Fender Hot Rod Deluxe III Hums (even with all tubes removed)

Started by Chumly, February 13, 2023, 05:37:55 PM

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Hi All,

It abruptly started this fairly noticeable, consistent hum, with no external cause that I'm aware of, and no inputs connected, and no problems with the 120 VAC.

The hum does not particularly change with channel selection, or volume control, and the amp can get loud, but always has a distorted sound to it (as long as all the tubes are in place of course).  I would most suspect the filter capacitors, but it came on so suddenly with no external cause that I'm aware of, and most of the time filter capacitors become problematic more slowly (not that they can't fail in a very specific and rapid manner).  I suppose it could be power supply diodes, but they're pretty reliable.

I understand it's likely to be the filter capacitors and/or diodes, it's just that the failure mode seems odd to me.

It's not a ground loop problem as there are no connections to the amp except 120 VAC. I have not disconnected the reverb tank, but that does not seem to be a likely cause.

I have not used this amp much at all, nor modified it in any way, and I bought it new. I haven't taken it apart. I have a nice scope so I could view the power diodes full-wave output, and I could view the actions of the filter capacitors, etc. and I am fully aware of all the HVDC concerns and how to address them. It could be a lifted trace or bad solder joint, and that would fit in with the abrupt nature, except it started when it was just sitting there, although the thermal action of the tubes might have exacerbated a continuity problem.

It just seems odd that the hum started so abruptly without noticeable cause, and that the hum remains relatively consistent even if all the tubes are pulled, and/or the volume is adjusted, and/or the channel selector is activated, etc.

I actually have three Fender Hot Rod Deluxe 112's.  One works perfectly, one the 6L6's red-plate (probably a screen resistor but I have not checked yet) and this one with it's hum that started so abruptly.

I like this YouTube quite a bit 
I think it's much more interesting to live not knowing than to have answers which might be wrong. - Richard P. Feynman


Common issue is open ground reference due to cold solder joint on internal PCBS

take pics, remove tubes with ability to re insert all in exact same order

After tube removal, reflow all PCB solder connections- check reverb pan cable connections and all pot to PCB and Jack to PCB and tube socket to PCB and ribbon cable to PCB  connections

The FX Loop return jack is a switching type which may fail (common fault) - lifts the ground reference, creates hum

Remember if amp is built after 2005, it's a WEE/RoHS lead free solder - requires a modern soldering iron and 450 degrees Fahrenheit temperature



Power transformer internal short ?
Doesn't blow fuses yet?

Or maybe it doesn't know the words.ha ha, tried not to use that one, but couldn't help myself.

I hope you find the source of the humm, and find a fix.
Always remember that you are absolutely unique. Just like everyone else.


If your Hot Rod has those cheap capacitors, like mine had, they should be replaced with better caps after 10 years or maybe some 15 years. 2 years ago, my HRD got new caps and the hum was away.


I think it's much more interesting to live not knowing than to have answers which might be wrong. - Richard P. Feynman