I've included a link which provides information on some of the possible causes for hum in an instrument amp. If all else fails to locate the hum, the hum may be caused by a bad filtering capacitor, possibly in the power supply. I suggest you have the amp diagnosed by a reputable musical instrument shop. Troubleshooting Guitar Amplifier Hum eHow
SOURCE: fender M-80 electric Guitar Amp.
You might have a ground problem. Does it do it in different outlets or when you take it to someone else's house? One easy solution to ground problems can be running a three prong to two prong adapter on the power cable. Give that a try and see if it helps.
SOURCE: Fender Hot Rod Deluxe making popping noise
Ah yes, the Fender Hot Rod Deluxe. I have seen more of these in my shop than any other amplifier!! I have even sent recomendations for design changes to Fender about this amplifier.
Sadly, your amp tech may be correct: while the amp is within working specs, you can still have an issue with a bad socket. The cheapest thing to try is to replace the preamp tube(s) you think may be causing the problem. If the problem persists, it may need to be serviced on this issue (by a different 'certified Fender tech'). In defense of the last tech you had work on this amp, these amplifiers have a brittle design and do not gig or travel well (there are a minority of these amps made that do perform). Fender tried to do too much for the price on this model: best advice is to fix it, sell it and get your tone with a more stable model made by Fender (like the new vintage re-issues). Fender, like Marshall, has for years been having problems when they get too far from thier 'working reciepe' of what made them great companies.
You likely have a socket that needs re-tensioned, or a preamp tube not behaving. If it is not these, then a tech had to dive in the house of cards that is this circuit. Sorry for the bad news on this, but it is the truth. I only take on Hot Rods that 'appear' to be fixable. I am a development engineer, build amps from scratch and fix them for the local music shops, and my success rate is a dismal 60%-ish percent on these...
Thanks,-mike
SOURCE: Fender Rumble 100 210 pop noise then hum after strong low e note
I just fixed my bass amp with a similar problem last week: the problem is related to a loose connection on the PCB board (likely).
A bass amp is a rough place for circuitry: the low rumble and power shake a bass amp more than a guitar amp.
In my case, the power transistors needed to be re-soldered and then the grounding bolts and screws needed to be removed and cleaned because the connection was bad. Remember that when two different metals needed for an electric connection are toughing, there is the likelyhood that corrosion WILL happen.
My bass amp did basically the same thing yours did: sounded good at low volumes, but at higher volumes it would act up.
In short, there is nothing you can do (unless you are a repair person): you need to take it to a local music shop and have a tech fix the intermittant contact problem: someone who knows what they are doing should only take an hour at most.
Hope that helps!
-mike
SOURCE: Fender Rumble 100 210 pop noise then hum after strong low e note
Replace C21 (10uF/50V) and C38 (2.2uF/50V) and it will work great.
SOURCE: I am having trouble removing head from Fender
I just removed the head from a rumble 100 I just got yesterday, you have to remove the 4 screws in the top, then the four that hold the handle on the top, and also the two from the sides. Slide the head out the back, but be careful of the wires attached so you don't rip them out (they are sort of short). Also be careful not to touch any electronic parts on the boards, some are extremely high voltage and can electrocute you (even when the amp has been unplugged)
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