Voltage discrepancy, buzz, and start up questions after build

[Sh7eleven]

I've finished the stereomour build and its making sweet music for two hours now, but I have three issues.

1. My voltage readings at terminal 2 and 14 are low (as are the corresponding OA and OB measurements).  When taking the readings from both terminals - they flash 220-230 for a split second and then settle out to the following: Terminal 2 reads 185 and terminal 14 reads 195.  Terminal 2's reading is about 20% off.  Terminal 14 is 15% off, which the manual states is ok. 

So can I live with 185 volts at terminal 2? is there a performance detriment to have the voltage this low? If it helps, I had difficulty with these same terminals during the resistance readings. I had a difficult time getting a wandering resistance reading - I kept getting a zero reading for a while.  I only got a reading after touching up some solder joints at the nine pine socket. Terminal 2 resistance reading according to my meter held steady at 3 M Ohm and terminal 14 steady at 15 M ohms, but they didn't settle at 486 or wander much.

2.  I have an audible buzz emanating from the area of the power transformer. Its not super loud, but i can hear it from a foot away or so.  I can't tell if the buzz is coming from the power transformer itself or the power supply pc board. So what might this be a symptom of? loose ground?

3.  On start up, for about 2 or three seconds there is a hum that increases in volume then abruptly drops off.  is this tube rush? After this when the speakers have been on, I can only hear a hum if I put my ear about three inches from the speaker, and it doesn't change when I turn up the volume. 

any help or advice is much appreciated. Oh yeah and

Thanks!

[Mike B]

I get that startup hum too, goes away as soon as the music starts.

As to the transformer, could one (or more) of the nylon standoffs fallen out?  I glued mine in so I wouldn't have to worry about them.  Also glued in the 8 fiber washers for the plate chokes and the output transformers.

[Sh7eleven]

No the washer are in, but I will keep a touch of glue in mind for next time to hold them in place - I always found that part tricky.  Though when I carefully read the manual this time around the method described - flipping the upside down transformer with top plate in place - saved me at least 30-40 minutes of hair pulling and steadily rising blood pressure because I couldn't keep those damn things in place compared to my crack and foreplay build . . .

[Paul Birkeland]

Hello Sean,

The lower voltages at OA/OB just indicate a fresher/stronger 12AT7 than is normal.

The hum at startup goes away as the 2A3 warms up and begins to conduct, this is normal.

A buzzing power transformer, however, is very abnormal.  This can happen if there is an unusually high current draw on the power transformer, but this would show up in the voltage checks.  Also, very loose hardware can allow the transformer to vibrate a bit, so I'd recommend tightening down those screws a bit more if you can.

If there are any other irregularities, do feel free to let us know.

-PB

[Sh7eleven]

Cool - i got a raytheon 12at7, which must be fresh, but I am excited about it for reasons completely unrelated to music.  The raytheon corporation was involved in one of the more famous US Tax law cases stemming from an antitrust suit against RCA for their tube licensing practices and my tax professor in law school has something from raytheon she keeps in her office, but its not a tube. Its her favorite case and she will flip out when I give her a raytheon tube. 

Anyway, enough about nerdy law stuff, the diagnosis of loose screws is spot on.  When I went over to and pressed the transformer bell the buzz stopped.  I painted the bell with rustoleum copper hammertone and in an effort not mess up the finish I might have gone easier on the screws than I should have. So I will tighten everything down.

Thanks!

PS I hate tax law and will neither listen to any questions about nor give any advice on taxes.

[Grainger49]

Sean,

Compulsive that I am, I wanted the top screw slots straight.  I hold the screws with a screwdriver on top and use a nut driver to tighten the nut on bottom.

[Sh7eleven]

Thanks for the tip - I tightened them down using a ratchet and the buzzing was still there at first, but seems to have died down now. 

I hear you about the screw heads all facing the same way - its a sign of craftsmanship . . . and OCD    - I did the same thing. 

[Mike B]

I don't care which way the slots are facing, but I'm going to get some model paint and a fine brush and paint mine black to match the covers -

[Sh7eleven]

I finally had the patience to paint the top plate, transformer bell, and the chokes ahead of time. Rustoleum copper hammer tone -Came out very nice. I'm going to build a base for it as well and will post pics in the gallery,

The buzzing has returned from the transformer bell - The buzz comes from the rear lip of the bell. I've tightened the screws down pretty tight. When I press on it, the buzzing stops - I was thinking a dab of of glue between the bell and the top of the transformer may stop it - any thoughts on that plan?

[Grainger49]

Sean,

Same scheme I did with my Eros build.  I did the three tube shields too.  But I forgot the base of the tube shields.  So there is a little brushed aluminum peeking out at the bottom.  The grounding through the base was necessary anyway.

How much does the transformer vibrate?  An overloaded transformer will have high vibrations resulting in any kind of sound. 

[Paul Birkeland]

I was thinking a dab of of glue between the bell and the top of the transformer may stop it - any thoughts on that plan?

If you have some varnish around and a syringe, you can squiry some in between the transformer and the transformer cover to fill the void.

Glue may work, but it's always hard to tell what exactly it might look like when it's finished.

-PB

[Mike B]

Did you put those 4 star washers between the bell and the transformer core?

[Sh7eleven]

I can't remember whether they were star or lock washers, but I got which ever washers i was supposed to in there.  I think a syringe and varnish may do the trick.  I'll give that a shot.

[Sh7eleven]

Update - the vibration noise stops completely when I place the bottle head badge on top of the transformer bell - so I'll be affixing it on top of the bell. I saw someone else do that, not a bad looking spot it and now it's functional too.