60hz hum develops after 6-8 hrs, and HOT

[aroide]

I'm having a strange issue with my BeePre.  I recently got new 300B tubes and have been playing music 6-8 hours at a pop to facilitate breakin.  I've had the preamp on that long before, but that was when it was relatively cool.  Its 107 outside and 78 or so in the room with the beepre.

After about 6 hours, I get a hum in one channel.  It is square-waveish sounding (sharp with higher harmonics).  If I try and touch the bepree top plate or transformers, I can burn my finger.  That thing is HOT.  If I turn it off and let it cool, then back on, no hum.  If I leave it on 6 hours or so and the room gets a bit warmer in the afternoon, hum.

What could be causing the issue?  An overheating component?  Which one is suspect? 

I do have my beepre raised about 1/2" on some vibropods.  And there is airflow front and back.

I'll do some experiments with a fan, but beyond that any help is appreciated.

Tony

[saildoctor]

Have you tried switching the tubes to see if it follows the hum?  What brand of 300b are they?

[aroide]

They are Sophia princess carbon plates.  I'll  switch and see after I complete my 'fan blowing airflow on the beepre on a hot afternoon' experiment.  It takes 6-8 hrs to develop and does not vary with volume or source.

[johnsonad]

Play with your RCA's in that channel too

[aroide]

more info:

It appears heat related.  I ran 6 hours in a hot room (room gets to be about 80 in the afternoon and evening), but had a dyson fan on lowest setting about 4 feet in front of the BeePre.  No issues and my touch test of the top plate was cooler (I could hold my finger on it for a several seconds... could not do that before).

The hum is not like a ground issue or inductive coupling.  I know those (and have had an inductive coupling issue when placing my components.  Movement of cables, check of RCAs, makes no difference.  I suspect under high heat that some component is outside its limits.  Maybe some AC is getting through the rectification, regulator and filter circuits under high heat?

I swapped the 300b's and will see what happens today.  And regardless, its time for me to order a quiet fan for long listening sessions in Phoenix summers.

[Grainger49]

I have often suggested a computer fan, 12V DC, run on 9V DC to lower the noise and RPM of the fan.  It has been successful.

The problem with most fans is they can easily be heard.  Lowering the speed makes them move less air but much quieter.

[Paully]

Putting in a fan is a fine idea of course.  It certainly won't hurt to run the electronics at a lower temperature.  But these are new parts that are heat rated.  If one of them is being affected by the heat, reducing the heat sounds like a temporary fix.  That part sounds faulty to begin with.  If the heat is getting to it, doesn't it sound like it is ripe for failure in the near future anyway and needs to be located and swapped out?  Anyone have suggestions for a likely culprit?

[aroide]

I just noticed that one of my EL84s is not glowing and is cool (warmish) to the touch.  It used to glow nicely.   Despite that he system is playing and sounds balanced and great.  That is kind of weird.

Swapping tubes, the non-glow follows the tube.  Interesting that this EL84 was not in the circuit of the channel that had the hum.  But maybe they are related.

In any case, I need to order some new EL84s.  Are they matched pairs?  What brand?  I'd like to replace with the same ideally.  I know they are only for the regulation, but I don't want to mess with any circuit magic juju.

[fullheadofnothing]

The tubes provided are Russian surplus 6∏14∏. They are not matched.

Did you re-do your voltage tests after you replaced 300Bs and started noticing problematic behaviors?

[aroide]

did not recheck voltages.  The issue appears to be related to leaving the unit on for long hours in a hot room.  I had never done that previously since I built it.  Since I need to replace a 300b socket that has a loose ceramic, I'll pull out the unit and recheck voltages.

The bad EL84 is the one tube that arrived with 'gunk' all over it.  I needed a healthy amount of windex and elbow grease to clean it off.  Maybe it was stressed already coming off that MIG ;-).

I've been looking and 6pi14pi NOS are hard to find.  Given where this is in the circuit, what about a new Mullard or something like that?  I assume that in the regulator circuit, impedance is an important factor, but I'm not aware of how much these tubes differ and how much different tubes affect sound.

[Paully]

When I have asked, the answer I received was that the particular EL84 shouldn't have much of an impact.

[Doc B.]

Yes, any EL84/6BQ5 will work. And we are happy to exchange that bad tube for you.

[Chris65]

I've been looking and 6pi14pi NOS are hard to find...

Maybe the wrong search term, try 6p14p. They're everywhere, start at about $5 a piece.
But as Doc says any EL84/6BQ5 will work.

[johnsonad]

I've got a large stash of EL84 and have found nearly zero change in sound when rolling them. Buy the Ruskies for spares.  They are cheap and when good, last a long time.

[STURMJ]

FYI: Im using new prodution Mullards, they work fine in the circuit