Hi Michael,
My 70/70 emits a slight mechanical buzz for a few seconds before the B+ relay clicks on, which sounds like what you are referring to as the "first hum (maybe more of a buzz)." It disappears completely when the relay clicks on, and I suspect is a mechanical phenomenon involving some of the several power transformers that are in the amp. So I don’t think that initial hum/buzz is an issue.
At this point all I can suggest in addition to what you are planning to do is to reiterate the suggestion of seeing if the hum is present with nothing connected to the amp’s inputs, if you haven’t already done so. Or better yet, with shorting plugs connected to its inputs.
Best regards,
-- Al
My 70/70 emits a slight mechanical buzz for a few seconds before the B+ relay clicks on, which sounds like what you are referring to as the "first hum (maybe more of a buzz)." It disappears completely when the relay clicks on, and I suspect is a mechanical phenomenon involving some of the several power transformers that are in the amp. So I don’t think that initial hum/buzz is an issue.
At this point all I can suggest in addition to what you are planning to do is to reiterate the suggestion of seeing if the hum is present with nothing connected to the amp’s inputs, if you haven’t already done so. Or better yet, with shorting plugs connected to its inputs.
Best regards,
-- Al