microphonic chassis problem

[EETStudent]

Ok, I just finished a new build, close to a jtm45 of sorts. Finished up the wiring and fired it up, and received major hum. This is by far my cleanest build, and tracing this thing is a piece of cake. All connections are fine, all grounds are as they should be and the wiring is correct.

I pulled the PI tube and noticed that the hum still resides in the power amp section. I tried a few things here and there, increasing input filter cap, redirecting ground wires, etc.. but no luck.

I then started tapping on the chassis and noticed that it was extremely microphonic, and I also noticed that if I press on a grounding point, for example one of the transformer bolts, the on/off switch nut, one of the circuit board bolts, then all hum and sound disappears. If i release, hum and major microphonics return. Does anybody have any experience with this sort of issue?

[Chuck H]

Do you have the three conductor power cord ground soldered properly? If so, are you sure the wall plug is grounded properly.

A long second, but... do you have the power tube filaments wired with all #2pins on side of the wind and all #7 pins on the other. If you get the filament leads confused here the amp will work fine but will hum. Though I can't imagine touching a ground would stop the hum in this case.

Chuck

[Dave Curtis, dB AudioTech]

"A long second, but... do you have the power tube filaments wired with all #2pins on side of the wind and all #7 pins on the other. If you get the filament leads confused here the amp will work fine but will hum. "

If the filaments are AC what would it matter?

[MWJB]

Sounds like a grounding issue, measure any resistance between grounded eyelets and your main power amp ground, remake any grounds that show whole ohms, you really want the meter to show default/0.0ohms.

How have you grounded the amp? How have you ground referenced the heaters?

Any pics?

Crossed heater wiring on a P-P amp power tubes can (but not always) contribute to hum. Being AC doesn't preclude heaters from inducing hum.

[yescaster]

Try tapping on each tube to see which one is the culprit. As for hum...couldn't say personally.

I will say though...some of the best sounding amps I've played have had a microphonic tube in them... my tremolux...has an old GE preamp..that is so microiphonic..if you drag the guitar cable over the cab...you can hear it in the amp. i 've gotten brand new tubes that were outrageously microphonic.

[Alex R]

Maybe it's not just when you press on the bolts but when you press anything that flexes the chassis. Some connection somewhere is making and breaking when you do that, or touching the chassis. Tube base connections are rather prone to that, especially 9-pin ones with the lugs so close together. So I'd say check for solder bridges, stray wires, bad joints, with a chopstick. I'm saying the microphonics might be a separate issue from the make/break of the signal chain.

[Chuck H]

I'm with MWJB. It's a grounding issue. The hum stops when you touch it because YOU are the ground. The microphonic issue could be related if there is AC potential on the chassis that YOU are also grounding when you touch it.

Dave, ever notice how single ended amps hum more per watt than PP amps? In a PP amp any hum induced in the power amp by the heater is cancelled because the two sides of the PP circuit are out of phase but the heater hum (if the filaments are wired properly) remains constant so it is cancelled by local NFB. If you wire the filaments on a PP amp opposite on either side the hum ends up IN phase and is not cancelled. This phenomenon is the basis for the "hum/balance" control on some amps. Because the hum on either side may be of different amplitude the control allows you to balance it so that maximum hum cancellation can be achieved. If AC heaters didn't hum, and PP amps didn't cancel it, this circuit wouldn't make any sense.

Chuck

[Dave Curtis, dB AudioTech]

Grrr, I replied to this post last night; apparently it didn't get posted. I should probably start using notepad for drafts and save the *.txt...

Sounds like a grounding issue, measure any resistance between grounded eyelets and your main power amp ground, remake any grounds that show whole ohms, you really want the meter to show default/0.0ohms..

I agree. From the description, it sounds like an open input; Is the shorting input jack working? Oh, and don't forget to deduct the meter lead resistance or use the the "relative" function on your meter.

How have you grounded the amp? How have you ground referenced the heaters?

Any pics?

Crossed heater wiring on a P-P amp power tubes can (but not always) contribute to hum. Being AC doesn't preclude heaters from inducing hum.

That's not what I meant. Being AC can cause hum (of course); being DC can't. I meant that AC filament wiring doesn't need to be polarized (it certainly can't hurt, and makes for better workmanship).

IME: I've never had crossed power tube heater wiring cause hum.

