DC Offset Issue in Don Sachs Preamp

Is this a new behavior?

With a capacitor coupled tube preamp, the output capacitor will develop a fairly large DC voltage across the cap. When the preamp is turned off, the voltage on the input of the cap goes to zero, but the DC voltage across the cap doesn't just disappear immediately. The larger the cap value, the more energy will be stored and the longer it will take to dissipate. If the power supply voltage drops faster that the coupling caps can drain, this will result in a DC offset for a short time which will cause a pop or thump if you still have the amp powered up. 

Amp is used (3-4 years old) but new to me.  

Today, I swapped out the CV181Zs for classic 6sn7s, and things may be better. Would that make sense? Doesn't the CV181Z have higher heater current value than 6SN7s?  

The only cv181 tubes that have a higher heater current are the nos tubes, the newer production cv181 tubes are identical to the 6sn7.

Don is still with us, contact him by email. My guess is the load resistor on the output of the cap has gone open, but I wouldn’t expect both channels to do that at the same time. Maybe something has gone wrong with the regulator. It would help to know what power amp you are using and how efficient your speakers are. (Khorns? Magnepans?)

The only way to be sure is look at the output with an oscilloscope and see how big that pulse is. You can get handheld Chinese models for about $200.