Hum and noise in small amp

[flatheadmurre]

Hi all

I´ve built a small 6aq5/ecc81 SE amp with ez40 rectifier for my compiuter.

Since it´s assembled from whatever was around the hum wasn´t to much of a surprice and i suspected the C-R-C filter, after adding another R-C stage it was a bit less but still there.

After some listening i found the hum to be pulsating (oscilation?)
and not like a regular 50/100hz hum.

Connected it to lab supply power hum still there so i threw in a bunch of ferrit beads and 1K gridstoppers to prevent any oscilation no difference.

Pulled all tubes, still on lab power pulsating hum still there.

I´m totally confused what´s going on any sugestions ??

[TheGimp]

Check your transformer locations. Are the laminates perpindicualr to each other? Are they in close proximity?

schematic would be helpfull along with photos of the top and bottom.

[flatheadmurre]

Yes they´re close and inline since it´s a small amp.

Power xformer is shielded and grounded.

I´ll try to draw up the schematic and take a few pics.

[flatheadmurre]

Some pics of amp and schematics

[Soonerorlater]

I like to rule out heater hum before I look any deeper, here's a good read on how to if you havn't already done so.


http://www.freewebs.com/valvewizard/heater.html


ps Lovely build

[flatheadmurre]

I tried adding a hum pot and elevating the heater no difference.

I was running it on lab power 250v regulated and 6.3v rectified and the hum still there. even with the power xformer out of the equation it has a low frequency pulsating hum.

Some kind of weird oscilation or interference betwen OPT´s ???

I´ll carry my big UPS from the computer into the shop just to rule out any chance of the mains doing something funny.

I´ve ran out of ideas.

[toprepairman]

Sounds like magnetic coupling between the transformers.
Unbolt the power tx from the chassis, and with no valves in, rotate the tx with the power on and you'll find the hum will have a minima with the power tx at around 90 degrees to the others.

[chrish]

For me, first thoughts for hum tend to be grounding issues. With that in mind, you appear to have a ground bus extending from your power stage towards the rear of the chassis towards the input RCA plugs. Hard to see where it attaches to because of the chassis lip. Connected to the signal input at the RCA jack?

[flatheadmurre]

The groundbus original started at power xformer an ended at rca,12ohm/,22cap to mains grounding point at xformer.

Under dehumming i rearanged it so HT is now fed through twisted pair from power supply with some extra film caps added,grounding point connects close to rca this time.

Right now after randomly throwing in caps and ferrit beads it´s total mess.

What pussles me is that level of the hum is pulsating slowly it´s not a steady 50/100hz hum.

It must be some kind of ground issue or oscilation but i can´t put my finger on it.

If someone have a good plan i´ll try it ASAP.

[jakruby]

i built a 12ax7 phono stage that also seemed to have a similar 'oscillation' to yours. yet mine had the power supply on a separate chassis and still problems. your circuit and mine don't use any feedback so (i think) oscillation should not be an issue. have you tried more decoupling between stages and channels? and can you trace where in the circuit your hum originates or is greatest? i have yet to solve my problem, increased PSU capacitance made no difference for me.