Hybrid Tube Preamp - Loud Pops and Crackles

Could be the tubes and the tube sockets to start with. If that is not the case could be the volume control. RCA cable connections?  Then a cap of resistor. If in the NYC area I can take a look at it once you have eliminated the above.

Thx, colleagues. I will tool with the tubes and tube sockets and play with the volume control ( a possibility) and the IC cables (RCA and XLR). Beyond my skill to deal with cap or resistor. I am in NYC and hope I may not bother you -- but I might. I am on CPW. Cheers. neal

I'm a bit confused by the suggestions that you mess with the tube sockets -- the process of removing the original tubes and putting in new ones should have taken care of any issue that was caused by corrosion build-up or a poorly seated tube.  Next, the odds of a different set of tubes exhibiting identical problematic behavior is unlikely (though not impossible.)

If the volume control, you'd almost certainly notice the problem as you turn it up or down.

Once you've toyed with everything and re-set the tubes and all other cable connections (not forgetting to plug/unplug at the amp end) the last thing I'd suggest is to disconnect the preamp from the amp. If the amp by itself continues to make noise, it's not the preamp causing the problem.

If the amp is quiet, plug in the preamp, but with no sources connected. If at this point the noise returns, it's time to take your preamp over to the shop for a professional look.

One colleague (oddiofyl) above suggested re-tensioning tube sockets.

I will follow your suggestions and see what happens. I too am surprised two sets of tubes causing the same noises, so it is unlikey the tubes are the problem. Thank you. Neal

Try this with your tubes (from a well known tube seller) 

Now, what if I heard something? For noise, at normal listening volume at your listening position you will clearly hear hiss, pops, crackles, rumble, whistles, or any combination of the above. For microphonics, at normal listening volume you will clearly hear tapping through the speakers as you tap on the unit. In the case of noise, you may be able to fix it yourself. First, locate which tube is noisy. You may have to substitute one tube at a time and try the unit out until the noise vanishes, indicating the last tube you took out is the one. If you don't have spare tubes, just swap the tubes one at a time from the left channel to the right channel and listen after each tube swap. The noise will follow the noisy tube to the other channel. Take that tube out of your unit and tap it on its side firmly on a table top. Then, hold it upright and tap it firmly with a wooden pencil. This will nearly always knock the offending flake of material out of the grid where it will cause no more noise. Reseat the tube and give it a good 12 to 48 hours of burn in time and the noise should be gone for good. I know it sounds extreme but in at least 85% of noise cases, this resolves the problem. Some audiophiles have said tapping a tube can CAUSE it to become microphonic. I totally disagree. I have tried this trick for 25 years with mostly successful outcomes, and never once has a tube become microphonic after tapping it.