New build: bad hum but hot damn what a sound!

[nee]

Thread title says it all, I guess. Quite a bad hum - (50Hz I think - I'm in New Zealand) - but the richness of tone when I plug in a guitar is incredible!

It's a slightly altered 'Wreck with KT77s - the lead dress is a little different. I know that's important with the TW, but chop-sticking the leads makes no difference at all to the quality of the hum.

I've biased the output valves as per Duncan's pages.

When I remove all the valves apart from the power valves it's dead quiet. When I put in the PI valve it hums. That seems to be where the problem is. I've swapped it out but no difference.

Add the preamp valves and the overall noise comes up some - increase the volume and the general "gainy" noise comes up.

I've checked and double-checked connections and chop-sticked around to no avail. I guess now it's methodical disconnecting and checking individual components.

[JamesHealey]

check all coupling caps have a DC voltage on one side and milivolts or nothing on the other side, to check they're blocking all the DC from the signal.

Check all ground connections with a multimeter (some have a beep feature to check this quickly)

Re-flow all solder joints, check your heaters are centre tapped or have x2 100ohm to ground if not.

Then finally look at your grounding scheme and make sure you've not grounded too many things in different places etc and created a ground loop.

James.

[ChrisM]

Does your heater have a CT or have you created an artificial one?

[nee]

Does your heater have a CT or have you created an artificial one?

CT.

First thing I'll do is re-examine the grounding, I think. At the moment I have the preamp components and filter caps grounded at one place (by the input jack), the PI and its filter cap at another, and the big filter caps and PT CT at another.

[Structo]

Yes, from as quirky as these amps seem to be you are best advised to follow Kens grounding scheme or another well published layout.

I found this in my files.

[M Fowler]

https://tubeamparchive.com/viewtopic.php?t=5691

If you follow the express build guide along with the information the guys have already given you should be able to get your lead dress under control.

Mark

[nee]

The hum was so bad I knew it must have been something major like a missing ground... and it was!

I'm using insulated jacks for the speakers and had not connected the jack socket body to ground. D'oh! No hum at all now!

Quite a lot of "gain" noise with volume on zero, but now I guess it's a bit of gentle tweaking here and there to bring it to perfection!

Yowser!