another humming cornet

buschfsu · « **on:** 14 Jul 2011, 12:45 pm »

Hello all,

Completed my cornet 2 build and fired it up. No heat or fire so i think im ok. (except some misvalued resistors which i sorted). Problem is significant 120-180Hz HUM. Heres what ive done.

•   Switched out tubes
•   Used plastic standoffs
•   Twisted mains wires
•   twisted signal wires
•   insulated RCAs from the case
•   used separate ground wires from each RCA to the board
•   i have a star ground near the TT ground post that all grounds attach to ( Earth from board, IEC ground)

So the unit hums loudly with and without TT attached. it continues to hum for a second after power down. pics below. Please help! im at a loss for ideas. Only thing i have tried from the suggestions on the forum is the grey wire inside the transformer bells? not sure about this one.

thanks

machine · « **Reply #1 on:** 14 Jul 2011, 10:48 pm »

Did the grey wire mod on both my cornets and cleared up my hum problem. Should have done this sooner...

Neither had as bad of hum as yours but what little there was is now gone.

Still need to work on my heater voltage...

buschfsu · « **Reply #2 on:** 14 Jul 2011, 11:33 pm »

Ok ill do that next. Yes 1.5r is only getting me 6.1v so i ordered a 1.8r

hagtech · « **Reply #3 on:** 15 Jul 2011, 07:06 am »

Try twisting the input wires with their related ground, rather than the other channel. You're picking up a direct connection from the AC mains wires.

jh

poty · « **Reply #4 on:** 15 Jul 2011, 07:14 am »

Quote from: buschfsu on 14 Jul 2011, 11:33 pm

Ok ill do that next. Yes 1.5r is only getting me 6.1v so i ordered a 1.8r

You should order lower values, not higher to obtain more voltage for heaters.
According humming. You should give us more information about this, because the source of hum can be anything.
I think you should begin with checking voltages, maybe capacitors in filters (bad joints, high leakage), diodes in the bridge rectifier of heating.
Then it is preferable to unsolder anything from input and close it (put a short wire between the two contacts on the PCB for each input).
From your pics I can't understand where is the transformer relating the PCB? It can be the source of the EMI pollution too.
My guess - a bad joint somewhere (most probably - capacitors in B+ filter, RIAA part, going to ground). Does the device produce any sound if it is connected to a turntable?

buschfsu · « **Reply #5 on:** 15 Jul 2011, 03:37 pm »

thanks for the responses all!

ok thats not intuitive. if i have the same I and want V to go up i thought i would increase R (V=IR). oh well 1.3R then.

As for checking and giving more information. i can...
check solder joints in the various B+ locations ( i have)
remove inputs and check for hum
wrap wires in channel/ground pairs instead of wrapping L and R together

It produces a wonderful sound when connected just hums

As for the picture. its actually the top of the case flipped upside down resting on the lip of the chassis. so its all there just easier to work with it that way.. thanks and ill report back

JayB · « **Reply #6 on:** 16 Jul 2011, 03:18 am »

What, may I ask, is the "grey wire inside the transformer bell" trick?

el34 · « **Reply #7 on:** 16 Jul 2011, 01:31 pm »

Ohm's law applies, but to the voltage drop across the resistor, so for example if I = 0.9,
and your target is 6.3v for the heaters, and V before the dropping resistor is, say, 8v, then
R = V/I
= (8 - 6.3) / .9
= 1.8 ohms

buschfsu · « **Reply #8 on:** 18 Jul 2011, 06:34 pm »

Ok, ive solved my hum problem and it wasn't that difficult.

these are things that reduced hum in my build

• Used plastic standoffs
• Twisted mains wires
• twisted signal wires to their ground wires for each rca
• insulated RCAs from the case using plastic spacers
• used separate ground wires from each RCA to the board ( red and black in my picture should be red/green and black/green twisted
• i have a star ground near the TT ground post that all grounds attach to ( Earth from board, IEC ground)
• take the black covers off the transformer, there is a small grey wire with a lug soldered to it, solder a wire to that and pull it down with the rest of the wires so you can attach it to the star chassis ground.

