Cyclic Hum....sounds like 'naaahhhhh-ummmmmm......naaahhhhhh-ummmmmmm, 2 sec rep rate

[nevetslab]

Every so often, servicing gear in the shop, I'll come upon an amp that has a bit more residual hum than I care for....Volume & tone controls CCW. This time, chasing a random click that may be in the driver stage of an Ampeg SVT-VR being repaired, and even with the power amp input dead-patched with a shorting plug, the random clicks are there. So after removing the power connection to the preamp (leaving the AC/StandBy/Chassis Gnd plug attached, powered back up, the clicks are no longer present (this time), but what IS there now is an old nemesis.....a cyclic hum that sounds like (phonetically).....'naaahhhhhh-ummmmmmmm........naaahhhhh-ummmmmmm......naaaahhhhh-ummmmmm' with about a 2second rep rate, wth the duty cycle around 1/4 (long 'ummmmmm', shorter 'naaaahhhhh'.)

Anyone ever hear that character in the background hum & noise?

Never found it to be related to line voltage level (ran the variac up and down from 100VAC to 130VAC). I've heard it on Vox AC-30's, Fender Twins', Marshall JCM 900's, a few others every so often, but have never found what the source of it was. I still think it's AC Mains related, as it's disappeared before. I haven't ever hung a current probe on the AC Mains, or looked between Ground and Neutral on the AC Mains to see if it's on the mains. The sound stage complex where I am is across the street from the Burbank Airport.

My random clicks have stopped, preamp no longer being powered up. After I had replaced a faulty bias supply cap in the power amp chassis (180uF/250V), and just to be safe, both of the 47nF/400V coupling caps to the Cathode Followers that carry the two adjustable bias/signal lines to the SVT-VR output stage, I was once again able to adjust bias without the power tubes turning on hard and pulling a lot of mains current. Then, found the output of the preamp with a different random LF pulse & hum. Seemed to be the last stage with both channels mix (a 12AU7....V4).

Nah....random clicks are back, so not from the preamp section. Just love a mystery!

[loudthud]

Is there a TV near by or on the same AC circuit? Back in the days of analog TV the vertical rep rate was 59.something Hz. I'm not sure what it is for digital TV.

Edit: Might also be a computer monitor.

[Jazz P Bass]

The sound stage complex where I am is across the street from the Burbank Airport.

Could it be the airport?

Nah!!

[nevetslab]

Is there a TV near by or on the same AC circuit? Back in the days of analog TV the vertical rep rate was 59.something Hz. I'm not sure what it is for digital TV.

Edit: Might also be a computer monitor.

There is a LCD computer monitor on the same phase, though different circuit at the AC Mains panel.

Later today, I noticed random groan being emitted from my General Radio W20M 0-140V/20A Variac, which was being used to power the SVT-VR amp. But, the rep rate of it's groaning is totally different in timing to that cyclic hum. Thinking back, I believe I've always heard this odd hum when powered from that variac. I'll give a listen tomorrow when I get back to the shop, bypassing the variac. I was still hearing it way in the background of the residual hum/noise of the amp.

[Audiotexan]

Restaurant/bakery nearby?
(my phonetics of it remind me of a cycling microwave for whatever reason)

[Mick Bailey]

I get this a lot with amps, sometimes enhanced by a fault condition, sometimes just the sensitivity of the amp. Its caused by my digital solder station bit-temperature sample rate.

[Leo_Gnardo]

Could it be the airport?

Nah!!

Jazz P, you got it, I think. Airport a big clue.

Next to Burbank? AND therefore the Skunk Works? You've got radar! Gonna take a BIG tin foil hat to cover all your gear.

Studio I was working in Ft Lauderdale, picked up an occasional blip-buzz on their EMT plate reverb. The only thing that happened simultaneously was an EB-2 passing over, one of those planes loaded with radar gear. Guarding the Florida coast against . . . what . . . Cuban submarines?

