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I've owned my modified ST-70 for nearly 5 years now. I recently retubed with a quad of Winged "C", a JJ rectifier and Sovtek 7199s. The amp is played for an average of 30 hrs/week. 3 days ago the amp began to produce both a mechanical and an electronic hum. The mechanical hum is from the power transformer and is quite worse with the cage installed (the cage appears to amplify the vibration). If I place my ear to a speaker, I hear an electronic hum of the same frequency and tone as the mechanical hum. I tightened down the power transformer with no results. I've searched through the archives and did not see anything similar to this so any input would be appreciated.

To some extent, most of my amplifiers have a mechanical hum coming from the power transformer. It makes me sad, but I try to get over it. You can try to clamp down harder on the bolts that hold the stack of laminations together. You can also try to isolate the PT from the chassis using felt or rubber washers, or some kind of clever bushings. Very few of my amps have little or no audible mechanical noise.

A few of my amps will even pick up the mechanical noise off the PT and reproduce it out the speakers. I think it happens because the vibrations generated by the PT cause the grids of the tubes to vibrate, and this ends up as a signal on the output.

The whole thing is a curse. In my experience, I've had very little success eliminating it completely. I expect a high quality, vacuum impregnated, potted, very expensive power transformer would suffer less from this problem. It is far more noticeable in a quiet environment. Play your music loud, and keep the amps away from the speakers and your seating position and perhaps it will be less objectionable.

If it is a loud hum, check the multi-cap's tightness to the chassis. Twist it counter-clockwise.

Well,

I was premature in my post. I spent more time last night observing and changed some tubes around. What I found does not make sense to my novice understanding. I noted that the hum is mechanical but that it is most apparent when the cage is installed. Well, I learned further that the hum is tied to a magnetic field and the cage is vibrated by that field. As I lift the cage on and off, it vibrates loudly when I hold it in close proximity to the power transformer. This did not occur a until a few days ago. I removed the cage entirely and the hum subsided and cannot be heard when standing next to the amp. But I would prefer to leave the cage on and I am concerned with the possibility of a problem. Any ideas?

Thanks!

Adam

Hi Adam ...

Did you check the polarization of the cage ?
It might be on backward. (North-South)
How about the screws that hold it on, did you replace them ?

... tom

Power trannies can vibrate/buzz for a variety of reasons. The simplest and most common reason is loose laminations, and tightening the screws usually fixes this. If the bobbin and windings are loose and not solidly attached to the core / laminations, this is a more serious problem. Sometimes this can be fixed by inserting thin wood shims between the bobbin and core, or using some glue to hold it fast.

The other source of mechanical vibes / humis more common with the horizontal mount power trannies, as their AC induced magnetic field can get out and make the steel chassis itself vibrate. This is more of a problem with thin steel chassis (like most Hammond chassis), and the problem can be a real "bear" to fix.

Thicker steel, and aluminum chassis are not so prone to this kind of problem.

I have had a couple of cases with horizontal mount power trannies, in which the top end bell itself would vibrate at 60 Hz or sometimes at a harmonic (300Hz) and the fix for this was to put a piece of adhesive-backed rubber inside the end bell, if it fits inside, or else on the outside.


This can look a bit ugly, but it does stop the "ringing" rather effectively. I suppose that double sided tape and some form or metal "badge" or label would also work, and look better.

/ed B

Hi ,

The most common cause of power transformer buzz besides loose laminations is 2 or 3 volts DC on the 120 AC line , which can be caused by a lamp dimmer or the transformer on the power pole to name a couple .


Mark

If I have a '70 apart for overhaul, and AFTER I paint the transformer covers, I will fill the PT top cover with RTV silicone to about 1/2 - 2/3 full. Then place a thin piece of plastic over the RTV so it doesn't get on the bobbin. Reinstall the cover and place washers between the cover and core and set it aside to cure. After a day or two, remove the washers and reassemble normally.

The RTV does a great job of dampening noise and vibration. YMMV!!
I like the Permatex Ultra RTV, as it doesn't stink!
I've done one stock ST70, and three 270HX PT's, and all are much quieter now, but still, YMMV.