Posted by Poindexter [ 4.63.6.54 ] on August 18, 2005 at 17:53:30:
In Reply to: One output transformer hotter than the other posted by hurdy_gurdyman on August 17, 2005 at 21:38:16:
Great Googly Moogly, they run those EL84s hard! Plate max.
Dave, since the 'bias take' resistor in the common cathode tail is 10Ω, your tubes will be at dissipation max at a common cathode voltage of 0.6v (60mA for the two tubes). Measure the DC voltage at either cathode of the pair, and see what you get. Compare to the other pair. I would say that you'd like to see 0.5 Vdc or so on each side.
If they're pretty close, turn the amp off and use your ohmmeter to measure the DC resistance of each half of the primary on each side, and compare likewise. You may have some corrosion or something in one transformer that is raising its DCR and causing the same amount of current to heat it up. Another way to check for this is to test the DC voltage on the plates, as a high R winding will cause more voltage drop across the primary. The plates are pin 7.
An odd thing; there's a DC balance pot after the bias adjust so you can set the OP of each tube in the pair, but I didn't see any way to test for this; or a bias take jack either. AFAICS, you'll just have to take off the bottom plate and stick your hot probe on the right spot. The cathodes are pin 3.
Questions?
Poinz
[ Tubes Forum ] [ Help ]