[Alex R]

random clicks - I had a vibrolux with that problem, took ages and many false cures before I found out it was the heater winding arcing.

[davohilts]

there is always the input shorting jack. try cleaning the contact and see what happens to the click/pop

[Jazz P Bass]

"'naaahhhhhh-ummmmmmmm........naaahhhhh-ummmmmmm......"

I got it, it's a Yoga Mantra.

[nevetslab]

Before I left for the evening yesterday, I did finally replace the Sextet of output tubes. The random clicks went away entirely. So, one or more of the aged 6550's was hacking up bursts of electrons. Glad it wasn't in any of the Xfmrs. It's been fine all morning, so just have the matching to check and I can put it all back together.

[nevetslab]

I wasn't hearing the cyclic hum this morning.....not until I inserted a shielded 100k termination plug into one of the preamp inputs and cranked the volume up. There it was again, just at a lower level than yesterday afternoon. Powered down, moved the mains cable from the Variac output to the wall outlet. Still had it. Though I'm not hearing the Variac groaning as it was late yesterday. No RF cage here, as I had at BGW Systems (there, we were on KNX Radio's AM Ground Plane from their AM station 5 miles away. That Lindgren RF cage we had vastly improved the coupled RF coming in on the AC Mains/Ground connections to the building power.

So, while not yet checking the Mains for the cyclic hum source, the field intensity seems to vary over the course of time. I wouldn't be surprised to find it IS RF transmission related. Haven't ever gone looking for it at night. I remember similar stories in recording studios picking up that sort of thing, and one of the engineers, having gone up to the rooftop to take a break, noticed a radar antenna rotating around at a nearby airfield, and the rotation rate correlated with their cyclic interference.

[Alex R]

I wasn't hearing the cyclic hum this morning.....not until I inserted a shielded 100k termination plug into one of the preamp inputs and cranked the volume up. There it was again, just at a lower level than yesterday afternoon. Powered down, moved the mains cable from the Variac output to the wall outlet. Still had it. Though I'm not hearing the Variac groaning as it was late yesterday. No RF cage here, as I had at BGW Systems (there, we were on KNX Radio's AM Ground Plane from their AM station 5 miles away. That Lindgren RF cage we had vastly improved the coupled RF coming in on the AC Mains/Ground connections to the building power.

So, while not yet checking the Mains for the cyclic hum source, the field intensity seems to vary over the course of time. I wouldn't be surprised to find it IS RF transmission related. Haven't ever gone looking for it at night. I remember similar stories in recording studios picking up that sort of thing, and one of the engineers, having gone up to the rooftop to take a break, noticed a radar antenna rotating around at a nearby airfield, and the rotation rate correlated with their cyclic interference.

not the amp humming then.

[nevetslab]

At least not the source of that cyclic character that I've heard here in the shop on multiple pieces of gear over the few years I've been here. That SVT-VR that I finished did have more (normal) hum than I've heard in other SVT-VR's. I had fixed a couple hum loops in the preamp PCB layout back in 2010, even re-routed the input signal to Ch 2 thru shielded cable. DCV Heater supply was clean, HT supply clean. Now, the Screen/Heater Pwr Xfmr on this amp ran quite hot, so there may be more field radiation on this amp from that. But, if it becomes an issue later, I know where to start from. Power Amp very quiet. And with my bass plugged in, I certainly wasn't conscious of it.

I haven't begun the witch hunt for the source of that characteristic sound that creeps in to some gear, but having heard it again, thought I'd bring it up for general discussion.

[Audiotexan]

"'naaahhhhhh-ummmmmmmm........naaahhhhh-ummmmmmm......"

I got it, it's a Yoga Mantra.

[ATTACH=CONFIG]31326[/ATTACH]

To be honest... the very first thing that popped into my mind was the intro from this song:
https://www.youtube.com/watch?v=fxvkI9MTQw4
But I was trying so hard NOT to go there... lol

But since you broke out the Yoga...I can blame you! ;P