Heat related hum

[Alonzo]

Looking for advice on what to check.  I have what I think is heat related hum, after playing an hour or so, the right channel of my BeePre to Kaiju will hum, a loud electrical static sound that is not responsive to the volume control on the BeePre.  I have switched cables, tubes, amps, moved amps and preamp, switched sources, put the BeePre on higher feet, flipped it and made sure the heatsinks were attached and tight. 
Turn off the system for a minute or two and it's still there but not as loud.  Then it gets louder and louder till it tops off.  Turn it off for the night and come back then it'll play for an hour or two then the hum comes back.
Where should I start?
TIA
Alonzo

[Paul Birkeland]

Are you running EML 300Bs that draw more heater current? 

Is this noise at all related to variations in line voltage?

Do you have 1N5820 or 80SQ045N rectifier diodes? 

How do the 10,000uF/10V caps mounted by the power transformers look?

[Alonzo]

Nope, EH 300B's, JJ300B's and EH Gold300B's.
I'll double check the line voltage but I don't suspect it.  It has happened late at night and all thru the day.  I'll check it now.
I'll check on the parts tonight.
Thanks,
Alonzo

[Alonzo]

PB,
You are right about the line voltages so far.  Another hour or so and I can declare it.  I've tracked hum/distortion that corresponds with my line voltage dropping to 120, 119 and 118 over a couple of hours.  Anytime it pops up above 121, I'm super quiet.  The biggest period has been between the 5:30 and 6:16 period where it consistently sagged every few minutes.  Unfortunately PG&E considers a 10% drop for 3 to 10 cycles okay.  So far I've run for over an hour and a half at 121 with no noise, up to my ear in the mouth of the horns.
So it looks like I need a solution to boost my voltages up.
Any suggestions?
Thanks,
Alonzo
 

[Paul Birkeland]

I can swap the diodes out to something a lot more efficient that will tolerate line voltage down to 116V I think.  They are kind of annoying to install since they are TO-220 parts instead of axial diodes like what's in the BeePre now.

Another option is to grab a 120:6 power transformer and wire it as a boost transformer to kick up your AC mains voltage.  The transformer doesn't have to be all that big, a 6V/1A transformer would be enough to feed a BeePre.

[Raymond P.]

I can swap the diodes out to something a lot more efficient that will tolerate line voltage down to 116V I think.  They are kind of annoying to install since they are TO-220 parts instead of axial diodes like what's in the BeePre now.

PB, would these more efficient diodes replace the ones for the heater regulator? Could you list the part number? I have experienced this low line voltage hum last summer, and I'd like to look into this diode replacement as a solution. Thanks!

[Paul Birkeland]

I believe I used DSS25-0025B.

These are very, very difficult to install. They really can't go under the board, because if the metal tabs touch anything you could end up with a very expensive problem on your hands. 

[Raymond P.]

I believe I used DSS25-0025B.

These are very, very difficult to install. They really can't go under the board, because if the metal tabs touch anything you could end up with a very expensive problem on your hands.

Thanks! Are heat sinks needed on the metal tabs? Just to be sure, these would replace the 80SQ045N used for the 10V regulator, right?

If I make this replacement, it sounds like using perforated prototype board would be a good way to go.

[Paul Birkeland]

Heatsinks aren't necessary.

[Alonzo]

So, the 10v caps under the power supply look good.  My BOM with my manual says I was sent 80SQ045N diodes, but by sight I can only see a "1629" number on them.
Checked the line voltage thru the night and it stayed over 121 and there was no hum or buzz.

I'll send you and email about getting on your repair list.
Thanks,
Alonzo