another BeePre hum problem...

[Mach2]

I finished my BeePre build today and it hums The hum is on both channel, and does not change with the volume or balance pot. All resistance and voltage readings are in acceptable 10% range, with a voltage rate of 227V. The BeePre and the amp are plugged in the same power strip. This is a completely stock build. There is music at the output, so at least the BeePre does what it's supposed to do.

I note that the readings of A2/B2 and A3/B3 are in reversed of the ones in the manual and thought that it's just a typo.

I tried swapping tubes, reflowing most of the solders with no luck. What should I do next? Help!

[Paul Birkeland]

What voltage do you have at POS-IN?

[Mach2]

I have 9.76V and 9.77V on the two PSU boards.

[Paul Birkeland]

That's at POS-IN?  What's your line voltage?

[Mach2]

Ugh sorry, those are POS OUT. POS IN are 12.21/12.29. My line voltage is 227v when i took those readings.

[Paul Birkeland]

Was your kit shipped with 1N5820 diodes or 80SQ045N diodes?

[Mach2]

I got 80SQ045N diodes in my kit.

[Paul Birkeland]

Just for kicks, what do you get at OA and IB on each C4S board?  How about each Kreg in back? 

[Mach2]

OA: 146/145
IB: 146/145
OB: 130/100
IA: 164/162

Kreg+: 146/145
Kreg-: 0/0
Kreg: 3.9/4.1

I remeasured all the other terminals as well, and everything is good, except that terminal 2 (OB) is measured at around 130V instead of falling inside 90-110. The other parallel terminal 17 is measured at 100V. I swapped the two 300b and the readings between 2 and 17 are swapped so that is caused by the (odd? OK?) tube. Not sure how it affects the system since I got hum from both channels though. The channel with the strange tube exhibits a kinda white noise in addition to the hum.

[Doc B.]

What amp and speakers are you running the Beepre into. It is a very sensitive amp?

[Paul Birkeland]

The 130V tube is somewhat distressing, especially if that voltage will follow the tube around. Does the silver flashing in the tube still look silver?

If you run the Beepre with the lights off, then look down into the 300B's, can you see a different number of filament wires glowing in one vs. the other?

[Mach2]

What amp and speakers are you running the Beepre into. It is a very sensitive amp?

I'm running the Paramount amp to drive a pair of Orca.

The 130V tube is somewhat distressing, especially if that voltage will follow the tube around. Does the silver flashing in the tube still look silver?

If you run the Beepre with the lights off, then look down into the 300B's, can you see a different number of filament wires glowing in one vs. the other?

The 130V tube looks fine, nothing odd about the silver flashing. I turned off the light to verify that it has the same set of glowing filament as the others'. In the dark it looks a little bluish, but the other EH tubes in my Paramount blocks also look bluish, so that can't be bad right? I tried taking a photo, but can't capture that blue tint.

[Paul Birkeland]

You might try swapping in an EH from one of your 300B's, just to get the voltages consistent. 

[Mach2]

I swapped in a EH tube from my Paramount and got a reading of 100V-104V on both channels. The hum is still there.

I then swap back the odd tube and observe the voltage at 2(OB). It starts high but dropping as the amp is heating up and finally is stabilized on the lower 120V after 10mins, still out of the 90-110 range in the manual, but nowhere as bad as 130+V initially. Do you think it's a bad tube?

[Doc B.]

It might be a tube that is new or has not been used in a while and just needs to cook for a while to get the filament emitting