HD3S hum problem.

[Rhod]

Hello, I'm new to the forum and to DIY audio so please forgive any ignorance or lack of etiquette as regards to both!
I built my HD3S xl and it worked beautifully for about 5 hours then developed a low hum through both channels, independent of the volume control. I've put shorted phono plugs on the inputs, tried it disconnected from everything except power, checked any earthing connections I can find with a multimeter, done a visual check for any loose wires, solder bridges and generally poked around with a chopstick to see if anything makes it better or worse, all to no avail. Could it be a component in the power supply or still an earthing issue? This is my first proper build (previously just a few guitar effects pedals) so I'm pretty much in the dark and any help would be much appreciated as the amp sounded great before this started.

[John Caswell]

Hi Rhod and welcome to the forum.
First things - are you absolutely sure you have wired it correctly?
Second - are the voltages all correct to within ±10%
Third - does the hum occur on both channels? - If not try swapping the valves over.

Try these and report back here please

John

[Greg.]

Welcome Rhod. We’re friendly here so let us know your real first name.

I’m sure the likes of John Caswell and Richard will chip in here very soon as they have the knowledge. My uneducated immediate impression is that as it is on both channels, the fault is in the power supply section. It could readily be a dry solder joint or a hum loop, although the later should be in second place because from what you say, it sounded great for five hours. I would revisit all the solder joints in your PSU section, reflow them and see if this is a solution. If not, as said, it’s more a Richard or John Caswell response you need which surely is on it’s way

[A Stuart]

Wikipaedia -
Rhodri is a male first name of Welsh origin. It is derived from the elements rhod "wheel" and rhi "king".

It may refer to the following people:

Rhodri Molwynog ap Idwal (690–754), Welsh king of Gwynedd (720—754) and about 20 others.

We can cope with Rhod, lucky he doesn't sign himself Molwynog.

[bob orbell]

Just to add my notes, I have built two of these for friends and both did have a small but irritating hum, I addressed both by twisting the transformer cables very tight and routing these and other cables to lessen the hum, I never did or could remove it completely at that time I had no scope. to see the hum. Bob

[Rhod]

Hi everyone, thanks for replying so quickly.

Thanks John, yes it's on both channels. Matthew checked my voltages as a couple were slightly high but he was happy with everything. As for wiring, I was very careful and it worked well and very quietly for the first 5or6 hours.
Hi Greg, real name Rhoddy which is actually Scottish so no Welsh king I'm afraid A Stuart . When I get a chance I'll reflow the power supply joints to eliminate that possibility.
I've looked at the transformer cables which i twisted pretty tightly, and tried moving them around with no effect.
The thing that gets me is that it's both channels and it was fine for a while, which makes me wonder if
as Greg suggests, it's the power supply. Could a faulty capacitor cause it?

[A Stuart]

I do apologise for mis-interpreting the name. We live and learn. No doubt you are fed up with explaining the whole time. I take it you moved to Carmarthen to stop Scottish people like me asking ?

Alastair

[John Caswell]

Hi Rhod,
Just as a silly, please measure the heater voltage on the valve base pins 4-5 and report back here.

John

[Rhod]

Originally Posted by John Caswell

Hi Rhod,
Just as a silly, please measure the heater voltage on the valve base pins 4-5 and report back here.

John

Thanks John, will do.

[Rhod]

Richard, I'm wondering if I might have 'cooked' one of the diodes (C3D04060A X4) or caps due to lack of soldering experience. I'll check for dry joints as you suggest, then look at replacing those one at a time. Thanks everyone for your input, I'll do some homework and get back to you. Merry Christmas.