[sushimaster]

I have a CLARINET that has developed a low level hum a couple of weeks ago that is bewildering me.
Otherwise it has been quiet since I built it 2 years ago.

I read in another Coronet thread where Jim said: Hum in a Cornet2 is not from this source.  You need to look elsewhere.  Ususally it is input wiring, magnetic coupling to power transformer, poor chassis connections (which can cause some pieces to be electrically isolated)... (I figure Clarinet/Coronet should have similar troubleshooting)

I have a few questions:

1. What is meant by "magnetic coupling to power transformer?" And what can I do to troubleshoot it?

2. Could the Clarinet power transformer become defective/worn out over time and emit hum? If so, Would a Plitron torroidal transformer have better silence/hum rejection properties over the Hammond one?

As for troubleshooting steps, I have done the following.

Replaced the rectifier tube, replaced both driver tubes. Checked the voltages against the printed reference on the bottom of board (seemed okay). I went ahead and replaced C106, C304, C305 with new caps, which had no effect. I did add a wire to connect one of the screws of the transformer to a screw on the side chassis which reduced the hum significatly, but didn't eliminate it. Changed interconnects. Changed the two prong power cable on my Dynaco/Van Alstine Ultimate 70 tube power amp to a three prong, by connecting the ground wire to the chassis, which made the hum louder, so I reverted back to a two prong. The chassis I have is the factory black painted one from Jim with all the decals. I check with my multimeter and it is electrically insulated.

[sushimaster]

Here are some pics of the unit.

[StereoNut]

I know why your Clarinet hums....

 

It's 'cause it forgot the words to the song! 

Sorry!  I just couldn't resist.

SN

[hagtech]

If touching the chassis in places quiets the hum, then perhaps the anodize is insulating the pieces and some are not getting Earthed.  Try scraping away the anodize where the screws and nuts (if that's what you can call them) touch the side bars.  Then for top and bottom use a counter sink to remove anodize from the screw holes.

jh

[machine]

I have 2 cornet2 preamps and I swear by the gray wire hum fix.

http://www.audiocircle.com/index.php?topic=68319.msg636694#msg636694

One I built with the black anodized case and was sure to ground the transformer legs.  It was quite from the get go.

The other I bought used and it was in a silver case.  Some hum issues which were cleared up by routing the gray wire to the star ground point.

Was so impressed that I took the time to do it on my original black case model.

Watch out for placement issues.  I also built a piccolo and had massive hum issues after deploying it in a new location.
Turns out that a 90 degree turn in placement cleared this up...

[sushimaster]

Thanks for the suggestions!

I took a cone shaped dremel bit and filed down the counter sunk screw holes on the case to expose bare metal.
Reassembled the case and tightened down all the screws. Then took a multimeter to check for continuity, by putting one lead on one screw and touching all the other case screws one by one. Got very low resistance readings, so that tells me all parts of the chassis are conducting well to ground. Fired it up, still hum.

Then I tool the transformer bell off and found the gray wire and soldered a wire to it and connected the other end to star ground. Fired it up, still hum.



Some other thoughts: When I turn off the Clarinet preamp, the Van Alstine Ultimate 70 tube power amp is very quiet. No other connections are plugged into the preamp.
I brought my Conrad Johnson CAV50 integrated tube amp from downstairs and plugged it into this system and it doesn't not hum through these speakers, so I don't suspect there being a problem with the AC line. Since it has not exhibiting this behavior in the past 2 years, until just recently, I'm led to believe something is malfunctioning or going bad. I suspect it's the Hammond power transformer.

Before I order a new one from AES, is there anything else I can try first? For example, install some rubber grommets under the transformer feet, so that it does not directly touch/vibrate with the chassis? Or even try to move the transformer away from the unit, put it into a separate box with an umbilical cord?

   

[sushimaster]

I was just perusing the Van Alstine circle. Maybe I need a Humdinger!

[Speedskater]

I doubt that a Humdinger (DC blocker) will help.  It's for certain  types of AC power line problems.  You could check by taking your system to a friends house (in a different neighborhood).

[sushimaster]

Perhaps, Speedskater, but at this point I'm getting desperate to get rid of it. The humbuster claims to: Cure hum issues causes simply by a defective noisy power transformer or hum from your speakers caused by any number of grounding or system defect issues. So I'm willing to give it a shot. If it doesn't help, I'll just return it.

[poty]

The humbuster claims to: Cure ... hum from your speakers caused by any number of grounding or system defect issues.

As far as I understand:
"...What the HumDinger will not do:
...
Cure hum issues causes simply by a defective noisy power transformer or hum from your speakers caused by any number of grounding or system defect issues."

I'd begin with some experiments. For example:
- pull your Clarinet from the mains. It will work for several seconds and you can listen how the hum changes;
- pull the rectifier from the device, power it on and listen to hum;
- pull the amplifiers' tubes (one by one), power on the device and listen to hum.

Of course - pulling tubes must be done on cooled and powered off device.

