Can hum be caused by transformer?

[dbishopbliss]

I'm preparing for the Bottleneck linestage competition.  When I breadboarded my linestage, there was a loud audible hum. After asking a number of questions, I thought it might have to do with not grounding the center tap of the filaments and not having raised the potential of the filaments.  Since time was short, I went ahead and re-built the circuit in the final enclosure but with those modifications.  However, I still have the hum.

One thing I noticed in both circuits an audible hum from the transformer even without being connected to an amplifier.  I can stand two feet away and hear it.  Could it be that my circuit is fine and my transformer is bad?

[Grainger49]

Transformers radiate a magnetic field.  That field can cause hum to be generated in the audio circuit.

The physical hum from the transformer can be one of a few things.  The transformer might be loosely attached and therefore vibrating.  It might need rubber grommets under the mounting "feet."  Feel it and see if it is vibrating.  It might need the laminations tightened (if bolted together).  It might be heavily loaded and the power transformers I am familiar with (many kVA) audibly complain when heavily loaded.

I hope this is some help. 

[Wardsweb]

If the hum is a physical oscillation of the transformer itself, this can be caused by actual DC voltage on your AC line. You can measure this with a multimeter to verify.

[dbishopbliss]

If the hum is a physical oscillation of the transformer itself, this can be caused by actual DC voltage on your AC line. You can measure this with a multimeter to verify.

Can you provide more details how to measure.  Do I connect the leads of my DMM to the AC input, but set it to DC?  If this were an issue, wouldn't it be an issue for all components?

[Grainger49]

David,

Most meters have an input that is common for AC volts, DC volts and ohms.  Maybe some other things too.  And then also have a common input for the black lead.  The other inputs are often for current (Amps) and temperature.  

Hopefully Wardsweb will come back with use the standard voltage and common inputs, set it on "DC Volts" but measure the incoming AC line at the power cord.  I have done this by mistake.  My meter when put on AC reads some very low DC voltage.  The analog readout of it, it is a digital display with dashes across the bottom, jumps all around.  There is no appreciable DC on my AC line.

[Wardsweb]

Can you provide more details how to measure.  Do I connect the leads of my DMM to the AC input, but set it to DC?  If this were an issue, wouldn't it be an issue for all components?

Yes, you just plug your leads into the AC outlet with your meter set to DC volts and see if or how much DC is present. Not all equipment is affected the same by the presence of DC. Some wil have mechanical oscillation and others not. I've had $100 amps not care and $7000 that you could hear across the room.

[dbishopbliss]

I will have to see if I can reduce the mechanical hum in some way.  Probably put the transformer on a gasket of some type, but I don't think that is causing my hum any more. 

After reading around some more about grounding techniques and star grounds, I believe my issue is I have things wired backwards.  That is, I have the power supply ground closest to the chassis ground and the signal grounds furthest from chassis ground.  I thought this was how the Foreplay I was configured so I copied it.  After reading a VALVE article, Doc recommended doing the opposite. 

Another mistake I think I made was to connect the ground wires of the input and output wires to the point were the cathode resistor is grounded.  So, I've got some work to do to rewire this thing.

One more question... I measured the voltage on my filaments and they are slightly over 7V.  I'm thinking this may have something to do with it as well.  The transformer is center-tapped and rated for 2A on the filaments.  The data sheet of the tube says the filaments draw 350mA.  Since I have two tubes (total 700mA), can I rectify my filaments?

[Grainger49]

With your voltage at 7V you might be able to go with DC on the heaters.  If the rectified/filtered DC is higher a dropping resistor, like the FP III, might be needed.

[dbishopbliss]

I added a 0.5 ohm resistor in series with each of the heater leads and that brought my voltage down to exactly 6.3V.  So, now the tubes aren't glowing as brightly and the hum has been reduced.  Still louder than I want.

Here's something I should have mentioned.  There is a grey wire labeled "shield" coming from the transformer.  Where should this be connected?

[Paul Joppa]

If you put a resistor in the primary of the power transformer, instead of the 6-v winding, it will reduce the voltage seen by the primary and might reduce the mechanical hum - at least a bit.

The transformer shield is probably between the primary and the secondaries. It should be grounded to the safety ground, not the signal ground, for the best performance.

[2wo]

For comparison sake, try a battery on your heaters. If you look you can probably find a 6V lantern battery, or 4 D cells slid into a paper towel core. you can ground or float one side or the other. try all of the possible combinations...John   

[ssssly]

Can also try bypass capping the diodes. I was mechanical hum issues with my PM1 transformers a while back. Came across an article about bypass caps for diode bridges and gave it a shot. If you search my posts you'll find the thread in there someplace. Don't remember the exact value I used. Tried 4 or 5. The value was slightly smaller than the one PJ suggested in the thread.

Didn't cure it 100% but it significantly reduced it. Also seemed to help dynamics a bit.

[dbishopbliss]

I think that the hum is related to the Heater to Cathode Voltage (Vhk) because it has lowered quite a bit since I have connected the CT of the heater winding (both the high voltage and heater windings of my transformer have center taps) to a voltage divider raising it 70V.  I do not have a bypass cap on the voltage divider, but I have found people suggesting to use a 10uF cap on another forum.  I believe my Foreplay I has a bypass cap from pin 9 to ground, but it was a lot smaller like 0.1uF (I don't have it here to look at so I may be way off on this).  Two questions:

Should I try bypassing the bottom leg of the voltage divider?

What value cap should I uses?

Thanks

[JC]

A couple of things:  Have you ever tried just grounding the center tap of your heater winding?  If you did, what were the results?

Second, I presume your Voltage divider is across an already-filtered supply, correct?  So, I would use a smaller cap as bypass in order to use something other than an electrolytic.  Of course, that generally sets the practical limit to less than 1 uF, but I don't think you are necessarily looking to filter out anymore ripple with the bypass.

Oh, I guess I have one other question: What are you using for heater wiring?

[Grainger49]

David,

I see a 1.0uF 50V Z5U cap in the parts list for the FP III.  I think that is what is across the voltage divider.  I think you can go to Radio Shack and find a 0.1 or 1.0 uF cap there for a few dollars to give it a quick try.  Since you have 70V across the bottom of the voltage divider you want a 100V cap.  It doesn't have to be soldered just crimp it at first to check then solder if it lessens the hum.