Power chord for SEX amp 3.0? (and other questions about SEX) [resolved]

S

slankoe

New member
Greetings!

I am glad to be joining the bottlehead builder's club soon as my SEXy amp is on the way to completion. However I am stuck at the current step and would appreciate a nudge in the right direction.

Here's my sitch, I'm on the powerline voltage test, on page 28 of the manual it is telling me to use the provided power cord, but I don't have that power cord. The only one I was given is not yet assembled, and I have figured that this is the C4S upgrade. So I open up the C4S manual and is gives a very strong warning not to assemble the C4S yet. So my question is what the heck do I do?

Can I use an ordinary power cord?

Thx,
-Dan
 
Oh right now I've figured out... I feel like such an idiot. I skipped the step where the fuse goes into the fuse holder, because I don't have a fuse holder.

edit: Wait hold on, I found it.  ::)  ;D

121.2 volts, perfect. One question, isn't that supposed to go away when I flip the on off switch of the SEX? Because my voltage stays at around 120 whether the switch is in the 1 position or the 0 position.

My SEX just won't get turned off?

edit 2: ok she passed the tube glow test. I think I'm good so far.
If I have any other problems, I maybe post here again, and figure it out myself :) This is kinda fun stuff, eh? I've never built anything like it before. Cheers 8)
 
The voltage test is done from the IEC, which is before the power switch.  The power switch turns off the voltage between the IEC and the transformer.


Cheers,
Jeff.


[Edit: and you're definitely not the first to try and run a voltage test without the fuse. ;)]
 
So it's very important that you unplug the power cord before continuing the build.  Like Jeff said, there is power on the IEC regardless of the amp power switch position. And if you continue the build with the amp plugged in you risk getting shocked by touching the IEC or power switch terminals.
 
Hey guys,

I have finally finished the SEX build (and yes, of course I only had the cord in for voltage testing! good advice anyway).
However I have run into a hiccup, actually at the very last stage voltage testing.

I began testing starting from terminal 1 and moving up like the manual said, same as when resistance testing. Everything seemed good on visual check. Resistance check was good too. Now as I got to terminal either 4 or 5 my readings were off (I can't recall what it was, wish I had recorded it...), and at terminal 6 I saw a spark. Looks like a short. I turned everything off and back on again, started testing again, this time I was getting low readings like 0.6V, or very high readings like 1.5 mV, and now I am not sure that my meter is reliable...

I did my resistance testing again just now, I got all the same results as before. However some were "unusual," but I let them slide because they weren't zero (0), where the manual that it's apparently not a problem unless it says zero, indicating a connection where there shouldn't be one. I guess I should have looked closer, as I might have assumed that it said kilo-ohms or something.
I've listed the current measurements below, and to my memory they look the same as when I first measured. 107.6 ohms is a recurring number

Terminals 30, 31, and 32 are 107.6 ohms.
Terminals H2, H3, H4, H5, and H7 are 107.6 ohms
Terminal H6, is 53.7 ohms

I also made sure that the capacitors weren't wired backwards, and looked everywhere so that nothing was crossed or otherwise causing a short. I'm at a loss for what to do, and I don't want to voltage test again if I risk screwing up my first tube amp ever, and the first one I've ever built.

Does anyone have any suggestions?

Oh yes, and I should say this also: When I was doing my final voltage testing, and that after the spark and wonky readings happened, the fuse was and is still intact.
 
Was the spark created by accidentally touching the test probe to two terminals at once, or did it happen by itself?
 
Doc B. said:
Was the spark created by accidentally touching the test probe to two terminals at once, or did it happen by itself?
Click to expand...
I don't see any other way it would happen actually, unless it's a ground fault... isn't that right? Anyway, a ground fault could electrocute me too, couldn't it? So if I touched the base with my right hand to adjust it's position while it was live and I didn't get zapped, it's not a ground fault?

Maybe the probe did touch two terminals, but my hand actually slipped just as I was about to touch terminal 6 (where the pop noise and spark happened). I am pretty confident you're right though in that two terminals were touched simultaneously.

And do the recurring numbers mean anything?!
 
I just want to establish what caused the spark. The most common issue is accidentally shorting two terminal together with the test probe, which seems to be the case here. But a wire that was only attached to a terminal but not soldered can also spark if it gets moved and makes intermittent contact.

this time I was getting low readings like 0.6V, or very high readings like 1.5 mV,
Click to expand...
Both those values are very low, with 1.5mV lower than 0.6V.

