Hiwatt DR103 Copy - Mains Hum and 120 Hz Harmonic Too

[ThreeChordWonder]

Mojototone DR103 100 watt head.

It's produces quite a noticeable mains hum, especially if the input volumes and master volume are set to max. Obviously this wouldn't be heard (much) when playing, but even at lower volumes its more than just noticeable. The Amp sits in a metal chassis, and I added a grounded aluminum cover to the otherwise open bottom, so I don't think it's being caused from outside.

Some quick measurements using a retail grade mid-price digital dB meter placed next to the cab:

Ambient / amp off 42.0 dB

Input volumes and master volume at zero 50.2 dB

Input volumes at 12 o'clock and master volume at zero, 59.2 dB

From here on, there's a 120 Hz additional buzz and some "ticking" noise added:

Input volumes and master volume at 12 o'clock 55.4 dB

All three volumes at max 77.0 dB

Input volumes at zero, master volume at max 50.8 dB

I think, therefore, I've got two or three issues going on here: preamp 60 Hz hum, power amp 60 Hz hum, and preamp 120 Hz hum, which seems to be coming from the Brilliant channel.

I'm going to check the filter cap grounds, and grounds generally, and the next step would probably be to change out the filter caps, maybe one at a time, to find the duff one, if there is one.

Any other ideas / suggestions would be welcomed.

Thanks in advance.

 

[Purgasound]

Always post photographs of your build and attach schematics for reference.

 

[Matthews Guitars]

The ticking implies that a charge is building up and then arcing over, grounding out, and then charging up capacitively, over and over. That's likely to be a ground that isn't well made. It could also be something as simple as a solder connection that hasn't been made or a cold solder joint. Retouch EVERY solder joint and paint every one of them with a permanent marker as you inspect them.

The hum at high levels could be filament hum. Might try installing a filament hum balance pot or reworking the filament string with a tighter twist and even better workmanship.

 

[ThreeChordWonder]

The ticking implies that a charge is building up and then arcing over, grounding out, and then charging up capacitively, over and over. That's likely to be a ground that isn't well made. It could also be something as simple as a solder connection that hasn't been made or a cold solder joint. Retouch EVERY solder joint and paint every one of them with a permanent marker as you inspect them.

The hum at high levels could be filament hum. Might try installing a filament hum balance pot or reworking the filament string with a tighter twist and even better worworkmanship.

Thanks for the advice.

I reflowed all the solder joints and I'm glad to say all the 60 Hz and 120 Hz hum has gone, as has the cyclic popping. You were right about fixing those issues.

I'm left with some hiss (white noise hiss, not hum), which really only get annoying with all the volumes cranked up to maximum. I guess that is fairly normal for a 50-60 year old circuit design. I'm not sure I want to tempt fate by doing any more at this point, but if you've got any ideas about hiss reduction I'd be glad to hear them.

 

[NickKUK]

If it’s full rectified then 120hz won’t be a harmonic of 60hz but the rectification fundamental.

Number of culprits to check:
* current draw too much for mains reserve cap capacity
* caps are degraded this places ectra loaf and causes hum/noise
* ground loops