Vta St-70 Main Power Transformer Hum

Hello All,

Another small issue has popped up. Since I've been paying close attention to my vta st-70 (see previous posts)I've noticed that the main power transformer has an audible hum to it that appears to resonate through the bell housing. It is not being transmitted through the speakers. Infact this amp is absolutely dead quiet! I have Klipsch Cornwalls and I can hear absolutely no hiss through the speakers at full volume. Obviously a testament to the high quality of it's components in the amp.

I have read that this hum may be caused by the transformers lamination's being loosened due to heat? Can anything be done?

Thanks Dave

So, you're saying you never really noticed the hum till you went all anal retentive about it?

Most transformers will hum - more like resonate - a bit - nature of the beast and usually due to the mechanical mountings. Either ignore it or maybe try adjusting the screw tension on the bell covers and chassis mounts. No rule of thumb really - you may have to go tighter or looser to minimize resonance.

Also could try some isolator feet on the amp. What you're hearing may be generated by the PT, but will be amplified by any other surfaces it contacts. Easy test there is to just pick up the amp a bit and see if the noise calms down.

PS ... since you're now obsessing, have you noticed that all the tubes have slightly different glow patterns and intensity?

.

Wondering if anyone ever tried using nylon bolts and nuts on the PT? Would ground be an issue?

Reason I ask, is there's still metal to metal contact between the chassis and transformers as there's only one isolation washer per bolt. I added some domed faucet washers when I built mine - never had it without, so not sure if that helped or not. The washers I used were undersized and drilled to match the bolt diameter which allowed them to slip into slightly oversized chassis holes - that also centered the bolts so those can't touch the chassis plate. Be a bit difficult to do that after the build though.

Another thought - get some hard rubber flat washers and cut a notch so you can slide them in between the PT mounting tabs and chassis on the top after you loosen the bolts. A wrap or two of black tape on the bolt itself where it would otherwise be able to go metal to metal on the chassis should help isolate things and reduce any residual hummmmmmmmmm ...

Still ... when all is said and done, it's not likely that little bit is gonna matter at any listening level. I know mine has a slight mechanical noise to it, but I can't say as I've ever noticed it even during ultra black passages on uber good source.

PS ... an example of how getting too close can drive you to distraction. I just replaced the water pump on my car the other day. That cured the little bit of a rattle that had been bothering me, but now I can hear a slight whine coming out of the power steering pump. Pointed that out to a neighbor when he stopped by while I was buttoning things up, who of course said "Noise" What noise?"

Hello All,

Thanks for chiming in everyone. A couple of observations here. I have recently moved and because of this two changes have occurred. The potential stability of my incoming line voltage has changed, and second I now have a cat roaming the house. As a consequence I now keep my amp caged, possibly haven't noticed it hum since I moved here.
If anyone remembers my previous issues with biasing tubes, I concluded that I had one bad tube. But I also became aware of the fact that I have as much as 119 volts coming through my outlets. I am wondering if this has become a contributing factor to the hum or not? As a result I noticed that my bias point would move throughout the day from the set 398-400mv to 425 during low power use times. As a consequence of these observations I have now biased the amp during low use times to avoid a spike in bias voltage. Is a variable power transformer or variac in order or am I obsessing?

Thanks Dave

The cage could certainly help act to amplify any mechanical noise ... think of it as a speaker cone and the PT as the driver ...

As much as 119VAC? I've never seen mine that LOW. That said, that's a very good range for a tube amp - most of us are fighting with over voltage issues of 122-124+ which can certainly have an impact on longevity ...

PS ... why do transformers hum?
(Because they can't remember the words)

My speakers have dc bias supplies and I could hear their transformers humming in the quiet of the night. Adding compliant washers between the transformers and chassis did little as the mounting bolts bypassed the washer, conducting the vibration to the chassis. What worked was enlarging the mounting holes in the transformers so I could insert a soft rubber grommet. I avoided over-tightening the hardware so the grommet could decouple the transformers from the chassis. The result was utter silence. I have not seen this done in an amplifier.

j beede wrote:My speakers have dc bias supplies and I could hear their transformers humming in the quiet of the night. Adding compliant washers between the transformers and chassis did little as the mounting bolts bypassed the washer, conducting the vibration to the chassis. What worked was enlarging the mounting holes in the transformers so I could insert a soft rubber grommet. I avoided over-tightening the hardware so the grommet could decouple the transformers from the chassis. The result was utter silence. I have not seen this done in an amplifier.

what about the weight of those speaker transformers? The ones used in the amps, especially the power unit, are quite heavy, using rubber grommets may not work in that manner due to that weight.

In the 80s....we used rubber washers between the chasy an output transformers ....gave better base
But i have found that the bolts can just be losen on the outputs tranfourmers an give a better diff sound than a titend bolt..an on the power trans..To tite can sound hard....an do funny things to the image............just like adding daping....can suck the life out of the sound...
As for hum in any amp preamp.. it can be minny things..if i turn on my toster oven i get hum in my Krell amp an in my OTL amp... not in my Dynaco MK3s..an type lites can case hum.... i work in a old Audio shop in the 80s-90.....Car Audio amps made some hum in the big Mcintosh MC1000....one more resion that i find so called high end.. car audio a joke!.. But some spent more money than home setup...but we did install  some of David Burning's littel OTL tube car Amps...Fun for me...Hummmmmmmmmmmmm...good luck

as has been said already, most 'standard' transformers tend to hum somewhat and usually that hum is not transferred to the audio circuit.
Once music is playing, that hum will be in audible anyway. So unless one really does not like the transformer quietly humming away, it should not be an issue.

most 'standard' transformers tend to hum .................

I have 6ea MK3s...An Jolita 60 watter..... An a real shocker a Conrad-Johnson Premier Four that was made in the 80 with all stock caps..........
you may find noise on a scope....only hum i can hear in or out of the audio circuit is if theres a Short or lose wire..a cap go out...