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I recently tried an experiment with my amp. I substituted a NOS Mullard GZ34 for a 5U4GB. Where there was NO noise or hum with the 5U4GB, the GZ34 hums. I have two of them and substituted one GZ34 for the other GZ34. The hum decreased but did not go away. I Thought full wave rectifiers are interchangeable as long as the voltage differences are accounted for.
My problem is that while it hums, I like the sound of the GZ34. Any ideas what might be causing this?
Yes, the increased voltage will also make more power and increase the gain of the drive stage and could also make a low level hum more evident.
You might want to verify that the B+ is taken off pin 8. That is the cathode terminal of a GZ34.
With a 5U4, the B+ can be taken from either pin 8 or pin 2, it does not matter because there is no cathode
You can get hum taking B+ from pin 2 of a GZ34.
The GZ34 will make more B+. You may need to reduce the idle bias of your output tube ---> to have idle wattage in nominal range.
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