Hot Rod Deluxe

SOURCE: Fender Hot Rod Deluxe. Turns on, sounds very good

More than likely a bad solder joint. Where is the next question. How confident are you with voltage measurments? The rails in tube amps are high and can kill you so if you are not inclined to do so please don't. The reason I suspect a solder joint is that it is a failure over time, cold solder joints expand and contract with heat. When they heat up they break connection. Not saying 100% that is your problem but it is very likley.

Testimonial: "Hey thanks for responding!...I am not savy at this so I will take it to an expert before those tuibes blow on my face. Thanks again!"

SOURCE: Popping sound when amp is on. A steady pop,pop,pop

You don't say WHICH lights dim with the popping. IF it is your lights in the house, IMMEDIATELY get an electrician to check your wiring as there is a loose connection or failure someplace in the sytem that would be a fire hazard.

IF the lights you are talking about are those on the amp, there is a power supply failure in the amp which is LIKELY a bad filter capacitor. Take in for repair or learn how to work on the eloectronics safely and do the repair yourself. The part is likely under $5,

SOURCE: I have Fender Hot Rod Deluxe and Blues Deluxe

I want you to do a test. while using the amp plug a pair of headphones into the preamp out... note you will only hear stuff on one side if they are stereo phones and the sound may be fairly weak... while the unit is operating correctely then unplug phones and use it. When it fades out plug in phones compare the level when it was OK... If the preamp output fades lower as well, then we know problem is in the preamp section...

We can do a lot of troubleshooting without opening the amp.

I see nothing in the scheamtics that should be an issue causing this...

Since it didn't fail for the tech, think "what was different?". Are you connecting something else? Foot pedal? Temperature? :Loudness of playing?

Now what else is in common to these two units... AHA! The guitar...

If your guitar has a battery in it, the 20 to 30 minutes fading would correspond to a battery failing and then recovering after you unplug the guitar for a while.

SOURCE: my fender hot rod deluxe

Realize that the number on the volume control is only relative... it means nothing regarding what the amp can produce. With a high level output guitar you can easily drive the amp into distortion at very low volume control level settings... what counts is the actual output sound volume and specifically the voltage output going to the speaker. Once the amp reaches saturation, any further input just pushes it into distortion or "flat topping".
An oscilloscope on the output will quickly show when amp starts to flat top.

SOURCE: Fender Hot Rod Deluxe Volume

Check solder joints send,return and input jacks. Have had this problem a number of times