Hum Help

[R.G.]

Thanks guys.
I "think" I know a little more now. Looking at the scope, it appears that the hum is 120Hz and not 60Hz. Although the bass control affects it and it sounds very low, the scope doesn't lie. I don't see much of a hum signal at the grid. There also isn't much of anything on the plate. It would seem that this should be power supply noise. Each stage in the amp is decoupled and individually filtered. I have subbed out the filter cap for that stage. The heaters have a center tap. I've gone over any solder joint that wasn't originally made by me as well as many others. I have also replaced the tube socket. It's nice and tight. More troubleshooting.........
Dave

*Exactly* where does the CT of the PT connect to the negative of the first filter cap, and *exactly* how does that point get to (a) chassis ground and (b) signal ground?

Also, are the central axes of the "E" of the power and output transformers and the filter choke all oriented at mutual right angles?

[Daver]

mfratus2001,
Removing the PI tube stops the hum. Dead silence. I've also tried different output tubes, too. Thanks.

R.G.,
The filter for the power tube plates and screens is a 50/50 uf JJ can cap. The negative has a 2" long wire going to a ground lug on one of the PT bolts. The PT center tap is also grounded at that point. The center tap is fused, so the wire from the transformer runs to the back of the chassis to the fuse holder, then to ground. I've also bypassed the fuse holder and attached the CT directly to the negative of the can cap.
The OT and PT have their lams 90 degrees opposed to one enother. They are separated by about 4". The choke is in between them and oriented in the same plane as the OT. Are you thinking there is power supply hum coming from the output getting amplified by the second stage? Thanks for taking the time to help!

The only thing that seems to help is adding negative feedback around the power amp. More feedback, less hum.

Dave

[Enzo]

Well, more feedback means less gain in the stage, so sure.

[mfratus2001]

If there's too much of anything, feedback will reduce it.
I don't think that is a problem area.

Try removing the tube ahead of the phase inverter, and if there is no change, than the PI is your problem.

What are the voltages on the phase inverter pins? Should be (from Fenderish designs) about 240 on plates, about 75 on grids, and 45 on cathodes that are connected together. The phase inverter stage is a problem in some amps. Since the cathode is at 45VDC, when you add signal to it, the voltage between the cathode and filament can exceed the tube's max specs.

This is such a problem on some amps that they float the center tap of the filaments, not at ground, but at some percentage of the B+, like 60-100V. They then bypass the voltage divider to ground with a fairly large cap.

If you haven't already, try a different tube for the phase inverter.

Try disconnecting the center tap of the filaments from ground and see if it rises to some high voltage. I have seen tubes develop a high resistance between some electrode and the filament, causing the whole amp to act strange.

When subjected to running into an open circuit from having no speaker connected, an output tube will often short or arc from plate to filament. That puts all tubes at risk for developing internal paths to the filament across the mica insulator.

Mike.

[Daver]

Mike,
If I remove the tube before the phase inverter (the second stage where the hum seems to be coming from) the hum goes away. I have tried replacing the three preamp tubes, power tubes and rectifier tube.
The PI is a LTP. The supply B+ for the PI is 345 volts. The plates are at 206 and 200. The cathode is 24 volts and the grids are at 17 volts.
I've tried floating the filaments on the output tube cathode voltage and making a virtual center tap with two 100 ohm resistors.

Enzo,
I should have mentioned again in my previous post that grounding the input or output of the second stage kills the hum, not just feedback around the power amp.

Thanks for the help, guys.

Dave

[mfratus2001]

I would look at the values you used for the divider. If they are high enough, it can cause all sorts of sensitivity to stray hum.

I take it this is built in a metal chassis. Is this point-to-point? Running your leads down along the circuit board to and from circuit elements shields them a lot. I have had disappointing results from shielded cable. Twisted pairing is more effective in many cases, from my experience.

If it is in a chassis, what happens when you put a bottom cover on it?

How is your filament supply routed? Twisted pair, above everything, dropping down only when needed to run to tube pins?

Finally, note what happens when you turn off the power (AC).

If the hum IMMEDIATELY goes away, it is from the line and is being radiated inside the amp, or conducted inside the circuit elements.

If it fades away, it is from the environment and can be shielded against.

Sorry for the wild-goose-chases.

Mike.

[mfratus2001]

I would look at the values you used for the divider. If they are high enough, it can cause all sorts of sensitivity to stray hum.

