Feedback problem

D

denti alligator

New member
My amp has been working fine for a couple months now, ever since a defective c4s board was replaced. But now, in explicably, and with no changes whatsoever, I've been getting feedback. But it's irregular. It's usually in the morning, when I first turn the amp on. It's pretty loud, but it's controlled with the volume so that I can turn it down and it goes away. And then, after a few seconds it's gone and the amp works fine.

Usually, then, I can use the amp on and off for a day or so without this problem. I have a feeling that it's becoming more regular. At first it happened with a week or two interval. Now it seems to be happening almost every day or so.

Any idea what's causing this?
 
Sam,

When you say "feedback" is it "acoustic feedback?"  That is the sound through your speakers feeds back into your turntable.  It produces a growling sound.  It is caused by the speakers and turntable being too close to each other.

Or are you meaning the type of feedback that makes a squealing sound?  

What is the source when this happens?  If you turn down the volume on the SEX amp before powering it up, wait 10 seconds and try to see if you get feedback.  

The C4S adds some gain.  You have probably noticed that.  In close quarters it might cause feedback that you hadn't had before.

Or is this like the "Help! Screeching From SEX" thread?

Also, when I turn on the TV in the morning to watch the news I notice it is much louder than what I perceived the night before.  That is because when we get up our hearing is more sensitive than after 16 hours of our noisy world.  It could be that you raise the volume over the day.  Or not.
 
This is squealing, for sure, but it's not like the last time I had screeching, because I can control the sound with the volume, which I couldn't do with the screeching.

So it's squealing and it's pretty intense. If I turn the volume all the way down and wait 10 seconds, I doubt it would ever happen. Source has been CD, PC, and TT. I think it's happened with all three. Not sure.

Amp sounds fine, so I'm not totally concerned.
 
If it's affected by the volume control, and if it occurs with no source connected (i.e. you're sure it's inside the SEX amp) then it must be connected to the grid of the first stage. Could be capacitive coupling of the second stage plate to the first stage grid - look to see if there are wires that are close.

Could also be a tube going bad - is it in just one channel? Does it move if you swap the tubes? Does slightly wiggling the tube have an effect (bad socket connection)?
 
So I've just not dealt with this, and have been turning the amp on ith the volume all the way down, waiting 15 - 20 seconds and then using it.

But then, yesterday I got the screeching! That is, volume didn't make a difference. It just wailed. So I went to work at it. I started by securing the Obbligato and Teflon caps that I had simply dangling. In doing this the one on the left got loose, suggesting that it might have been a bad connection. I resoldered the connections, secured the caps, and it worked again.

I used it on and off (meaning I turned it on and off again, and on again and off, etc.) throughout the day and again this morning. But just now I turned it on and got the screeching again, but only for about 1 second--too quick for me to pull the speaker plugs or cut the power. So now I'm listening again and it sounds fine.

What do you think is going on here?
 
Resistance readings fine, EXCEPT for 31, 36, and B2, where I can't seem to get ANY kind of reading. The resistor between 31 and 36 is fine though.
 
I did. And it sounds OK, though I kept the volume down for the first 20 seconds again.

I'm puzzled by the lack of resistance readings at those terminals. I checked the voltages there and they were fine. How could I not be getting a resistance reading? Believe me, I've tried it numerous times. It's just not reading anything.

The only thing I do hear is a very, very slight soft crunchy-like sound WAY in the distance and only when I turn the volume ALL the way up with no music playing. I have a feeling I haven't solved this one yet, though I'd like to be wrong.
 
What I understand is that you have a problem most often with the volume pot near the bottom of the volume range.  Start with the pot all the way down.

To check the pot find the lug where the input comes in.  Clip a lead on there and to one probe of your meter.  Then clip to the center lug of the pot and to the other probe of your meter.

Very slowly run the pot from bottom, off, to the top.  For this exercise it is best to have an analog meter, which I don't own.  The needle jumping tells you that you don't have good contact with the wiper.

Using a digital meter takes a little more patience.  It will "autorange" on you a few times throughout the range of the wiper. 

 
You can add a high value resistor (470k-1m) between the grounded lug and the middle lug on each deck of the pot.  Instead of screeching, you would just lose output if the pot wiper lifted. Testing for a lifted contact can be tricky if there is only a small spot on the pot that is bad. 

Another thing you could try would be to connect a clip lead between the two middle lugs on each deck.  This would temporarily make your SEX amp into a mono amplifier, and it would take the bad half of the pot out of the equation.
 
But if there's a only a small spot on the pot that's bad, wouldn't I only get the screeching when I hit that spot, not at other random points?

Also, the last time it happened it stopped on its own after a second, without me even moving the pot.
 
Sam,

You are right but these things don't act as we always think they should.  You should try checking the pot since it was suggested by the guys who know the amp best.  The same thing for the grids as Paul Joppa suggested.  I posted the tube pins for the grids of each tube. 
 
I checked the grids, which looked fine, though I could reflow them, too.

I will check the pot tonight and reflow the terminals where I'm not getting resistance readings. In checking the pot, am I setting my meter to resistance?
 
Paul Burkeland's suggestion is a safe way to keep an open wiper from causing a problem.  You just solder the resistor to the outer lugs of the pot on each channel.  Clean and pretty easy. 

You don't need it if you find the wiper is not opening.
 
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