Build question

[odamone]

I'm on page 13, trying to install the terminal strip in front of the power transformer hole.

It calls for a 4-40 x 1/4" screw which is not on the parts list. Should I use one of the 4-40 x 3/8" screws supplied?

It also calls for slipping a #6 lockwasher over the end. Is that right or should I use a #4 lockwasher?

Thanks.

[fullheadofnothing]

Yes use the 3/8".

There are no #4 lockwashers (we don't stock them) only #6 and #8. What you think are #4s are #6s.

[odamone]

Thanks, so I should use the smaller of the two lockwashers here?

[fullheadofnothing]

Yes.

[odamone]

Thanks! I'm soldering the OT-2 - do I just do the same thing for each transformer? the instructions don't spell that out. I just do everything two times, one for each OT-2?

[Paul Birkeland]

Thanks! I'm soldering the OT-2 - do I just do the same thing for each transformer? the instructions don't spell that out. I just do everything two times, one for each OT-2?

Yes, you will want both of the ouptut transformers to be the same (otherwise you would end up with left to right channel differences).

[odamone]

New question:

I've completed the soldering, however nowhere in the instructions does it tell me to solder 17L. Should I leave that connection unsoldered or should I solder it?

Thanks

[Grainger49]

Everything should be soldered.  It is probably in the instructions, or is called out in a  correction sticky in the SEX folder.  But regardless, if the build is finished, solder it.

[odamone]

Thanks Grainger, I soldered that.

So I did the voltage check and everything was fine except everywhere that is supposed to read -3.15 reads -3.96 and everywhere that is supposed to read 3.15 reads 2.36

These are not within the 15% tolerance, so I'm wondering what's up.

The amp sounds fine and all other readings checked out.

Cheers

[Grainger49]

Oliver,

This may be a case where the tube needs the cathode formed.  That means you should run it for 50 hours to see if the voltages come back in line. 

You are right, the target voltage is greater than 15% off from your measurements.  But I don't have a SEX 2.0 manual and don't know where the measurements are taken in the circuit.

[Paul Joppa]

These would be the heater power, pins 7 and 8 of the tubes. That power supply is on the back right of the power transformer, four fat Schottky diodes, a resistor, and a 10000uF capacitor. Since the winding center tap is grounded, one would expect a symmetrical voltage.

It is possible that there is a bad connection to one of the four diodes, breaking the symmetry. They all have fat, heat-conductive wire leads which makes them difficult to solder - they suck the heat from the soldering iron pretty fast. Another symptom of this is some mechanical buzzing from the power transformer - that is not always obvious, but it's a bad sign; things will get too hot and lifetime of the diodes will be shortened if this problem exists.

A less worrisome possibility is that the ground connection (transformer pin 10 to terminal 3L) is not solid, allowing the power supply to drift. Note this is a chassis ground; make sure that the signal grounds (T13 and T23) are also well connected to the chassis, i.e. measure the voltage T3 to T13 and T3 to T23.

[odamone]

I'm stumped.

T3,T13 = 0
T3,T23 = 0

A7,B7,C1,C4 = -4
A8,B8,C2,C5 = 2.4

These measurements are after I (sort of) desoldered and re-soldered:
PT4
PT5
C5U&L
C4U&L
C1U

Desoldering with the copper wick was tough, but I think I got 80% of it. The solder iron was good and hot and the I felt these were solid solder connections when I was done.

I guess I could re-do C1L and C2U&L.

What do you think I should do??

I rechecked all other voltages and all else was fine.

One oddity. After I re-soldered:

PT4
PT5
C5U&L
C4U&L

I had bad measurements at C2 and C4 - the voltage was 0 but it wasn't stable. That's when I went back and re-soldered C1U. Then the measurements came back as above. Maybe that's a clue?

Paul, it wasn't clear, should I take Grainger's advice and just keep the amp on for 2 days and re-check?

Thanks.

[Paul Joppa]

Upon more reflection, I suggest you just ignore this issue, as long as the power transformer isn't shaking and buzzing and the solder joints are good - which you just confirmed.

The 0.1-ohm resistor will offset the DC voltage at the capacitor, so some difference is normal. I don't have a SEX 2.1 on hand to check - my personal one is much older, so I can't check but I would not expect the real voltages to be exactly as in the manual. I suspect that the measured voltages were close enough that they were replaced with what the tester expected, probably not even consciously. By the time the production prototype is ready for voltage checks, everyone involved is a little punchy!

[odamone]

Thanks, Paul. I've got a low level hum in the headphones. Is this normal? It reminds me of the hum I would hear on my guitar amp. Anything I can do to eliminate it?

[odamone]

It's. Probably a 120hz hum.  I just read docb's write up on hums and will redo some solder joints to see if that helps.