Hum neutralizing circuit

[sluckey]

Recent discussion over on TAG. I found it very interesting. If I had a single ended amp I would definitely experiment. Video is worth watching...

     

[dwinstonwood]

Thanks for that sluckey. I'm going to try it on my AC4.

So, cathode of EL84 power tube to cathode of EF86 input tube.

Does the bright green wire with the 4.7K look right?

[sluckey]

Looks right, but... Don't you have a tremolo circuit that is injected at the EF86 cathode? That could complicate things. Nothing should blow up. Go for it.   

[dwinstonwood]

Yes, tremolo is connected to EF86 cathode. I thought about that. After all the effort to get it working I don't want to knock it out of balance. I'll try the hum fix, but, if it messes up the tremolo, I'll take it out. As it is, the hum is very low; you have to listen for it. I'm just curious to see how well it works.

[sluckey]

Use a good meter or scope to monitor hum at the speaker.

[kagliostro]

If I've to try the FeedBack resistor as the author say 4.7K till 15K I'll use a resistor with an in series trimmer to listen to changes and decide which value is the better spot

If there is unwanted interaction with the tremolo and so no feedback resistor, I suggest you to give a try to this, who can say, may be you solve the residual hum



Board measure 50mm x 30mm - Till 100mA Load no HeatSink, if more use one or bolt to the chassis

Franco

[HotBluePlates]

Recent discussion over on TAG. ...

The squiggle at the grid-input of V2 was flipped in the drawing.  The grid-input of V2 is in-phase with the cathode output, and the plate-output of V1 is in-phase with its cathode-input.  However, the drawings everywhere else were correct, and the approach will work.

I still prefer killing power supply hum with an extra stage of filtering before the output transformer feed.

[kagliostro]

Ciao HBP

Do you mean that it is as if you have something that is not well fixed to a support that is ringing and instead of fixing the thing that is ringing, you put acoustic insulation on top of it so as not to hear it ?


Franco

[HotBluePlates]

Ciao HBP

Do you mean that it is as if you have something that is not well fixed to a support that is ringing and instead of fixing the thing that is ringing, you put acoustic insulation on top of it so as not to hear it ?

Yes, that is a good comparison.

The Neutralization that Sluckey linked is to reduce hum, specifically in single-ended amps.  It uses negative feedback for neutralization.

I prefer reducing the hum as much as possible by simply adding a stage of filtering in the power supply, as in all single-ended Fender amps before their last-tweed-version.

    I suspect the feedback will have unexpected consequences for the amp's clean tone, its transition from Clean-to-Dirt, and an abrupt increase of hum when distorted (because the feedback is overwhelmed).

    I do not know these things will happen, but they are typical for other cases of feedback in the power section.

    In other forums, people gripe about greatly increased hum intermodulation with their signal when an amp is driven to distortion.  Feedback neutralization of hum seems like I might result in that particular issue.

[shooter]

i've done enough tinkering, agree stopping it at the source is the better option.
I look for <2Vac at PA, <20mVac at V1.
that said, in a quiet room, there will be enough "residual" hum to let you know, ahh, forgot to power off the amp