HELP! Screeching from SEX

[corndog71]

Have you tried different tubes?

[Paul Joppa]

The thread has gotten too long to figure out what has already been checked, and it's unclear whether the C4S has been replaced or not. (At one time the noise was thought to have originated in the C4S?) So maybe a status summary would help. Is it in one channel or both?

From the description, it seems most likely to be a mechanical connection problem, such as a bad solder joint, bad tube socket connection, bad pot, broken part, etc. Nothing bad enough to be obvious, just enough that the connection comes and goes. It might be time for the chopstick test.

[denti alligator]

C4S has been replaced. LED lights on those boards still light up, so they appear to be fine.

I only have these tubes, so I can't switch them out. I can't tell for sure (because I don't let the squealing continue for more than a split second, if I can help it) which channel, but it appears to be both.

What's a chopstick test?

The question is: what would work fine for a year and then without any changes, suddenly act up again? Surely not a solder joint? I'm wonder if the heat has anything to do with it: since when it's cold in the morning it acts up; but when it's warmed up it appears to work fine. Could that be affecting something.

[Paul Joppa]

Solder, unlike a diamond, is not forever. Funny things happen, especially if the joint has a hidden flaw.

Chopstick test is to poke each joint and component with a non-conductive stick such as a chopstick, while the amp is running and you are listening. This helps detect anything that is mechanicall sensitive, indicating a possible bad connection or damaged component. You have to be careful, since all the dangerous high voltages are exposed, and it's a bit of a pain to get everything connected correctly with the amp upside down so you can do the test - it has to be stable enough to actually push the component, not just touch it as for a voltage test.

[2wo]

Might be worth acquiring a spare set of tubes...John

[denti alligator]

Will run some tests

Where's the best place to get tubes?

[Paul Birkeland]

It sounds like a bad joint or a bad cap.  Did you melt any of the film caps while building the amp? 

You never mentioned if it is one channel, or both channels? 

It would be most likely that this is a bad joint, flipping the amp over restores connection for a while, and everything is OK.

Cleaning a tube socket or a pot every day will just wear them out, I wouldn't consider that a solution or an indicator that the problem is in either of those components.

-PB

[denti alligator]

I left it on today with the oscillation and it subsided after a couple seconds. Not sure if that helps explain the source.

Any suggestion for finding the faulty solder joint? Is the "chopstick method" the best way to go?

[Paul Joppa]

I thought the tubes had been cleared of guilt - I have heard such a noise from tubes warming up, though not often. Pick up a spare pair from Antique at http://www.tubesandmore.com/ - they are usually inexpensive and reliable. I don't think anybody offers tested matched gold-pin cryo'd 6DN7s anyhow, an it would be overkill for a SEX amp anyhow!  :^)

[denti alligator]

So I turned it over and turned it on and let the oscillation subside (after 1 1/2 seconds) and plugged in my phones and began prodding. I immediately found a problem at 36 and 37, both of which make loud unpleasant sounds when prodded. So I reflowed the solder. Same problem. I then reflowed and added some fresh solder: same problem. Could it be the cap? How to check?

I need someone to be so kind as to post the resistance and voltage readings for the SEX 2.0 (the one before the newest version) so I can check these. Or I can just check them all and post them here and hopefully get some feedback. Thanks, and merry Xmas! I got a ton of vinyl (or, as we call them, because of their shape: wall calendars), but with my wounded SEX I'm not able to enjoy them

[Paul Birkeland]

Hello Sam,

PJ and I have asked several pointed questions, and I would recommend answering those first before going any further. The answers to these questions will help determine if the cap at 36/37 might be the problem.

Otherwise, you are chasing your tail (and we are too).

-PB

[denti alligator]

I can't tell what channel, because the feedback is so loud and strong I'm trying to turn it off so as not to destroy my speaker or phones drivers. But I'll go ahead and see if I can identify it.

And no, I didn't melt any film caps during the build. At least I don't think I did.

I'll get back to you with resistance and voltage readings and (I hope) channel ID. In the meantime, are there any other questions I forgot to address?

Thanks!!

[denti alligator]

resistance:

1   1.65M and steadily decreasing
2   1.50M and steadily decreasing
3   0.2
4   0.614M and slowly rising
5   0.4
6   2.484M
7   
8   0.3
9   622
10   0.2
11   1.70M and steadily increasing
12   622
13   
14   249.4k
15   0.3
16   1.59M and steadily increasing
17   0.3
18   0.2
19   1447
20   can

[Paul Birkeland]

I'd recheck 39/B4 while you are turning the pot. 

Can you describe the nature of "all over the place"?  Do you have any spare resistors around in the 100-500K resistance range? (any wattage will do), we can do a little experiment to narrow the suspects down.

-PB

[denti alligator]

I think I have some spare resistors.

Now I'm getting fine readings at 39 and B4. Goes up steadily with the pot until it reaches 2.075 (same at both places), which is about 12 noon on the pot, and then drops out (no reading).

I find the complete lack of readings at B2, 40, and 36 very odd. There's an earlier thread from about a year ago when I had this same reading, and thought it might be connected to the problem. Could it be?