Many years ago I had asked about a noticeable hum that from my W 532 amp. I checked out a few things but never resolved the hum problem. At the time I was new to tube amps and wasn't quite sure how much if any hum might be normal for these amps and just left it go thinking that some hum might be normal. Now that I have had experience with a couple more tube amps, I realize that the hum is not normal and have decided to dig in and try to resolve the hum problem.
Symptom: Hum that does not change with the volume. Sounds like 60hz to me and not sine wave since it has bit of a buzz to it.
I've narrowed it down to the circuit for the 6AU6. Hum goes away when the amp is muted. Removing the 6AN8 does not change the hum. Shorting the Aux Amp connection has no effect on the hum. Removing the remote volume plug has no effect on the hum. I did go through all the resistors that did not require flying a lead to verify their values. I did find a few that are marginally out of spec reading approx. 10% too high (R34, R36, R37). Replacements are on order, but I am not convinced these are the culprit, but my plan is to replace them to be sure. I think this is a case where I wish I had an oscilloscope to probe around to see if I can narrow down the source of the hum. I verified all the grounds for this circuit and tried swapping out the tube. My next plan is to go back and check the B+ and then check all the component values and connections for this circuit. Since these are all on the PCB, I can't imagine there being not being connected correctly but who knows. Am I missing anything obvious?
Carl Haines
Carl,
A lot of good troubleshooting! The muting is after the 6AU6; since the hum goes away when the amp is muted and that any "muting" ahead of the 6AU6 (shorting the aux amp, pulling the volume plug) that says that you're on the right track. I'm suspecting a heater cathode short in the 6AU6. Try pulling the 6AU6 tube, the hum should go away. If you have another one try swapping it. I agree, the out of spec resistors, while they are in the 6AU6 circuit, are not the issue. 2 of them (R36 R37) are in the tone shaping T filters and the 3rd (R34) is the plate resistor. None of these would cause this problem.
Karl
-----Original Message----- From: Carl Haines via Jukebox-list Sent: Wednesday, February 01, 2023 6:22 AM To: jukebox-list@lists.netlojix.com Subject: [Jukebox-list] Wurlitzer 532 hum revisited
Many years ago I had asked about a noticeable hum that from my W 532 amp. I checked out a few things but never resolved the hum problem. At the time I was new to tube amps and wasn't quite sure how much if any hum might be normal for these amps and just left it go thinking that some hum might be normal. Now that I have had experience with a couple more tube amps, I realize that the hum is not normal and have decided to dig in and try to resolve the hum problem.
Symptom: Hum that does not change with the volume. Sounds like 60hz to me and not sine wave since it has bit of a buzz to it.
I've narrowed it down to the circuit for the 6AU6. Hum goes away when the amp is muted. Removing the 6AN8 does not change the hum. Shorting the Aux Amp connection has no effect on the hum. Removing the remote volume plug has no effect on the hum. I did go through all the resistors that did not require flying a lead to verify their values. I did find a few that are marginally out of spec reading approx. 10% too high (R34, R36, R37). Replacements are on order, but I am not convinced these are the culprit, but my plan is to replace them to be sure. I think this is a case where I wish I had an oscilloscope to probe around to see if I can narrow down the source of the hum. I verified all the grounds for this circuit and tried swapping out the tube. My next plan is to go back and check the B+ and then check all the component values and connections for this circuit. Since these are all on the PCB, I can't imagine there being not being connected correctly but who knows. Am I missing anything obvious?
Carl Haines _______________________________________________ Jukebox-list mailing list Jukebox-list@lists.netlojix.com http://lists.netlojix.com/mailman/listinfo/jukebox-list Searchable Archives: http://jukebox.markmail.org/
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Carl Haines -
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