Mechanically isolate power transformer?

[Paul G.]

I have an amp that hums (what else is new?). After checking everything I noticed the power transformer is actually vibrating. I pulled the transformer away from the chassis and checked that all the bolts holding it together were tight (they were). If I hold the transformer in my hand and run the amp, there no hum. When I bolt it back in, it hums. It's not terrible, but it bothers me.

If I mount the unit on rubber washers, with double nuts to lock it in place, is there any possible long-term harm? Rewind is going to be expensive, as is a replacement transformer. I was hoping to use this as a gigging amp.

[Stevem]

Just unbolt and crack open each Bell end by 1/2” and fill it as much as you can with black automotive silicone form-a-gasket.

[ChopSauce]

So the vibration can be cancelled from the inside?

If I mount the unit on rubber washers, with double nuts to lock it in place, is there any possible long-term harm?

Automotive-style: with nylock nuts?

The only long term harm I see is warpage within the bounds of washer thickness. Hardly significant...

[R.G.]

Just speculating a little on why the transformer would be humming...

Is this amp a vintage one that might might have been made in the days when AC line voltage really was 110Vac? If it is, the transformer could be telling you that the actual line voltage is too high for it to be comfortable. Noticeable hum from AC line transformers is often from magnetostriction (!?) and that can be a symptom of over-driving the core.

Has it always done this?

[Paul G.]

It is indeed an old amp. When I bought it someone had bumped up the reservoir filtering from 35uf to 110uf. It's solid state rectifier but I'm thinking that could have put some strain on the PT. There is a bit of goo that leaked and hardened again on the outside bottom.

Again, it's faint until you turn up the volume. At home volume it's pretty quiet actually, but I think playing at volume to cover a drummer it may be an issue.

[Stevem]

What you are detailing in your last post Paul is sounding to me like the Transformer has a issue like 1 shorted winding which will only rear it’s ugly head and make noise when the transformer is called upon to output / transform its max current.

I think now that even if you do muffle it or cushion it the thing is just going to burn itself up likely sooner then later if you lean on it hard!
By the way, what amp is this?

[Paul G.]

Yet another AB165 Bassman. It had mods, which I undid, and I put it into a combo cab with the intention of having a clean but compact gigging amplifier.

[Stevem]

Compact as in one speaker?

[angelodp]

A possible cause may be old solder joints on the B+ connections that are going cold. I had this very same thing on a new Marshall build. The chassis was vibrating, very low, along with hum. After reflowing the connections to all the B+ connections this vibration/hum went away.

[Paul G.]

Compact as in one speaker?

2x10. Smaller than a Deluxe Reverb or Vibrolux Reverb (speakers are angled).

bassman combo.jpg

[Paul G.]

A possible cause may be old solder joints on the B+ connections that are going cold. I had this very same thing on a new Marshall build. The chassis was vibrating, very low, along with hum. After reflowing the connections to all the B+ connections this vibration/hum went away.

All solder joints have been reflowed.

[Stevem]

Note that if it’s the coil winding pack that is loose on the core ( which does happen at times ) then isolating washers will not improving things one bit and your only hope will be filling it up with the sealer / gasket maker I posted about.

A few years ago I got a BF 1967 Bassman that needed a PT so from Mojo tone I got a low voltage Marshall 45 transformer and it was a nice change.
It took then amp from being a too loud to crank up 48 watts of clean power down to 34 watts.

Size wise it was also a drop in for the original!

[Paul G.]

Note that if it’s the coil winding pack that is loose on the core ( which does happen at times ) then isolating washers will not improving things one bit and your only hope will be filling it up with the sealer / gasket maker I posted about.

A few years ago I got a BF 1967 Bassman that needed a PT so from Mojo tone I got a low voltage Marshall 45 transformer and it was a nice change.
It took then amp from being a too loud to crank up 48 watts of clean power down to 34 watts.

Size wise it was also a drop in for the original!

1. You are correct, isolating it didn't work. Maybe reduced the noise a bit, but it's subjective.
2. I've ordered a Hammond 290EX PT to put in there. Eventually I'll get the original repaired professionally, and put it back in.
3. I really, really, really want a big-sounding clean amp in a small package. When I do want a bit of drive, I use a Klon-type unit in front, but I never, ever, ever go really dirty.

[Phil_S]

2. I've ordered a Hammond 290EX PT to put in there. Eventually I'll get the original repaired professionally, and put it back in.

I'm not convinced you'll hear a difference with the Hammond unit. Try it and see.The 290EX is specifically listed as a replacement for AB165. Unless it is something very simple, you are probably looking at an expensive rewind and it probably isn't worth it.

[Stevem]

If I recall right a customer some 12 years ago gave me a Bassman and the 290ex to replace his fully toasted one and I think I had to play with bias resistor on the AC side of the diode to drop a bit more voltage and get the bias pots range back.

I think I also upped the wattage of that resistor also.

The Hammond does have a tad more mass to its core then the original Fender so you might maintain a bit more punch before the output current level starts to nose over!