50Hz hum DIY project

[pellek]

Hi, i've build a self designed amp, and i am having a hum problem.

It is not an overpowering kinda hum, but just a bit annoying if your home alone.

The power amp itself, is dead quiet. But when i plug in the PI tube, there is a 50 Hz hum.

Also, when i insert the send/return tube, there is a slight increase in hum.
Except, when i turn the master volume, completely up or down.

Inserting all the other preamp tubes, does not change a thing.

When i turn the humdinger, the intensity changes, but does this has to mean that it comes from the heater supply?

What can cause this problem, and what can i do to get rid of it.

I already changed the grounding from star to bus, with no significant result.

[M Fowler]

Or bias supply if not a cathode bias amp.

[matt h]

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[pellek]

If the bias was wrong, than it would also hum without the PI.

Output tubes are 5881 biased at 40mA.

I have a seperate bias adjustment for each tube, and a 1 Ohm resistor at each side to
measure the bias.

[pellek]

No, with the master volume completely up, there is no extra hum added to the hum from the phase inverter.

Yes, there is a cathode follower for the send part of the amp.

[tele_player]

I'm assuming you're in a part of the world with 50Hz mains cycling--this suggests it is heater related rather than a fixed bias supply issue (which would be rectified and therefore be giving you 100Hz buzz.)

A half wave rectifier, typical for bias, has a ripple frequency equal to the input frequency.

[matt h]

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[matt h]

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[pellek]

I'll give it a try, i also grounded the humdinger, and the rectification of the switching device, just at the nearest place, can this cause some troubles?

The DC- heater is for the V1 only. The reason to place the humdinger in that part, is to make a proper ground reference in the DC part of the heater circuit.
If it was placed before the rectifier, i assumed it would cause some troubles.

[matt h]

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[Colossal]

You might consider decoupling each bias supply node with a film cap to ground. Add 0.1 - 1uF to ground from each of the bias adjustment pots.

Elevating the heater supply to DC (not pure DC supplied) works well to lower the noise floor.

[pellek]

Ok, i removed the DC-supply for the heaters of V1. Certainly a big improvement!!

But still 50Hz. A 20mVpp at the speaker output (not really loud). If the volumes are set to max, there is a noticeable hiss.

Now i have an other question.

If you have a full bridge rectifier, in parallel with for example, some heaters,
and you make the ground reference after the rectifier, would the other heaters, (the ones before the rectifier) be ground referenced in the correct way?

Can, referencing before the full bridge rectifier, be harmful?

Sorry 'bout me English.

[martin manning]

What about magnetic coupling from the power transformer to the output transformer?

For the FWB ground, I believe as long as you only have one ground reference, either on the AC or DC side (and not both), you are ok, and the AC and DC heaters will be properly ground referenced.

[pellek]

Ok, i moved my grounding point. With some difference in the good direction.

What is the easiest way to find out about magnetic coupling of the transfo's,

and what is the best remedy to get rid of it?

The transfo's are 12cm apart, and with a different angle. 90°

Thanx for the help already!!!

[martin manning]

If it's quiet without the PI tube (I went back and read the earlier posts), then it's not a PT-OT magnetic coupling problem. With it being 50Hz hum, maybe keep looking for coupling with the heater wiring.