Summer project troubleshooting

[goldenGeek]

Hello
I have had this project this summer where I've built this Fender-ish amp from scratch. A friend of mine was interested in an amp with one good clean channel that looked like a Mesa. I figured I'd give it a go and I have spent lots of hours this summer bulding the headshell and the amp. Anyway, it was a really fun project and it sounds pretty good. It's based on the clean channel in the MK-series, which I guess is pretty much like a Fender, hence the placement of this post. I haven't drawn the schematic, it's all in my head, but I drew up a layout in DIYLC.

The problem is some hum, I think it's 100Hz, between the PI and the output. If I pull V2 (PI) the hum is still present. If I short the two 0.1uF caps to each other the hum disappears. Also if I short the one 0.1uF furthest to the left on the layout to ground the hum goes away. If I short the 0.1uF to the right on the layout nothing happens. Is there some obvious thing I have overlooked that cause this hum? There's also some slight 50Hz hum in the pre amp section when the gain and volume are dimed, but I think I can live with that for now.

Btw. the negative feedback in the layout is not connected.

Mesa-ish clean.pdf

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0AE55FD5-2F44-4F19-AC25-CCEA4F7D985F.jpeg

905FD373-74A0-426F-8408-E1ADCD3EC436.jpeg

F0E16A73-5C65-4371-B2F7-2EBE338C385C.jpeg

14CCB8E4-37B6-42F9-8240-A93C0A00AC8B.jpeg

[bepone]

is this norway language? im now in norway, nice 15-18 degrees!

you are mixing preamp and output currents in the ground, do this 2 mods for the start, and see if is better

21.png

[goldenGeek]

Yes, it's norwegian
It's actually, well kind of, wired that way. The bottom/filter board ground is not grounded like that, it's connected to the same ground point as the ground bus. I think you can see it in the pictures. But I can try to wire the cathode bypass directly to the filter cap negative instead.

[bepone]

hi neighbour !
then fix the ground point close to input jack:

22.png

in this way you have 2 separate ground points, hi and low current, should be better,.. and best results noisewise are always when you have gnd point close to input jack..

[goldenGeek]

hi neighbour !
then fix the ground point close to input jack:

in this way you have 2 separate ground points, hi and low current, should be better,.. and best results noisewise are always when you have gnd point close to input jack..

Thanks, I'll give that a try later today

[bepone]

but wait, before all of that, do you have heaters grounded? 100R-200R from each heater line to the ground?

[goldenGeek]

but wait, before all of that, do you have heaters grounded? 100R-200R from each heater line to the ground?

There's a center tap for the heaters on the PT..
(edit) and the CT is grounded

https://www.tube-town.net/info/datenbla ... -v2020.pdf

[bepone]

ok update on dwg, you need to remove HV center tap wire (gray) from the chassis or any other point, it need to go directly (and only) to first filter cap in supply!

then ground the 2. filter cap to the chassis how it is on your sketch. preamp gnd to the input jack, remaining. heaters doesnt matter or preamp or chassis gnd (but not where is power supply cap grounded)

22.png

[pdf64]

To DC elevate the heater circuit and so mitigate for 50Hz hum due to imperfect heater cathode insulation, the heater winding CT could be moved to the output valve cathodes.

[Stevem]

Your B+ center tap should go right to the negative of the first filter node before it gets landed down to the chassis.

You next two filter nodes can be chained to each other and then they should have a separate ground wire running back to land next to where the B+ is grounded on the chassis.

[goldenGeek]

Thanks @Stevem, @pdf64 and @bepone. Here's an updated layout, does this correspond with the tips you provided?

Mesa-ish clean - V2.png

[bepone]

not really,
this would be, lines in red added. you need to be very careful which capacitor to ground..here is grounded C2. you are trying to ground C1 which is the worst case...

123.png

[goldenGeek]

not really,
this would be, lines in red added. you need to be very careful which capacitor to ground..here is grounded C2. you are trying to ground C1 which is the worst case...

Ok, thanks. While we're at it, what's the best place to ground the output jack?

[goldenGeek]

Now I have moved all the grounding as suggested, but it did (next to) nothing with the hum at idle. The hum when gaining up is maybe a tiny bit better

[goldenGeek]

Now I have moved all the grounding as suggested, but it did (next to) nothing with the hum at idle. The hum when gaining up is maybe a tiny bit better

But wait... If I connect the negative feedback the hum pretty much disappears It's still present, but it's at a "normal" level for a tube amp I guess. At least I got to learn how to properly set up the grounding this time