HT-1R 60 Cycle Hum

[nigeltufnelyyc]

My HT-1R starting losing volume recently so I decided to swap out the stock tubes for brand new JJ ECC82 and ECC83 tubes. That is when the 60 cycle hum started. It is making me crazy.

Here are the symptoms:
The hum responds to the volume control. If I turn down the volume, the hum disappears. This leads me to believe that the hum is originating in the preamp, before the volume pot.

The hum is present in headphones when I plugin to the speaker emulated 1/4" output. I've read that the power amp section is bypassed if the 1/4" output is being used. Is this correct? If so, then it clearly isn't the new JJ ECC82 (12au7) tube that is causing the issue.

I suspect that the hum is originating in the ECC83 (12ax7) tube. Would it make sense to hit the store and try a new 12ax7 tube in the preamp? Unfortunately, I don't have any laying around to test with, and I really don't want to spend the money unless it makes sense...

[thephantum]

Your logic is correct. Since the hum started when you replaced the tubes, one of the tubes is most likely the culprit. Since the power amp is not used when taking a signal from the EO, that rules out the power tube. Making the ECC83 the most likely culprit.

Have you tried putting the original ECC83 back in? The symptoms you initially described indicate a failing power tube. So your old preamp tube is most likely still good.