only about 2 hours in all. and the sound. with my smooth plate telefunkens and rca buffer tube it sounds amazing!!!!!

buschfsu · « **Reply #9 on:** 18 Jul 2011, 06:41 pm »

Quote from: el34 on 16 Jul 2011, 01:31 pm

Ohm's law applies, but to the voltage drop across the resistor, so for example if I = 0.9,
and your target is 6.3v for the heaters, and V before the dropping resistor is, say, 8v, then
R = V/I
= (8 - 6.3) / .9
= 1.8 ohms

but if my voltage is too low as in my case.. 6.1v and

R=V/I
= (8-6.1)/.9
= 2.1 ohms

doesn't jive with

You should order lower values, not higher to obtain more voltage for heaters

WGH · « **Reply #10 on:** 18 Jul 2011, 08:15 pm »

There is a lot of info in the Cornet2 heater voltage thread:

http://www.audiocircle.com/index.php?topic=41430.msg369374#msg369374

el34 · « **Reply #11 on:** 19 Jul 2011, 03:08 am »

> but if my voltage is too low as in my case.. 6.1v and
>
> R=V/I
> = (8-6.1)/.9
> = 2.1 ohms
>
> doesn't jive with
>
> You should order lower values, not higher to obtain more voltage for heaters

Not sure what you mean here... Are 8 and 6.1 your actual measured voltages? And is 2.1 the
measured resistor value or what's printed on it?

Think of it this way, if you have a very low value resistor, as that resistor becomes near zero, it should
drop almost no voltage across it, so you would then be very close to the voltage at the junction
of the bridge rect. output and the first set of capacitors.

buschfsu · « **Reply #12 on:** 19 Jul 2011, 05:12 pm »

ok that makes sense. and matches what im reading. just trying to force the math to match what im seeing.

off to listen and enjoy!!!!! thanks

buschfsu · « **Reply #13 on:** 30 Jul 2011, 09:14 pm »

one more thing guys. after a few days of playing sometimes the unit will start producing a loud sputtering crackly distortion. when i tap the 12ax/au7s they make that sputtering crackling sound. usually to make it go away i tap the tubes gently and it goes away. multiple tubes have produced the same.

any ideas

mjock3 · « **Reply #14 on:** 2 Aug 2011, 01:50 pm »

Is the sputtering in one or both channels? I have been trying to fix a problem like this. It is like a static crackling sound. It was suggested to me that it was probably a bad solder joint though I have not had success in finding it and have re soldered a number of joints.

Mark

el34 · « **Reply #15 on:** 2 Aug 2011, 11:24 pm »

I agree, check the solder joints. 2 more possibilities: dirty tube pins or sockets (much more likely with vintage tubes than new),
or sometimes the 12au7 can develop a heater--cathode short, it is stressed a bit in the cathode follower. I had that happen with a couple of 12au7's in my cornet. Usually that will be only in one channel.

Oh yeah, better to just try to re-seat the tube than ding it which will risk damage.

buschfsu · « **Reply #16 on:** 4 Aug 2011, 02:09 pm »

yes im thinking dirty pins also. im going to scotch pad them and rub with ipa.

hagtech · « **Reply #17 on:** 8 Aug 2011, 04:08 am »

Indeed, crackling or scratchiness is a bad connection. Sounds like the tube pins as you described. What is IPA? India Pale Ale?

jh

buschfsu · « **Reply #18 on:** 8 Aug 2011, 01:23 pm »

Ha!! no its an engineering term for Isopropyl Alcohol.

regap25 · « **Reply #19 on:** 10 Aug 2011, 01:58 pm »

I just finished my Cornet 2 build and its fantastic, its also ansolutley dead quiet. I built mine with the 2 incher hammond chassis. I made a video with the cornet with under 2 hours of use. I know how to edit videos but have no idea how to edit the sound. in the video you can hear the hi8 tape hiss from the camera.

I'll be on vacation, so i won't break it in until the beginning of next month..dang

http://www.youtube.com/user/Migkiller1971