hum and mayhem! When I am doing this type of resoration work I also replace the screen grid resistors and the grid stopper resistors on the output tube sockets. Add a properly grounded 3-prong AC power cord and this amp will be brought back to life! Once it is up and running, I’ll also check some of the plate resistors and pots. Nearly 50 years of deferred maintenance, getting done!
The bottom most (3rd) photo shows that the electrolytic caps are leaking (two of them) and the other two are showing signs of leaks beginning to develop a leak. So, all these caps are bad, not a surprise as they are not meant to last more than 15 years or so. With new power supply caps this great guitar amplifier is going to come to life!