Hot chassis - normal?

[markmalin]

Hi all,


Just curious.  I'm working on an AB763 build running KT-66's and a tube rectifier.  The KT plates are at about 409 VDC, and I have them biased at about 34ma.  If I run this chassis hard, like let it sit for an hour with the volume on about 8 it gets really hot.  The chassis is nearly too hot to touch around the area where the output tubes are.  I know it's probably a silly question, but this is pretty normal, isn't it?


Humbly,
Mark

[eleventeen]

Not that normal. You know that a KT66's EACH draw 400 mils more than 6L6? Total, 800 ma add'l. That's not an incredible lot more but it's not nothing.


I can only assume your add'l heat is from the power transformer, that is, unless you already have the amp in a cabinet and the confined heat of the [upside down] KT66s' is heating up the chassis. You are not running them especially hot @ 410 B+ and 34 ma each. That should be just above a medium cruise for them.


It is normal for a transformer to get "too hot to comfortably touch"....but definitely not "searing".


Ultimately, you may have to add a fan or downshift to using 6L6.

[markmalin]

I thought about adding a fan.  It's not in the cab now, just on the bench, but I was simulating the cabinet by putting a board over the top (chassis upside down the way it would be mounted).


The last one I built had Hammond transformers rather than Mercury.  I suppose the Merc could be running hotter, but that said, the tranny isn't too hot to touch.  Mostly the chassis above the tubes.  And the chassis isn't so much searing, just too hot to leave your finger on it.  I remember reading something Gerald Weber wrote about back in the 60's playing a 3 hour gig and having to use a pick to turn the amp off because the switches were too hot to touch. That's not the case here...but I do wonder  what it will be like when the owner plays a 3 hour gig.



What are my options for fans?  For example, is an 115 AC fan going to add electrical noise?

[eleventeen]

"For example, is an 115 AC fan going to add electrical noise?"


Shouldn't. Can you recheck your heater current specs to see if you are not overdrawing the filament winding?


In general, even a small fan will make a pretty large difference. Because if you are not having a real fault of some kind, it will tend to prevent the chassis from heating up in the first place. You can tear one out of a dead PC power supply and run it from a 9-12 volt wall wart as a no-cost experiment. I have such a thing mounted above my bench to suck solder fumes away.

[markmalin]

Shouldn't. Can you recheck your heater current specs to see if you are not overdrawing the filament winding?

eleventeen, I'm not sure I follow.  Sorry to be daft, but do you mean recheck the heater current specs of the tubes I'm using?  (they are JJ's)  Are you thinking the KT filaments might be drawing too much current for the PT to handle?  (i.e. it's overheating due to more current being drawn through it than it's spec'd for?)  It's a Mercury Magnetics FBFS-P40X. I spoke with Patrick and mentioned I'm running KT-66's and he recommended this transformer.  The OT is set up as a 6.6k primary and using the 4 ohm secondary.

To quote Will Smith from Seven Pounds..."explain it to me like I'm a 6-year-old"

Mark.

[eleventeen]

That's what I meant---it was a reco to check with the mfr or the tranny spec sheet to see if the add'l draw is within specs.


I can't seem to find that tranny, but if you've checked, you've checked.

[sluckey]

I think it's normal for the tubes to heat up the chassis in an upside down chassis configuration.