AO-28 hum only at low volume!

[ReindeJong]

Hi There,

I have encountered a weird problem: my A-100 seems to give more (much more) hum at low volumes than at high volume. In fact the amp is completely quiet at 'full volume" but at very low I can hear a 50Hz hum. Is it a right assumtion something is wrong between the 12AX tube (A) and 12AX12 (B) ...there are soms resistors and capacitors in between...

Who knows this problem?
Kind regards from the Netherlands,

Rein de Jong

[geoelectro]

It could be filter capacitors. It’s possible when you raise the volume the added hum is out of phase with the residual hum cancelling it out.

Geo

[ReindeJong]

It could be filter capacitors. It’s possible when you raise the volume the added hum is out of phase with the residual hum cancelling it out.

Geo

Thanks Geo, I think you mean the "tin can" capacitors in the power supply. That's possible because I only replaced the other old capacitors and resistors above 100K a few years ago. I'm not sure if measuring them up with a multi-meter is reliable in this case. In my Leslie 147 it was an easy job just pulling it out and replace it with a new one. So ...this could be it! As soon I have the time I'll look into this.

Thanks again.

Rein

[Jyvoipabo]

Look at the diagram.
There are 2 ways mixed by the pedal.
At the top, the high volume way.
At the bottom, the low volume way with reinforced bass. And so reinforced Hum.

What you observe is perhaps normal.

JP

[geoelectro]

Thanks Geo, I think you mean the "tin can" capacitors in the power supply. That's possible because I only replaced the other old capacitors and resistors above 100K a few years ago. I'm not sure if measuring them up with a multi-meter is reliable in this case. In my Leslie 147 it was an easy job just pulling it out and replace it with a new one. So ...this could be it! As soon I have the time I'll look into this.

Thanks again.

Rein

A simple way to "test" the filter capacitors (in the tin can) is to take a 33uf @ 450V cap and attach it to two clip leads. Clip the ground to the chassis and clip the positive to the filter in question, or each one at a time. Power it up and see if the hum goes down.

Geo

[ReindeJong]

Hi JP,

this is also a possibility...I could suspect the lower line after testing the power supply. To be seen is that I placed the air capacitor near the swell pedal; also a thing to be concerned about...But i used a good grounded cable and also sleeved it. Thanks!

[Wes]

> I'm not sure if measuring them up with a multi-meter is reliable in this case

It is not.

This job will be more difficult than in your Leslie 147, since the cans are not in sockets. But it is the same principle.

Wes

[Jyvoipabo]

Hello,

On the diagram, only the bottom channel (Soft) have a connection to ground. May be it is the source of hum.

Check the wiring for ground loop. Only one side of the shielded câble should be grounded.

JP

[ReindeJong]

Hello,

On the diagram, only the bottom channel (Soft) have a connection to ground. May be it is the source of hum.

Check the wiring for ground loop. Only one side of the shielded câble should be grounded.

JP

Thanks Jyvoipabo, Thanks a lot everybody!

With your tips I am quite sure I am able eliminate this hum. With this knwoledge it's possible to look at different stages in the amplifilier. Doing all this my insight in tube amps will grow. My education is no more than conservatory and some physics a long, long time ago.....But I'm very eager to own some stunning good HammondsO:-):-B