Second considerations - the source of hum. If you disconnect interconnects from the output of Clarinet (powered on) - what happens with hum? The hum is from both channels or maybe only from one? Are the "new" tubes really new and function flawlessly? (The SRPP layout sometimes reveals tubes problems) Could you measure marked voltages? ...

DC blocking works only for some transformers with non-obvious internal problems (or problems with their usage) - I should add - mechanical problems. From your description I could guess that your hum has electrical nature.

[hagtech]

Have you tried different interconnects?  Does moving the placement or orientation of the Clarinet affect the hum level?  Is it a hum or buzz? 

jh

[sushimaster]

I been using BlueJeans LC1 interconnects (1 meter) I just tried using Monoprice interconnects, same hum.
It's definitely a hum, not a buzz, that I hear through both speakers, from the woofer. As soon as I turn off the Clarinet and leave the power amp on, hum gone.

I borrowed my dad's Fluke digital voltmeter (my Simpson 260 is hard to measure accurate values)
These are the measurements I obtained.

resistor   printed    actual reading
R107       375v        372v
R311L     140v        126v
R311R     140v        125v
R314L     280v        280v
R314R     280v        279v

I also checked the AC reading from the wall and it was 123 VAC.
I tried moving the Clarinet around, still hums.
I also just re-tightened the screws on the bells of the transformer, really tight. To no effect.
I think I should order a new transformer just to eliminate my suspicion.
 

[Charles Xavier]

Check your tonearm wires , headshell wires and clips on the cartridge. I had an annoying hum and it turned out to be the connection to my cartridge.

[poty]

As soon as I turn off the Clarinet and leave the power amp on, hum gone.

Most interesting is what happened between the two point (Is the hum gone straight after you turn off the Clarinet or diminishing gradually with draining the capacitors)?
Your readings about voltages are OK for me. It seems you used last mods so the R311 low voltages are just refecting this.
In my opinion - if the hum came suddenly - it is either tubes or filter capacitors. But more concrete guess I can give only if you answer some of my previous questions.

[sushimaster]

Poty, When I turn off the Clarinet while leaving the power amp on, the hum gradually recedes in about 6 seconds. Then I don't hear it anymore.
I swapped rectifier tubes with some GE NOS that I purchased off eBay. I had a Sylvania in there that I bought new from Tubesandmore.
The 12AU7 I have swapped are NOS RCA and GE that I bought on eBay.

Just to make sure I understand you correctly, you want me to try taking the rectifier tube out of the preamp, and listen for hum.
Then put back the rectifier tube and remove one of the 12AU7, listen for hum. Then remove both 12AU7's and listen for hum.
Also, forgive my ignorance, but is it safe to remove the rectifier tube while leaving both 12AU7 tubes in and powering the Clarinet on?

[sushimaster]

Okay poty, so I took out both 12AU7 but left the rectifier tube in. Turned the preamp on, and there was still hum.
Then I put back both 12AU7, and removed the rectifier tube. Turned it on, no hum.

I had just bought two supposedly NOS GE rectifier tubes off eBay that I used to test ever since this hum business started.
Are all 3 of my rectifiers bad in this regard? Perhaps I should have gone with a brand new Sovtek 5Y3GT.
Or is it possible I need to clean the rectifier tube socket and or check the solder joints or even clean the tube pins itself?
But does almost certainly point to the rectifier causing the hum?
 

[poty]

Hello sushimaster,
Firstly I should say: the fact that hum was eliminated by removing rectifier tube doesn't point to the rectifier as the problem. It maybe still possible, but the sentence:
"When I turn off the Clarinet while leaving the power amp on, the hum gradually recedes in about 6 seconds."
says that the main problem is not rectifier. When you turn the Clarinet off the AC from the transformer instantly stops, so the rectifier is not working through the period. Could you say how did you power off the Clarinet - by power switch or pulling the power cable from mains? I suspect some coupling through the transformer.
Other suspects - external EMI or bad shell joint in an input RCA (maybe even in the volume pot). If you can do a small trick: while the Clarinet is powered off - connect a short wire across the R302L and another - across R302R. Then power on and listen to the hum.

[sushimaster]

Poty,

Just to clarify (b/c I don't know what I'm doing at this point. I do not have an electrical engineer background)
Do you want me to connect one piece of wire with one end connecting to R302L and the other end to R302R?
Or Two small wires to essentially bypass each resistor? I think the latter, but I just to be certain I understand you correctly.


Also, I went back and tried to power off the Clarinet again, with my ear right up against the speaker.
The hum is gone immediately, both when I turn the switch off and when I pull the power cable out of the wall.
Not gradually like I mentioned before. I performed the test twice just to make sure I wasn't imagining it.

[hagtech]

It could also be the first electrolytic in the B+ supply.  If blown, ac would be injected by rectifier and go straight to both outputs, whether or not 12AU7s are installed.

jh

[hagtech]

Oh wait, according to your last post, then it should be a power cord issue?

jh