On more than one occasion, I've "phased" the filament wiring on pre-CBS Fenders (you know; the non-polarized cloth-covered green wires), with no measurable (or noticeable) change in hum. AC filament voltage changes polarity at the same frequency as the line voltage; 60 (or 50) times per second. A hum balance pot is essentially an adjustable pseudo-center tap, as is a pair of fixed 100R resistors, and serves the same purpose as an actual center tap: humbucking. A P/P OPT is naturally 'humbucking' as well. Leo didn't polarize his filament wiring, but I normally do on my builds; just because.

On another note; I've had Sprague Atoms that were quite microphonic... They had me thinking all the preamp tubes I had were bad.

[ampcabinets]

microphonic capacitor(s) in amplifier

I have 2 amps with the same problem, one has hum to go with it, the other doesn;t.
I suspect the first has heater wiruing issues (not twisted, etc).
the second is a 1963 showman, and the capacitors a the mixer are extremely microphonic.
I hear some low hum but lots of high hum, as in an open input somewhere.
there is no open input. I have been over and over this amp, but I have not changed any parts. The main part seems to be the 500pF cap in the middle of everything.
The woring to the preamp tube is also microphonic to say the least.
NO tubes are microphonic, I have been over and over them.


anyone have any experience with this effect happening?

[mac dillard]

When ever I have had a build where the chassis was microphonic it turned out to be a cold solder joint at the preamp tube socket.

[Enzo]

If a part is microphonic, it is microphonic. Doesn;t matter if the part isn't "supposed to" get that way. If you can tap on something and it comes out the speaker, then replace the something. Lots of things can get that way that might surprise you, but pretty common for disc ceramic caps to get microphonic.

[soundguruman]

You don't have a ground.
You are making yourself the ground connection.
Your electricity is messed up, dude.
The white wire from your power cable, is tied to the green wire from your power cable, at the circuit panel.
But somehow, that tie is missing. You have an open ground in your power receptacle.
you get one of these gizmos, and it will help you trace it down:
Test, Measurement & Inspection | Electrical Testing Equipment | Gardner Bender Gfci Outlet Tester, 120 Vac | B548121 - GlobalIndustrial.com

That fixes the hum, next please.
So fix the ground and get rid of the hum FIRST.
Then, start working on the microphonic problem.

Any wire that is tied to the input grid (control grid) of the preamp, is extremely sensitive and will be microphonic.
Any cap that is connected to a control grid in the audio chain will also be microphonic.
That's normal.
Maybe you can find a less microphonic cap, but they are all microphonic in some way.
Some more than others. A really expensive hi fi cap is built to be less microphonic.
A silver mica cap is a relatively cheap, less microphonic solution.

[woodyc]

...On more than one occasion, I've "phased" the filament wiring on pre-CBS Fenders (you know; the non-polarized cloth-covered green wires), with no measurable (or noticeable) change in hum...

Me too. There is a measurable difference. It was about a ~1mVrms difference at the output in my setup (two 6L6s.)

But the result I got was the opposite of what Chuck H said. The one time I measured it, having the heater polarity opposite was quieter.

I still have the chassis I used -- I just checked again with different tubes and got the same result. I imagine it could go either way, but it would take more testing.

To first order, there shouldn't be an effect since the heater wire goes up and back down inside the cathode, and the currents should cancel. So I'm guessing its some kind of second order asymmetry thing, and the polarity might be a coin toss for any particular tube?

Here's a pic of my setup -- I had a polarity switch on each heater...

[soundguruman]

If you want as little hum as possible, you want DC heaters.
You want to use silver mica caps, especially on the volume control bypass, or bright switch.
BUT get your power receptacle grounded properly FIRST.
USE the receptacle tester, to confirm there IS a proper ground FIRST.

[Chuck H]

Me too. There is a measurable difference. It was about a ~1mVrms difference at the output in my setup (two 6L6s.)

But the result I got was the opposite of what Chuck H said. The one time I measured it, having the heater polarity opposite was quieter.

I still have the chassis I used -- I just checked again with different tubes and got the same result. I imagine it could go either way, but it would take more testing.

To first order, there shouldn't be an effect since the heater wire goes up and back down inside the cathode, and the currents should cancel. So I'm guessing its some kind of second order asymmetry thing, and the polarity might be a coin toss for any particular tube?

Here's a pic of my setup -- I had a polarity switch on each heater...

[ATTACH=CONFIG]20364[/ATTACH]

I digress. It just seemed to me that since AC (not DC) IS when you need opposite polarity for phase cancellation that the push pull circuit would be the place to do it?!? Somehow it's obvious to others that it isn't important but I'm still confused about why.