Buzzing Hot Rod Deluxe III PT (wax pot?)

[Iplayloud]

In 2010 I bought the new version 3 HRdlx and it never left the house, never played loud, babied literally. It always made a light buzz when I turn it on (even with stdby off) but the sound of the guitar burries it and I just ignored it. Now I picked up a beat-up 2012 model in local classifieds just to flip it and keep the speaker, because it has a Celestion Blue alnico. Long story short, as I prepare to sell it I notice the PT is 100% quiet on this one. I made a few tests and with my 2010 and the whole amp also vibrates when it's on, while the 2012 is still as bedrock. I wonder should I wax-pot my 2010 PT ? I'm considering swapping them honestly.

[Greg Robinson]

Absolutely 100% do NOT wax pot your power transformer. This will almost certainly degrade the primary-to-secondary insulation. Waxes are also flammable, the proper materials are flame resistant.Full potting also causes heat buildup in power transformers - as such, the transformer must be designed for potting from the start for it to be appropriate.

Transformers are typically dipped in transformer varnish (rated for high insulation/low flammability), sometimes under vacuum. They are infrequently fully epoxy potted (again, with insulation/flame retardant rated epoxy).

First, simply check that the mounting bolts are tight. I frequently find that fasteners on amps are not properly torqued straight from the factory.

[nevetslab]

And also the Core Bolts, unless they are one in the same on that model. I'd have to open one of the HR amps in the shop at the moment to be sure.

[Greg Robinson]

And also the Core Bolts, unless they are one in the same on that model. I'd have to open one of the HR amps in the shop at the moment to be sure.

They serve both functions in this model.

But on that note, they have jam nuts that hold the core and endbells together which end up between the interior endbell and the chassis, so you may need to pay special attention to those if the laminations are loose.

[Mick Bailey]

I had a vacuum potting setup along with a commercial pump but gave up trying to silence noisy transformers. Wax doesn't work because the mechanical forces involved overcome the film strength of the wax. Epoxy is expensive and messy. Urethane varnish is perhaps better - I bought a can of transformer varnish to try out but the cure time is long and it still works out quite expensive and the results are unpredictable. It depends where the noise comes from and mounting bolts are a common source of noise.

[Tom Phillips]

... they have jam nuts that hold the core and endbells together which end up between the interior endbell and the chassis, so you may need to pay special attention to those if the laminations are loose.

Good comment.
If the bolts are loose they could be the cause of or contribute to the buzz so they are a good, and relatively easy, thing to check first.
You do not need to unsolder any leads or completely dismount the transformer. Just loosen the chassis mounting nuts enough to make room to get an open end wrench in place to hold the jam nuts and tighten the screws as necessary. Then re-tighten the mounting nuts.
Let us know how that works out Iplayloud.

[J M Fahey]

I had a vacuum potting setup along with a commercial pump but gave up trying to silence noisy transformers. Wax doesn't work because the mechanical forces involved overcome the film strength of the wax. Epoxy is expensive and messy. Urethane varnish is perhaps better - I bought a can of transformer varnish to try out but the cure time is long and it still works out quite expensive and the results are unpredictable. It depends where the noise comes from and mounting bolts are a common source of noise.

I always have more or less fresh transformer varnish because of regular use but in a pinch (Saturday afternoon when I find the can empty or simply with inadequate 1 or 2 inches left at the bottom) I have used good quality home grade wood/deck/furniture rated varnish bought at Home Depot or a local hardware store for very low price.
Works fine.
Only problem it takes all night to steep in deep, then equivalent time to drip excess out (hanging from a broom handle across 2 chairs, back into the can and then over old newspaper), it is sticky to the touch for a day or two and STINKS for a week whenever transformer warms up, but it does its job.

Stopped using air dry rated transformer varnish because it includes some kinky chemical added to gel it after a few minutes drying (to stop endless dripping and make it fully dry faster, within an hour) because after the can is opened and some Oxygen got in, the whole can gels in a week and becomes unusable.
Oh well.

Never got to making a vacuum chamber but guess one can be kludged by using an empty 20 liter paint or glue can and adapting a vacuum cleaner.

[Mick Bailey]

A paint can would suck flat under a decent vacuum. I used a large aluminium pressure cooker that had a reasonably flat top and cut out the middle the made a thick rubber gasket ring, eight stainless bolts and a disc of 8mm acrylic as a window. Even the 8mm bowed in when it was pumped down. My first pump was a small domestic refrigerator compressor with the inlet connected to the chamber. This worked really well - as good as the proper pump I later got and I was perhaps mistaken in replacing the fridge unit. It's really satisfying to see the foaming as the air is pulled out.

I also tried regular polyurethane varnish - something here called Diamond Glaze which dried very hard. They changed it to water based though so no longer any use.

[SoulFetish]

I wager it's non of those things. Almost every hotrod/Blues * amp that comes in will do that. It's actually the ceramic snubber caps they install in the rectifier circuit (often in parallel with the diodes). Ceramic capacitors can behave piezo-electrically. But in this application, the reaction is inversed. I suspect when they are subject to the voltage switching, it results in a mechanical/physical stress. You are hearing the acoustic noise of the 120Hz switching. Probably amplified off the PC Board. Totally normal, but annoying.

edit: sorry, read fast and didn't notice that it would happen with the standby off. in that case, it's probably what these guys said
carry on....

[nevetslab]

Interesting. I have Hot Rod series amps flowing thru the shop all the time from our rental inventory, along with those from a very busy client, and i don't recall any of those amps buzzing audibly. Most amps begin on the check-out bench at the end of the shop, but the Hot Rod Series and the Blues series combos start at my test bench, with the rear panel removed so I have access to all the inner workings to start with. On hot days, I have a dual 120mm 105CFM Fan box running next to my desk computer, but the rest of the year don't need that noisy fan running.

I just looked at photos I have of the main PCB of some Hot Rod Deville amps, along with Hot Rod Deluxe amps, and there's only a single 8200pF Ceramic cap across the Primary HV winding. Now in the Blues Deluxe, I see that same 8200pF cap across each of the 1N4006 rectifier diodes in the discrete bridge of the HV supply

Fender-Blues-Deluxe-Reissue-2004-Schematic-Rev-A.pdf

Hot_Rod_Deluxe_Schematic.pdf

[SoulFetish]

Dude, every model/version/reissue in this line all kind of blend together into one big heat damaged PCB to me. I've worked on so many. Some are worse than others, but the ones with the the 4 caps in the rectifier seem to exhibit the most noise, but the ones with the single will buzz as well if my memory serves. It's definitely audible in when the back is off, but it doesn't couple through to the speaker, so most of the time it's no big deal.

[nevetslab]

'One big heat-damaged PCB'......boy you have that one right! The PCB quality of those boards is JUST AWFUL! As is the Tube PCB down below it!! Shame Fender also seems to be having the same PCB source producing the boards for the Twin Reverb, Deluxe Reverb and who-knows how many other models. Solder pads peel away with the greatest of ease, traces lift to add to your misery.

Now, it may just be my aging ears not picking up the singing ceramic caps on this units. My right ear has lost an octave in the past six years, no longer hearing pitch at 14kHz. Left ear still has sensitivity out there, but is slowly suffering aging, being 74 yrs and counting. I'll have to move one of my lab mics in to see what I've been missing when I get around to that. You now have me curious.