Thank you for the tip. I'll look for that in the morning.
cheers

No, unfortunately I don't have an o'scope.

I did monitor V+ and A voltages and they held steady when the fault occurred. V+ at 26 VDC and A at 22
I have changed both C10 and C20

I should add that these schematics are very helpful! Thank you for creating them.

Okay - I did replace C12 once already.  I'll try another.

I have attached the schematic I am using - I downloaded it from this string of posts.
All the resistors are new, but there could be one that is failing under load.
There are some differences in this schematic and my two 40XL amps, but none that I see within the tremolo circuit.

Now for the tremolo problem: the trem comes on strong for a minute or so, and then fades out to nothing.  The audio is not effected, just the tremolo effect.  Using the schematics found in these posts, I have replaced each component in the trem circuit, one at a time, but the problem still exists.  I have also examined the board closely for bad foil traces and solder joints but everything looks good.  When the trem fades, it seems to speed up first just before fading out, which leads me to believe a failing capacitor, but there are only four cap's in the trem circuit and I have swapped each one with no change.  I also noticed that if I shut the amp off and leave it off for a while before powering back up, the tremolo lasts longer before fading, whereas if I shut it off and right back on it cuts out quicker.  The longer I leave it turned off, the longer the tremolo lasts before fading.  Again, this seems to point to cap discharging.  Has anyone experienced this issue with the 40XL tremolo?

Yes, that is correct. It's just that the reverb sounds brassy and shrill rather than a good reverb.  When I put the repaired PCB from the "inop" amp into the working 40XL chassis, the reverb in that amp works fine, so I am convinced the reverb drivers on both PCBs are working well.  The only difference is in the quality of sound coming from the two reverb tanks.

I have a second, fully-functional 40XL that I use as a comparative tool. The two amps are identical, with the same PCB revision (134-5 rev C).  The reverb actually sounds fairly good on the working amp.  I have swapped the PCBs as a test of the chassis and reverb on the "inop" amp, and with the working PCB installed, the reverb on the inop amp works but sounds brassy and a bit distorted - very ugly - but not necessarily weak.  That makes me think the tank itself has an issue.

Got it. Thanks. I have been restoring this 40XL that was totally inop when I started. Now, everything works except the tremolo, which will work for a minute or so and then quit. It's proving to be an interesting and elusive problem. I also hope to improve the reverb as the original tank sounds terrible.

Has anyone found or created a schematic for the SS 40XL Silvertone amp?  I have one with a tremolo.
Thanks