Has anyone figured out the Vox AC4HW hum?

[peckjed]

I've seen a few posts looking for solutions to the AC4HW hum issue and was curious if any owners of Vox AC4/15/30HW amps have figured it out. I believe its a 60hz hum as it does not alter when turning the amp up or down.

Below are a stock photo of what just about every AC4HW looks like... and the AC15HW schematic. I could not find the AC4HW schematic, but the main difference other than an extra EL84 is the AC15HW has tube rectification and a clean channel.

Any guidance into getting that hum down would be great!

original board overvw AC4hw1.png

Capture.PNG

Vox AC15 Handwired.pdf

[peckjed]

I’ve found the service manual with schematic: http://dealers.korgusa.com/svcfiles/AC4HW1_SManual.pdf

[xtian]

SE amps hum because they don’t know the words.

I would try adding an extra filter cap and resistor, CLC style.

[peckjed]

SE amps hum because they don’t know the words.

I would try adding an extra filter cap and resistor, CLC style.

Lol. I'd happily teach the AC4 the words if it would stop humming! Any suggestions for additional filter cap/resistor values?

[sluckey]

Refer to the service manual you posted.

1. Break the connection between the junction of the rectifier diodes and point B+.
2. Install a 250Ω/10W resistor between the junction of the diodes an point B+.
3. Connect a 100µF cap between the junction of the diodes and ground.

For even better hum filtering, use a choke rather than a 250Ω resistor.

[peckjed]

Refer to the service manual you posted.

1. Break the connection between the junction of the rectifier diodes and point B+.
2. Install a 250Ω/10W resistor between the junction of the diodes an point B+.
3. Connect a 100µF cap between the junction of the diodes and ground.

For even better hum filtering, use a choke rather than a 250Ω resistor.

So something like this?

AC4 w choke.png

And, would a 270ohm choke suffice? http://www.classictone.net/40-18082.html

[xtian]

Yes, your drawings looks good, but remember that first B+ after the rectifier diodes will not be connected to the OT, the second B+ will, after the choke.

[peckjed]

Yes, your drawings looks good, but remember that first B+ after the rectifier diodes will not be connected to the OT, the second B+ will, after the choke.

Ah, right. I forgot to erase that b+ before the choke. Good catch.

Any foreseeable issues with this choke? 270ohm choke http://www.classictone.net/40-18082.html

[xtian]

Any foreseeable issues with this choke? 270ohm choke http://www.classictone.net/40-18082.html

That's overkill. You don't need more than 50mA current. This one will work great: https://www.tubesandmore.com/products/f ... e-vibrolux

[peckjed]

Any foreseeable issues with this choke? 270ohm choke http://www.classictone.net/40-18082.html

That's overkill. You don't need more than 50mA current. This one will work great: https://www.tubesandmore.com/products/f ... e-vibrolux

Perfect, thank you. Should adding the additional filter cap and choke have any negative effects on an amplifier? Or other areas that I would need to compensate for in terms of perhaps, lower B+? (still new to a lot of this)

[pompeiisneaks]

Any foreseeable issues with this choke? 270ohm choke http://www.classictone.net/40-18082.html

That's overkill. You don't need more than 50mA current. This one will work great: https://www.tubesandmore.com/products/f ... e-vibrolux

Perfect, thank you. Should adding the additional filter cap and choke have any negative effects on an amplifier? Or other areas that I would need to compensate for in terms of perhaps, lower B+? (still new to a lot of this)

Nope the filtering will just stiffen up the power supply a bit, and the choke is an equivalent replacement to the resistor for the voltage drop but you also get the hum rejection of the choke added to the B+ supply.
Win win The B+ might go up a touch just due to that added filtering giving a bit more available storage in that capacitor, less 'dips' on heavy current usage in the amp.

~Phil

[sluckey]

Just a little sidebar... Back in the '60s hum was not a problem with the AC-4, Champ, or other similar small SE practice amps with minimal filtering. Oh, the hum was there alright, but that cheap 8" speaker could not reproduce it very well, so you just couldn't hear it. Then people started connecting a better quality 12" speaker and all of a sudden there was hummmmm.

[xtian]

Just a little sidebar... Back in the '60s hum was not a problem with the AC-4, Champ, or other similar small SE practice amps with minimal filtering. Oh, the hum was there alright, but that cheap 8" speaker could not reproduce it very well, so you just couldn't hear it. Then people started connecting a better quality 12" speaker and all of a sudden there was hummmmm.

I did not know that! I just thought they sucked.

[pdf64]

The original ac4 looks to have a pretty well filtered HT feed https://el34world.com/charts/Schematics ... ox_ac4.pdf

[frankdrebin]

In the old days of tube radios, they used an output transformer that allowed to connect the screen grid out of phase respect to the plate,turn ratios exactly the same of the current ratio between plate and screen,that way no hum.
Why nobody in the amp business never made it tells a lot.