I take it this is built in a metal chassis. Is this point-to-point? Running your leads down along the circuit board to and from circuit elements shields them a lot. I have had disappointing results from shielded cable. Twisted pairing is more effective in many cases, from my experience.

If it is in a chassis, what happens when you put a bottom cover on it?

How is your filament supply routed? Twisted pair, above everything, dropping down only when needed to run to tube pins?

Finally, note what happens when you turn off the power (AC).

If the hum IMMEDIATELY goes away, it is from the line and is being radiated inside the amp, or conducted inside the circuit elements.

If it fades away, it is from the environment and can be shielded against.

Sorry for the wild-goose-chases.

Mike.

(Don't know where my previous reply went... if it appears, that's where it went!)

[Daver]

Mike,
No problem about chasing the geese. I've stepped in enough goose poop with this thing already it doesn't matter!

The divider resistors are 470K/470K.

The amp is in an aluminum chassis and is built on an eyelet board. The filaments are tightly twisted and against the chassis up against the fold where the side meets the top. All the connecting leads are running against the chassis.

When powering off, the hum fades after a second or two. It doesn't immediately go away.

When the amp has a bottom sheild on it the hum is somewhat less. It helps, but doesn't cure it.

Thanks much for the help and interest.

Dave

[Enzo]

There is a valuable clue. Power supply ripple will stop the instant the power switch goes off. But the amp continues to amplify for a second or two as the heaters cool and the power supply caps discharge. SO your hum is coming from something other than power supply.

[loudthud]

There is a valuable clue. Power supply ripple will stop the instant the power switch goes off. But the amp continues to amplify for a second or two as the heaters cool and the power supply caps discharge. SO your hum is coming from something other than power supply.

Don't forget the business about the hum going away when the chassis was touched. Are you in a room with fluorescent lights?

[Daver]

Enzo,
OK, that's good to know. That will at least help me focus on something else. Thanks!

loudthud,
No fluorescent lights in the room. Thanks.

Dave

[mfratus2001]

Did you use a DPDT power switch?
When you pull the plug from the outlet is there any change with the hum just fading away? If so, it could be you just need to switch the power transformer's primary windings, as it has leakage from the winding to the core.

In general, though I think filament wires, twisted or not, should be run away from everything else, including chassis walls.

I had an amp, major manufacturer, with a stubborn hum. Even though the pots were grounded with a star washer, the hum would not go away until I ran a ground wire from the pot's metal covering to a local ground right at the same circuit the pot was involved in. Grounding to different parts of the chassis with a clip lead brought forth differing levels of hum. The star ground that the manufacturer had made wasn't any better. Only a local ground to what appeared to be a ground buss worked.
Come to think of it, maybe I was just grounding the buss to the chassis, not the pot to the buss!

It's an important clue if pulling the plug kills the hum immediately, where turning off the switch does not.

Mike.

[Daver]

Mike,
No DPDT switch, just a SPST. When I pull the AC cord the hum stops immediately. So is this really that much different than using the power switch to turn it off? If the PT has leakage why does it only seem to affect the second stage? That's one of the confusing things about this problem. It seems to only affect the second stage. Thanks for helping! Maybe we're getting closer.
I should add that if I get near the grid of the second stage with my hand or Sharpie while moving wires around the hum gets louder. If I'm also touching the chassis at the same time, the hum gets quieter.
Dave

[mfratus2001]

Well, there is your answer.
When you just turn off the power switch, the hum fades away.
When you unplug it, the hum stops immediately.

The hum is coming from the AC line. Probably your chassis, which you assume is true ground as far as the circuit is concerned, is really at some AC potential.
That is why all this grounding doesn't work the way we all think it should have.
Betcha if you unmount the power transformer, the hum stops.

Got a grounded 3-wire cord? My house doesn't have grounded outlets, so it doesn't help.

Swapping theprimary wires on the power transformer is only cheating. The real problem (if that takes away the hum) is leakage from primary to core.

Mike.

[EFK]

"Swapping the primary wires on the power transformer is only cheating. The real problem (if that takes away the hum) is leakage from primary to core."

Can you elaborate on this in lay language? I'm having what I believe is a very similar problem, hum that cannot be eliminated by any means. Old Selmer amp. Is the leakage just symptomatic of an old/poor quality transformer? how does swapping primary wires affect the hum? I'm very interested. Thanks!