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Hot PT in BF Bassman....

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12/4/2005 8:51 AM

Hot PT in BF Bassman....

I picked up a 66 Bassman yesterday for a good price. It was in a 2-10 combo cabinet, but the seller also threw in the original head cab. I didn't like the sound of the 10's, so I hooked to my 2-12 cabinet, & jammed for about 3 hours with some friends.

When I got home, I removed it from the 2-10 combo cab, & put it back in the original cab. I noticed that the PT was still warm. This was about 15 minutes after we had quit playing. The power tubes & everything else had cooled down, but the PT was still warm. While we played there were no problems at all with the amp.

Is this something I should be concerned about? What would be the logical things to check?

Thanks,
Steve

12/4/2005 10:23 AM
Bruce /Mission Amps	"Warm" is so subjective Steve. Warm to hot is really normal. We all like to think the PT should never get very warm but I've had many vintage and new amps with power trannys hot enough that after an hour or two of use, I can't grab the top "like a baseball" more then 5-10 seconds due to the pain... they never failed and never sounded bad from being so hot. Bruce

12/4/2005 1:06 PM
SteveB	Thanks so much Bruce. It was very warm, almost too much to hang on to. Thanks again, Steve

12/5/2005 6:25 AM

Transformers run near their rating run hot. In fact the power rating of a transformer is *determined* by how hot it gets. A transformer with Class A temperature rise insulation system (A is the lowest temp rise capable rating)will get to about 130F at full power. 130F is just hot enough that you will not keep your fingertips on the surface - Bruce, you were touching transformers running just about where they were designed.

The same iron and copper with Class H insulation could run happily at the boiling point of water and put much more power - and do it on a continual basis. It's the insulation temperature rating that's the weak link, so insulation temperature rating defines th power rating.

And transfomers are dense. The thermal time constant of a ten-pound transformer is about an hour, so it takes a while to get them hot and also a long time to cool down.

I agree with Bruce - the observed action is normal unless you find some other problem.

12/5/2005 9:06 AM
Bruce /Mission Amps	Thanks for the confirmation... Over the many many years of doing this I can probably count on one hand the number of times I've run into a failed power tranny. Not so with output trannys though. Hot to the touch just doesn't mean that much to me... hot, stinky and much much lower secondary voltage then expected would be another thing! Bruce

12/7/2005 2:16 AM
Mark Burness	Hi Bruce, If you had a PT with some shorted primary windings, wouldn't this result in much higher secondary voltages (as well as excessive heat)? Mark.

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