Low hum from BF Vibrolux

[Randall]

Yes, sorry. The last sentence about the burst going away is not correct. The burst does not totally go away. Even with the PI tube removed along with all preamp tubes, the burst is still there, as measured on the speaker jack, but it is about half in size as when the PI is installed. With PI removed I cannot hear the hum.

Tried clipping a fresh 22uF across each filter cap using verified grounds with no change.

I have also tried new power tubes and rectifier.

[Chuck H]

Scope settings are 5 mV/div and 2 mSec/div.

If I'm seeing the picture correctly that would put the burst at 270Hz. Which seems odd.

[Chuck H]

Even with the PI tube removed along with all preamp tubes, the burst is still there, as measured on the speaker jack,.. .

Let's consider what this means. There is absolutely no signal conduction to the power tube grids with all the 9 pin tubes out. And I can't think of a single thing in the amp that would oscillate in bursts like that and at that frequency. I think you should try the amp in a different environment just to be sure. Make sure your cell phone isn't with you. That sort of thing. Typically environmental gremlins need some amplification to be audible. Maybe there's some dirt in your power supply or even the wall?

[Helmholtz]

If I'm seeing the picture correctly that would put the burst at 270Hz. Which seems odd.

Bursts repeat after 4.2 divisions. So period is 4.2 x 2ms = 8.4ms, meaning a frequency of 119Hz.

Agree that it looks like an environmental EMI effect.
Never seen such noise pattern produced by an amp itself.

Either there is a strong noise riding on the mains voltage (like with mains signalling) or a nearby source directly couples into the OT.

[nosaj]

What kind of light do you have over your bench? I've had some florescents do it and ones with the LED bulbs.
nosaj

[Chuck H]

Ah. I used an incorrect number in my calculation for the frequency. Thanks for the correction.

[g1]

My take is also that the burst is environmental and that it is a red-herring getting in the way of the troubleshooting.
It has been established that the burst still shows up when the hum is not present. Focus should be on the hum.

[Chuck H]

Agree. But doesn't it seem coincidental that the hum must be entirely encompassed in the image of the oscillation bursts? I would have expected to see some hint of a hum signal imposed there. Rather, there are clear gaps between the bursts and no apparent other wave form imposed there. Randall said that the sound was 120Hz "but it looks like this". Implying that the hum was present when the scope image was captured.

[g1]

Since it has not been said outright, can we get verification that the amp is otherwise working ok, putting out proper full power with guitar at input?

[Randall]

OK, back at it, hurricane and all.

It seems I was led astray by my LED tube lighting close by. Turning it off ceased that strange burst. I have never seen it effect anything before, so that is a bit odd.

I have approx 120vac sawtooth on the resevoir caps as well as the screen node cap. I shows as 3.15vac on my fluke, and approx 4 vp-p sawtooth on my scope. I haven't measured output, but the amp sounds good and loud as it should. It's just this annoying hum. I have swapped out the filter caps one at a time to no avail. I have treid new power tubes and PI tube. It does it with only those three installed, and it gets worse when all tubes are installed.

Could I have a faulty choke?

[Helmholtz]

I have approx 120vac sawtooth on the resevoir caps as well as the screen node cap

What?? You probably mean 120Hz.

Can we see a scope pic of the output hum?

Yes, a shorted choke would increase ripple but happens very rarely. Does ripple increase if you do short the choke?

[Randall]

I can't get a photo of the wave, my probe crapped out.

Controls have no effect. It is louder with all tubes in, lesser with just PI and 6L6s. Yes I meant 120Hz.

It makes no difference with the choke shorted, so I'd say we have a bad choke, Would explain why the wave looks the same on either side of the screen node cap. I ordered a Hammond.

I agree this would be a rare problem, since I have never come across it before. Also, it bears mentioning it was running on a SS rectifier plug in, and had just been replaced with a JJ GZ34 that shot some lightning a couple times while I have had it. Replaced it.

[Randall]

Well, it did turn out to be a shorted choke. The new Hammond measures 185 ohms, the original measures 9 ohms. And as I've said jumpering the original in circuit had no effect. I would usually expect an old choke to open, not short, but here we have it.

Amp had two IC 80 uF caps in the reservoir postion, and had been fitted with a diode rectifier before the owner put a GZ34 back in it. It arced twice on my bench, so I replaced it. I don't know, but it seems like either the diode or large filter caps damaged the GZ34, or the bad GZ34 damaged the choke, or some other version of it. Anyhow, they are all fresh now, and the amp is working fine.

Thanks for the help.