Mechanical Buzz from Power Transformer on BeePre2

[BrucePoi]

Hello,

I've had a mechanical buzz coming from the power transformer since I built my BeePre2, and I've finally started troubleshooting it.  My unit has all of the upgrades but no part substitutions or modifications.  Here's what I've done so far:

- read through the forum thoroughly, first.
- checked incoming AC voltage as someone had an undervoltage condition around ~114VAC which caused a buzz.  From the wall I'm getting 121.4VAC.
- verified the correct sequence of star/lock washers and plastic/fiber washers/inserts in relation to the screw, end cap, transformer, chassis plate, nut, standoff, etc.
- cranked the hardware to within an inch of its life using an 11/32" wrench on the nuts.  No change.
- the latest attempt was installing nitrile o-rings as soft mounts, both underneath the end cap and between the transformer and chassis plate.  No change.
- tried re-torquing the hardware at different levels after the o-rings.  No change.

Still, the problem persists.  It's a mechanical buzz at 120Hz.  It does not come through the speakers, and in fact you can hear the buzzing with the preamp on but the amp (Kaiju) off.  It's not obnoxious, but once you hear it you can't un-hear it.  It's audible at the listening position ~9-10' away with no music on if you listen for it.

The other thing to note is that with the power turned on but the tubes removed on my test bench, there's no buzz.  The buzz only starts when there's a load on the secondary. 

Does anyone have anything else I might try?  I can take pictures if they'd be useful.

Thanks,

Bruce

[Doc B.]

We got a batch of transformers in which a few were a little noisy, in the early days of BeePre2. Did you buy your BeePre2 kit a few years ago?

[BrucePoi]

Looks like I purchased it on 5/25/2023.

[Paul Birkeland]

If you rest your hand on the metal cover of the power transformer, does the noise diminish?

[BrucePoi]

Paul,

I just did some testing.  If I rest my hand on the end cap, the noise does not go away.  I tried a light touch, heavy handed, etc.  I left it on for 10 minutes to warm up to see if that made a difference, but no change.  Also, I didn't notice any vibration on the end cap, transformer, or chassis.

With the nitrile o-rings as soft mounts, I have a ~0.110" +/- 0.010" gap between the flange of the end cap and the laminations on the core itself.  I found if I can wrap my fingers and palms to close that gap, the noise goes away completely.  As soon as a little gap opens up in my "seal" though, the buzz comes through.  This makes me wonder about removing the soft mounts, but I was getting the buzz in the standard configuration too.

Let me know if you have something else I can try.

Thanks,

Bruce

[Paul Birkeland]

On some different transformers I've been using from a different vendor, I've been struggling a bit with the transformer cover rattling against the bits of the transformer under it.  For your Kaiju, it might just be that the varnish underneath is in just the right spot that the top cover can rattle against it, or it could be that bits of the bobbin are sticking up just enough to contact the top cover. 

One experiment you could try is just running the amp without the top transformer cover (definitely put the screws back though) just to see if that offers any kind of relief.  If so, maybe post a picture of what the top of your PT looks like and maybe it's just a matter of cutting some of the plastic away or adding some goop under the cover to keep it stationary.   

[BrucePoi]

On some different transformers I've been using from a different vendor, I've been struggling a bit with the transformer cover rattling against the bits of the transformer under it.  For your Kaiju, it might just be that the varnish underneath is in just the right spot that the top cover can rattle against it, or it could be that bits of the bobbin are sticking up just enough to contact the top cover. 

One experiment you could try is just running the amp without the top transformer cover (definitely put the screws back though) just to see if that offers any kind of relief.  If so, maybe post a picture of what the top of your PT looks like and maybe it's just a matter of cutting some of the plastic away or adding some goop under the cover to keep it stationary.

Paul,

The transformer cover wasn't contacting the transformer itself on my BeePre2 because I had already tried inserting nitrile o-rings as some sort of soft mount between the two on the screws.

I took the end cap off and remounted the transformer, but I'm still getting a buzz directly out of the transformer.  The short video below has a clip of me switching the preamp on without tubes--and it's dead silent.  The next clip in the same video shows me turning it on, with tubes in, and you can hear the buzz.  I'm also showing you my line voltage.  I used the 115-130V option when wiring.

How do I quantify how loud it is, given it's a cell phone video.  Well, my Kaiju's power transformer really is dead silent, even with tubes in and my ear right up to it.  It's louder than the buzz from 1971 Fender Twin Reverb in standby.  At my listening position 9' away, you can hear it--especially if you know what to listen for.  As I said in my first post, once you hear it you can't un-hear it.

Let me know if you have some other things I could try.

Thanks,

Bruce

https://youtube.com/shorts/m824NZHDAic?si=BqbWcAeil9nehqa-

[Paul Birkeland]

I think it's worth giving a replacement transformer a shot.  I believe we will be getting a fresh batch in this month.

-PB

[BrucePoi]

If you can confirm the correct address to ship it, I'll get the old transformer returned ASAP.

Thanks,

Bruce

I think it's worth giving a replacement transformer a shot.  I believe we will be getting a fresh batch in this month.

-PB

[Doc B.]

Bottlehead Corp.
26272 12 Trees Lane NW
Suite D
Poulsbo, WA 98370