Could be that you have lost DC to the heaters. Are your tubes glowing?


 
My tubes do glow still. And I should also say that the 1.5 reading was a capital m 1.5MV as in MEGAvolts? But that seems insane. Its probably millivolts.
 
Those resistances aren't really conclusive. Only one seems a bit off from what I measure on a S.E.X. 3.0 here at the lab, that being the 53.6 ohm one. I would suggest starting the voltages tests over and reporting anything that does not meet spec with the values in the manual.
 
The voltage for terminal 2 is good (85 or so), but then terminal 3 is completely weird. It goes in the millivolt range, between 200mV and 2mV sometimes there is a minus for example 02.4mV was just measured. The number keeps changing. what the hell is going on?

I'll try the rest of them...
 
I hear a faint buzzing in the right channel sounds like oldschool radio. Left sounds good. Switched tubes, same thing, so its not the tube. Also when I turn off the amp, it still plays. Then fades away. I guess this is how tube amps work? Kinda neat.

Just quadruple checking the connections here and they seem ok so far
 
Did all my voltage checks again. All were pretty normal looking. Except much of the 385V were bumped to 400V and likewise the ones that were supposed to say 400 were closer to 425. The only one that isnt normal is terminal 3 which shows in the millivolt range, which is not ZERO, but then again all the ones that were supposed to say zero as per the manual actually are close to zero, but none of them are zero. I'm assuming this is just because of the sensitivity of the volt meter or what?

Also, maximum volume on the amp with Sennheiser HD650 only reaches comfortable listening. I can't crank it any more.

With that old radio sound in the right channel, it's volume is contant no matter what I set the amp volume to.
 
Terminal 3 is the connection from the parafeed capacitor to the output transformer.  If you've no voltage there but terminal 10 reads the correct voltage, then I'd check your solder joints at terminal 3 or 10.  Assuming your resistance check at terminal 3 also passed, then it's either one of those or your parafeed capacitor is blown.
 
JeffYoung said:
Terminal 3 is the connection from the parafeed capacitor to the output transformer.  If you've no voltage there but terminal 10 reads the correct voltage, then I'd check your solder joints at terminal 3 or 10.  Assuming your resistance check at terminal 3 also passed, then it's either one of those or your parafeed capacitor is blown.
Click to expand...
I checked the resistance of 3 and it was as it should be, there is the correct voltage at 10. I checked my solder joints and re-soldered them even. Played around with them a bunch, and am very confident that there is a proper connection.

Same test results. I guess the capacitor is fucked? Maybe I should switch them and see if that helps.

Too bad I have to go to work tomorrow, I'll be away from two weeks. But that is just enough time to order a new capacitor. I'd like to verify that this is indeed the problem.
 
Yes 3 is connected to the primary. But that is not where the problem would lie, and I doubt that your parafeed capacitor is "blown" if it has 400v on one lead and 0 on the other. Caps short when they fail.

If you look at the schematic you can see that the low end of the OT primary winding connects to the cathode of the output half of the 6FJ7. The cathode should be around 18V and that would float the end of the parafeed cap up close to that voltage. So check pin 7 of the 6FJ7 on that channel and see if you have around 18V.
 
Doc B. said:
Yes 3 is connected to the primary. But that is not where the problem would lie, and I doubt that your parafeed capacitor is "blown" if it has 400v on one lead and 0 on the other. Caps short when they fail.

If you look at the schematic you can see that the low end of the OT primary winding connects to the cathode of the output half of the 6FJ7. The cathode should be around 18V and that would float the end of the parafeed cap up close to that voltage. So check pin 7 of the 6FJ7 on that channel and see if you have around 18V.
Click to expand...
My knowledge is lacking o really understand what i going on here. but if I am to understand what you say, I must test pin 7 the voltage at the white circular socket that holds the tube, under the chassis, and on the side with no audio?
If that is the case, when I test pin 7, I get a very low reading in the tens of millivolts range, but oddly it starts to ramp up and keeps going to around 200mV within about 15 seconds, and I guess it would keep doing so. But that must be the meter doing funny stuff?

What does it mean? What is up with the slowly climbing or descending readings, and why are some readings preceded by a minus (-) symbol?
 
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