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Dirty Tube Sockets?!?!?!


MechEngVic
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Just a little while ago, my amp (Dynaco ST-70 Series II) started crackling and popping during low volume listening, no red-plating or arcing tubes, no resistors burning, no speaker blowing boom of a capacitor failing, just crackles and pops and the high-low bias LED's flickering. 

 

(You set the bias of this amp by turning a trim pot until both LED's are of equal brightness, they also pulse in time with the music when it's loud. These LED's are sensitive to AC fluctuations and will go up and down in brightness. I say all this about these LED's so you'll understand why I mention them flickering with my issue)

 

I instantly turned off the amp and did a quick look and sniff. No burning smell nor anything that looked wrong. 

 

I decided to pull the six tubes, I inspected the socket seats and saw what looked like a tiny bit of carbon deposit on a couple of them. I grabbed a thin wood dowel and some isopropyl alcohol and gave them all a good reaming. Then I retensioned the seats and slapped the tubes back in. I turned on the amp and it is running smooth so far, cross my fingers.

 

This is the first time this type of crackling and popping has happened and I fixed it (hopefully) with socket cleaning and retensioning. I had a very similar type of crackling and popping happen a different time and it turned out to be a bad driver tube. Anytime I remove and install a tube I re-tension, but never did much in the way of corrosion removal. These sockets are about 1.5 years old and this amp runs several hours a day.

 

Is what I experienced typical of what happens when tube/tube socket contact is compromised? 

 

Also, what do you use to clean tube sockets?

 

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8 hours ago, Les Lammers said:

Deoxit on pipe cleaners what I use to clean sockets. There are also small wire brushes made specifically for this purpose too. 

Have you had a similar issue to mine, where you actually started getting contact failure? So far I think that's what happened, the amp has been running smooth since cleaning.

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I need to clean the contact on my 2a3 sockets... they seem really loose as well, but I hate pulling them out to work on them.

 

I also have an old knight integrated, never been able to find a schematic, but I can't do anything with the sockets (retension) and the pins on the newer el84 tubes seem smaller than the original tubes.

 

I guess I could replace the sockets, but that would be a pain to do.

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8 hours ago, MechEngVic said:

Have you had a similar issue to mine, where you actually started getting contact failure? So far I think that's what happened, the amp has been running smooth since cleaning.

 

Contact failiure could also be due to 'dirty' tube pins, especially on old production tubes. I clean the pins with Scotchbrite pads.

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5 hours ago, Marvel said:

I need to clean the contact on my 2a3 sockets... they seem really loose as well, but I hate pulling them out to work on them.  They can be 'tightened' with a small screwdriver.  You don't have to remove them to do this.

 

I also have an old knight integrated, never been able to find a schematic, but I can't do anything with the sockets (retension) and the pins on the newer el84 tubes seem smaller than the original tubes.

 

I guess I could replace the sockets, but that would be a pain to do. Take it to a good tech. 

 

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Welp,

My amp started crackling again. It was a power tube. The crackling was so subtle I didn't think it could be a power tube, there was no arcing or red-plating... Until I turned off the lights. Then I could see the faintest of sparking from within the EL-34. It took a couple of days and several on and off cycles for the problem to go from occurring after several hours to occurring a few minutes into the warm-up cycle. The tube is replaced and I'll be watching the amp carefully for the next several cycles.

 

The many ways a tube can fail surprise and confuse me!

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