troubleshooting ?- Hum

[Rattler66]

i'm getting hum in my power amp (no pre or pi tube). can i ground the screens and grid etc... through a cap to remove any ac to see where it is coming in?

i figure if it quiets down that should show where in amp the hum is being injected.

thanks

also is it safe to ground the output of bias ckt to ground any ac ripple, thanks

[R.G.]

i'm getting hum in my power amp (no pre or pi tube).

You have one of a few problems.
1. ripple or hum on the bias voltage
2. radiation of hum from PT or choke to OT
3. leakage of heater to grids
4. Bad tube (which I should have put in first)

If the amp has ever worked before, chances are it's a tube or noise on the bias.

can i ground the screens and grid etc... through a cap to remove any ac to see where it is coming in?

Yes.

...
i figure if it quiets down that should show where in amp the hum is being injected.

Good thinking. I use that one all the time.

...
also is it safe to ground the output of bias ckt to ground any ac ripple,

NO!!! It is never safe to remove bias from your output tubes!! They'll overcurrent and die in a very short time.

Look at http://geofex.com/Article_Folders/st...ing%20amps.pdf for some ideas about hum.

[Rattler66]

Thanks RG. I will check Saturday, it must be something simple. Just got in new tubes also.

[Enzo]

To add to RG's list, unbalanced output stage. A bad tube could certainly do it, but also severly mismatched power tubes can do it. And that includes where one side is not running. For example if the previous tubes were shorted, a screen resistor might be burnt out. Now when a new good tube is installed, it gets no screen voltage and so does not conduct. Push pull output stages naturally cancel hum in the stage. Turn off one side and it no longer cancels. So check for screen voltage at each socket.

[Rattler66]

thanks guys, i'm going to open it up saturday and take some pics and measure voltages etc... hopefully i can get this, otherwise this amp rocks.

[Rattler66]

found entrance of hum.........

if i ground this node with a cap to ground the hum goes away. right now the wire from the tube socket to these points are disconnected. the only thing connected is B+2 (353volts). if i ground the node between the 81k/92k/10k there is just a slight slight reduction in the hum, practically nothing. But again if i ground right before coupling cap or after then the hum disappears.

what may it be, i'll brainstorm tonight it's gotta make sense somethings is happening at that point

thanks

[Enzo]

That is not the only thing connected there, the bias supply is also connected, it is just on the other side of the caps, and so is the power tube grid.

1. When you measured the ripple on the bias supply, what ripple voltage did you get?

2. When you measured the DC voltages on the power tubes, what reading did you get on each plate, each screen, and each control grid?

3. When you tried different power tubes, was there any change? Did you try the existing power tubes, but with only one at a time installed?

4. How much ripple is on each B+ node?

[Rattler66]

i will check tommorow and post voltages etc...

thanks

[Rattler66]

allright, i think i got it. i moved the screen wire around and the hum went away, it's a layout problem, so now i just gotta find the right routing for it.

i'll post back............thanks

[Rattler66]

it's fixed............

it was a wire that had to be moved. thanks so much guys.

only one more question, the original hum was about an 8 on a scale of 1-10, now it's down to almost a one. how much hum is too much, would you guys keep fishing around till it's dead quiet? thanks

[R.G.]

What I would do is of no consequence to you at all. You should reduce hum until YOU don't care any more.

[Rattler66]

yeah stupid question..............i guess the question is, is there not supposed to be any hum at all or is a little hum normal?

[txstrat]

With a ac filament you can probably always hear a faint hum if you get your ear close enough to the speakers. I expect the hum to be less loud than the faint hiss when the volume pot is fully turned up.

[Rattler66]

thanks, i'll probably look further into it when i need yo change tubes and rebias, it's practically nonexistant, you have to be looking to hear it. thanks again