New Build 120hz Hum

[Andy Le Blanc]

you can bias a pair of 6l6 on the two 12ax7 heaters dressed series,

Bias and free DC, the draw back is the controlled heater warm up time.

it take a little longer to get up and running, any way...

simple humdinger is real nice.

[ampgeek]

Excellent! Sure feels nice when a small tweak makes a dramatic improvement.

Is it possible that it was actually 60 Hz hum as opposed to 120 Hz?

Is the Humdinger a voltage divider off of the HT like ~1/2 down here?

http://www.freewebs.com/valvewizard/heater.html

Replace fixed 100 ohms with a 100 ohm pot and snag the filiment winding CT with the wiper?

That looks like it could be a tidy little set-up!

Dave O.

[Andy Le Blanc]

60 and 120 can be freq centers or harmonic, it more a matter of getting
the level down critically.

120 usually is a result of rectification, 60 usually comes through the heaters

I've seen excessive 60 come through leaky PT winding/insulation
bad rectifier/sockets...wacky ground issues... watch out for old PT
could be real sign of danger, a shorted winding will lift you if the ground
is not there for what ever reason.

pushpull- feedback will cancel some. I like the cathode bias as a voltage source
because there's some voltage regulation associated with it, but you can use
any positive voltage, it also helps heater to cathode voltage limits.

A 100 to 500r hum pot can be real nice no matter what the voltage source.
really dial it out...

Keep in mind that other countries have different line frequencies.
although its more pertinent to PS values than heater hum.

If you after a voltage divider, any thing from 20 to 100 v.
if your rig has a cathode follower figure the v on the cathode v...
plus the max signal v at the stage.

[surfsup]

Replace fixed 100 ohms with a 100 ohm pot and snag the filiment winding CT with the wiper?

Having redone my heaters and CT a hundred times, i believe the wiper goes to ground and a CT on the transformer should be taped off. You wanna just run the 100 Rs off the heaters (usually at the indicator).

I wound up buying a batch of 100Rs and measured till i found two that DMM'd the same and used those. No need for 1% Rs. Pot is better but more $$$

[passfan]

I am about to assemble my first single ended build and am following this with interest. Plenty of PP's under my belt and I haven't yet had to deal with humming issues.

My layout includes an elevated heater center tap which appears to have solved (for the most part) the OP's original issue.

But...doesn't the technique only reduce 60 hz hum from the tube's filiment supply? I do understand the theory behind how that works.

Seeing that it help reduced 120 hz hum has me believeing that I may not truly understand the technique and/or theory.

TIA,
Dave O.

Ampgeek, 120 hz is the first harmonic of 60 hz. You also have 240 hz, 480 hz, 960 hz and so on. To get to the next harmonic you simply double up. Computers inject 5th order harmonic noise in ac lines and so we dedicate circuits and grounds to them in an attempt to keep their noise out of our power system. It doesn't surprise me to see elements of the other harmonics present in our hum issues.

[ampgeek]

Good stuff. Thanks as always for sharing.

Here is the graphic that I was referencing above. Specifically, the voltage divider off of the HT supply option. I was envisioning replacing both 100 ohm resistors with a 100 ohm pot.

And...yes...now I see that the wiper finds it's way to ground through the supply side and the 6.3 V winding CT (if present) would be tied off and isolated.

Thanks for pointing that out Surf!!

Cheers,
Dave O.

[ampgeek]

OK!! Now I think I am beginning to see the light!

Harmonics, just like those pleasing ones from our vibrating strings, are also derived from less desireable sources (e.g., 60Hz from heaters).

How do the amplitudes of the harmonics compare with those of the "base" frequencies? In our "pleasing" cases it appears that they are much lower. Is it possible that in some cases they are higher?

That would appear to be the case if by neutering 60 Hz at the filiments we hear a dramatic reduction in 120 Hz at the output.

I just went out to the BFDR'ish build currently on my bench (no humming problem discerned) and slapped my amp stethoscope onto the filiment supply (60 Hz) and onto the first B+ node (120 Hz) just to calibrate my ears.

Hell...I am not so sure that I could have confidently identified the difference if I wasn't able to quickly A/B them! Now I am wondering if my hearing isn't AFU!!

Cheers,
Dave O.

[surfsup]

No prob, glad I finally helped someone. If that doesn't work the next step is to build an elevation circuit perhaps which can be added pretty easily using a terminal strip as I did in the top right of this photo. You can see the two 100Rs coming down in a "V" off the light on the right, and grounded at the first filter cap. A similar schematic is page 2 here (on mine I paralleled two Rs on each end to achieve a similar resistance b/c I didn't have the proper value): (btw, neither worked for my SE, I ultimately had to redo the ground scheme)

http://ax84.com/static/comanchero/Coman ... 110323.pdf

[img1500]http://chicagocadcam.com/ChrisHahn/GND.JPG[/img]

[passfan]

OK!! Now I think I am beginning to see the light!

Harmonics, just like those pleasing ones from our vibrating strings, are also derived from less desireable sources (e.g., 60Hz from heaters).

How do the amplitudes of the harmonics compare with those of the "base" frequencies? In our "pleasing" cases it appears that they are much lower. Is it possible that in some cases they are higher?

That would appear to be the case if by neutering 60 Hz at the filiments we hear a dramatic reduction in 120 Hz at the output.

I just went out to the BFDR'ish build currently on my bench (no humming problem discerned) and slapped my amp stethoscope onto the filiment supply (60 Hz) and onto the first B+ node (120 Hz) just to calibrate my ears.

Hell...I am not so sure that I could have confidently identified the difference if I wasn't able to quickly A/B them! Now I am wondering if my hearing isn't AFU!!

Cheers,
Dave O.

Neutering 60 hz in the filaments and all the harmonics associated with it does not affect signal at least to the extent that your describing. We're simply eliminating ac as a noise source in the heater circuit and not the signa. l

[passfan]

Perhaps I misunderstood your previous post. Merlin gives an example in his book of a 1khz signal containing 90% 1khz and 10% 2khz distortion. Through the use of feedback we cut our original signal by 6 db and distortion increases proportionally to 55% 1khz and 45% 2khz distortion. Could distortion be considered a harmonic of the 1khz signal, perhaps. It certainly shows one possibility of controlling the balance.

[passfan]

Perhaps I misunderstood your previous post. Merlin gives an example in his book of a 1khz signal containing 90% 1khz and 10% 2khz distortion. Through the use of feedback we cut our original signal by 6 db and distortion increases proportionally to 55% 1khz and 45% 2khz distortion. Could distortion be considered a harmonic of the 1khz signal, perhaps. It certainly shows one possibility of controlling the balance.

[passfan]

Perhaps I misunderstood your previous post. Merlin gives an example in his book of a 1khz signal containing 90% 1khz and 10% 2khz distortion. Through the use of feedback we cut our original signal by 6 db and distortion increases proportionally to 55% 1khz and 45% 2khz distortion. Could distortion be considered a harmonic of the 1khz signal, perhaps. It certainly shows one possibility of controlling the balance.

[passfan]

Perhaps I misunderstood your previous post. Merlin gives an example in his book of a 1khz signal containing 90% 1khz and 10% 2khz distortion. Through the use of feedback we cut our original signal by 6 db and distortion increases proportionally to 55% 1khz and 45% 2khz distortion. Could distortion be considered a harmonic of the 1khz signal, perhaps. It certainly shows one possibility of controlling the balance.