Dirty Tube Sockets?!?!?!

Yesterday, 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 I was able to fix (hopefully) this type of crackling and popping 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?

Apart from impaired contact of tube pins with the sockets, internal tube issues can cause crackling noises, from  expansion and contraction as tubes heat up to tiny pieces of metal breaking away from electrodes etc. Some people have reported more dramatic symptoms when pins intermittently loose contact with the socket.

In your case cleaning and retensioning has done the trick.
I use various  tobacco pipe cleaners dipped in isopropyl or Deoxit.

Welp,

My amp started crackling again.

It was a power tube.

The crackling was so subtle and steady 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 coming from within the EL-34. It was just a steady non-escalating crackling sound and light sparking from the tube.

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. That's why I was thinking my socket cleaning had fixed it. I'm glad I took the time to do the cleaning anyway, and I'm ordering some proper brushes for the job.

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! This was a new one to me and I've had tube amps for almost 30 years!

The same thing happened to me a couple of months ago, a rustling noise in one channel. The EL34's had been running well for  over 7 years, but I took it as a signal that the tubes had had there day and replaced the complete set after cleaning the sockets with pipe cleaners and Deoxit.

I agree with you entirely Bob, all good and relevant points you make there.  
I only use my tube gear when I sit in the hot seat and seriously listen to music. My music listening sessions are up to about 2 hours, 5 days a week, that's roughly 520 hrs a year multiplied by seven =  3640 hrs. I use a desktop silicon based system for background music the rest of the time.

Bob, what driver tubes do you recommend for the VTA-120? The sleeve of one of the ones that came with my kit said USA Made 5963 / 12AU7 ,and RCA. Which of the many iterations of RCA's are these? The Amp sounds great, and I would be happy to replace them with the exact same thing when these wear out. I already have a spare matched quad of Sovtek 6550's, and a spare rectifier.

Here's a good guide to the various tube variants.
http://www.vacuumtubes.com/12au7.html
On the left-hand column is a purplish area with other tubes. Just click on a tube type and you will have a guide for all the variants available with that tube.

There is another website that has descriptions for the various variants and manufactures types. Subjective of course. I'll see if I can locate it.