2002 UK AC30TB 60HZ Buzz

[garytoosweet]

I have the 5W 100R pot installed and the amp is now very quiet. The pots position is typically almost all the way to one side, although you need to adjust it depending on what channel you are using and where the volume is set. I thought the humdinger's purpose is to balance the heater voltage to an even amount on each side of the tubes filament, so I'm not sure why its setting would change depending on channel/volume. I'm happy it's working and quiet though.

I tried shielding the PT with a grounded steel sheet but it didn't make a difference. On this amp there PT and OT are on opposite sides of the amp with opposite orientations.

One trick (in use of the amp) I've often made is with nothing plugged into the Normal Channel, or either of the other two channels, the amount of hum heard in the Bright Channel can be somewhat nulled by turning up the Normal Ch volume control. It only helps a little bit, but does lower the hum. I haven't yet installed a humdinger pot on the heater supplies of any of our AC30-6TB amps, but did on one occasion install a separate heater xfmr, disconnecting the heater wiring from the main power xfmr. While that did lower the hum significantly, the down side was sit increased the plate voltage of the EL-84 power tubes, which are already running way too hot. We have a number of AC30-6TB in our rental inventory, so I will be visiting this humdinger pot before too long.

I did notice that same effect with the channel volumes. Strangely, after completing the mods that no longer happens. The video from my original post (from 'Psionic Audio') has some interesting info on taming the hum/noise in these amps. Not sure if it's similar to what you do.

Huge improvement over stock, I think this amp is in a good place now.

Thanks again Helmholtz.

[Helmholtz]

I thought the humdinger's purpose is to balance the heater voltage to an even amount on each side of the tubes filament, so I'm not sure why its setting would change depending on channel/volume.

Thanks for your feedback, glad I could help.

If optimum humdinger setting would always be in the middle, two equal value balancing resistors would do as well.
A humdinger allows to vary amount and PHASE of heater hum.
Seems the amp has more than one hum source. Also Brilliant and Vibrato channels connect to opposite phase PI inputs. So you need to compromise.

I think there's good reason, vintage AC30s had a humdinger.

[g1]

The hum balance pots are meant to be 'set and forget', and it is usually done with all volumes turned down.
Like Helmholtz said, because there are different hum sources, the hum pot will allow you to cancel some of them out (phase cancellation) if you play with it every time you change the amp settings.
But that is not how it is normally used.