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Why is my PT getting hot?

1/18/2004 8:16 AM
Ricky Why is my PT getting hot?
Hello all, I just finished a PP 2x6l6 amp and I had a concern about the PT. First off the amp seems to be running fine....but I left it on for a little over an hour today for a bit of a 'burn in' and noticed the PT getting pretty hot. Hot enough to where I wouldn't want to leave my hand there for longer than about 5 seconds.  
The amp is using a single 12AX7 for the preamp, 12AT7 for the PI, a 5ar4 tube rectifier, and a pair of 6L6's. the power transformer is a Hammond 270FX(275-0-275 @150mA)The plates are running at about 360v, and I've biased so the 6L6's are getting about 58mA each...this amp is cathode biased, by the way.  
The PT should be fine with the amount of current it has on tap, right? I don't understand why its getting so there anything else I should check?  
P.S. the OT is a Hammond 1645 (30watt), the filter caps are all three 40uf, and the filter supply has a 9 henrie choke in it...not sure if any of this matters, just thought I'd add it in case it did.
1/18/2004 12:18 PM
Dave H

Are the 6L6s right next to the PT? They are dissipating over 40W. That would heat up the PT. What is the primary impedance of the OT? If you could make it 8k the 6L6s could run at 44mA each (17W).  
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1/18/2004 6:22 PM
Wild Bill

Ricky, the heat is caused by excessive current draw from the PT. You have only 3 windings - HV, 6.3v and 5v.  
If the 6L6's drew too much current you would see much higher idle currents through the cathode resistor(s). If your calculation was seriously wrong you'd see red spots on the plates that would give a strong clue! :)  
Could you have something wrong on the filament line(s)?  
A bad connection or a partial short that is sucking power? Not enough to blow a fuse but enough to heat up the winding? If your meter will read AC amps you can open up a filament line and put the meter in series to measure the current. If it's in line with what you'd expect the tubes to draw then you'll know that's not the problem.  
could there be a wiring error around the later B+ taps feeding the PI or the preamp? Again, not a short but maybe a high resistance path from plate to ground that will pull a lot of current from the supplied HV. Turn the amp off (and if necessary quickly bleed off any stored filter charge!) and feel if the PS dropping resistors feel too hot. That will help you isolate which stage is drawing too much current.  
You might also measure the voltage drop across the 6L6 screen resistors. You should find only 3-4 ma at idle. A bunch more indicates a problem.  
Again, it's current that makes the heat. You could also have a bad PT winding. Hammond rarely has such a problem but anything is possible. Some folks do win lotteries, after all.  
Something else I just thought of - you could remove the main fuse and 'gator your AC current meter across the fuse clips. You already know your idle currents and expected filament draws. Convert everything into watts, add 'em up and then calculate that wattage @ 120 VAC line voltage. This will give you the total current draw and you can see if it makes sense or not.  
Good Luck!  
---Wild Bill
1/18/2004 8:22 PM
ted m
I would lower the B+ for slightly less dissipation, but you may also want to use a weber copper cap which wouldn't draw any power from the PT, and emulate the 5AR4 pretty closely.
1/22/2004 4:34 PM

What you are experiencing is normal temp rise, IME.  
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