Audible hum just from power transformer and power switch hooked up to mains

It is your power transformer making the noise and not the switch. You could have a loose winding, lamination, the transformer inducing current into the chassis vibrating it and when you press the switch you dampen it down. Fix it now or it will bother you forever. Just have to troubleshoot it and isolate things until you find it.

 

I have owned several new amps in which the PT's would audibly hum (ringing is the correct term I believe) when the amp's are powered up. It worried me at first but after a bit of reading on the subject, it seems that many power transformers exhibit this phenomenom. All of the amps have performed flawlessly for several years now with no issues.

 

I just a finished a Dumble #102 and I have the exact same issue. There is an audible hum - although I'd describe it more as a buzz - that seems to match the 60 hz of my single coils. This buzz is in the amp chassis and not coming out of the speaker at all.

I posed a very similar question on a different forum and the initial thought was possibly loose laminations of the PT. Although you're not supposed to overtighten the PT nuts, I did add a bit of torque to them. Didn't help. I proceeded to double-check that the PT was bolted tight to the chassis and I've chopsticked the heck out of it to see if anything would change the buzz.

Like you, I've been wondering if it's the IEC potentially. My only other thought was possibly the acoustic hum you can get from ceramic capacitors in parallel with diodes - this was mentioned by a member of the other forum where he stated that it sounds like a fish tank pump buzzing in the background. This is the exact sound I'm getting and I noticed that when I move the ceramic capacitors on my rectifier board, I'm getting a change in the buzz. So, I've ordered some high voltage film caps to put in place of those. Should be here sometime next week and I'm hoping the buzz problem will be solved.

Sounds like you don't have your rectifier board in so the above paragraph may not be of help. I'm just trying to say that I'm right there with you. I'm going to troubleshoot these capacitors next to try and take care of this.

For reference, I used a Hammond 290FX. The amp sounds fantastic and I will say it's probably my quietest build to date with none of the buzz getting to the speaker.

Perhaps it's just a big old talking PT - I don't know and I don't have enough experience to say definitively.

I'll be staying tuned to this thread and will also post updates as I figure things out and / or rule things out.

 

I think that's probably the main reason and potential for stripping.

My PT is also brand new - it's a Hammond so completely different brand too. I thought about seeing if there was any sort of warranty but now that you're mentioning a similar problem with a completely different brand that is also brand new, I'm wondering if it's maybe something else or inherent to the layout or something. The more I listen to it, the more it sounds like a mechanical buzz, which makes sense since it's not coming out of the speaker.

I'm feeling strongly that it's the capacitors in my case. My new ones should be here today. Hoping that I'll be able to get them in tonight and I can report back.

If it's not those, I guess I'm down to thinking IEC or PT.

 

I would finish more before I would worry too much. But what do you have soldered? Is the center tap grounded or anything?

 

It's OK - it's just because it doesn't know the words.

(rimshot)

Seriously though, I've had two power transformers in my 5e3 - one was an old fender DR trans, and the other is a brand new hammond 290BX. Both of them have or had just barely audible hum upon power up that you can hear before the tubes heat up, which sounds like what you're experiencing. It sounds kind of like a fridge almost - the aquarium sound you describe also sounds right. you can juuust barely hear it, but you know it's on.

Anyway, I can tell you it hasn't been a problem for my amp because I ripped out the entire circuit with the DR transformer, including the whole board and the switch, and both it and the hammond made the same little noise at about the same volume, which isn't much. I figured it was just a transformer quirk - the hammond has been great so far, but it's about 5% above voltage spec.

Best of luck! As others have said, watch your grounds, and have a great time.

Edit: ah, i just saw you're building an ODS and not a tweedle dee - sorry about that! i deleted some of my post to avoid any confusion.

 

Ive had two hammonds start buzzing shortly after customers recieved their amps. Im replacing them just to be safe.

 

Can be normal, in my experience. Instead of trying to mount it even harder, try tightening down the bolts holding the actual laminations together. That will often help.

 

Power transformers hum, that much is normal. (google magnetostriction if you're curious). Some do it more, some less.

It's one of the reasons potted transformers have always been a thing.

Output transformers experience the same effect, but since they're passing audio instead of 60Hz they "sing along" with what you play.

 

Some progress to report to you itsGiusto: the capacitor replacement solved my fish tank pump buzz. My chassis is now quiet as a mouse. For further clarification, these were the 4 ceramic capacitors that were wired in parallel with the 4 diodes on the full wave rectifier board:



Sounds like in my case, it was "singing" capacitors. This is the first time I've experienced that.

I guess the big difference between my issue and yours is that your rectifier board is not hooked-up and mine was. I could still rule out everything after the standby switch because I had the buzz with the amp in "standby," and could tell it was in the area of the PT. Chopsticking those 4 capacitors changed the tone of the buzz.

I'm not sure if it's possible for your capacitors to "sing" by just being near your PT / high voltage.

If it's something you decide to pursue, here are the capacitors that I replaced the ceramics with:

https://www.digikey.com/en/products...gTCBcDaIAoFkCiYBsBGAUgIQKzoAYCBpAJQGYCQBdAXyA

They're rated 1kvDC and 500vAC. I thought the 500vAC was cutting it a bit close with 325v coming in each line but it's my understanding that each set of series capacitors should share the AC burden.

Like I said, since your rectifier board isn't wired to anything, this is probably a shot in the dark for you. Hope your build goes well!