We are a bit nervous about anyone using our equipment or playing around inside it plugged in and with the cover off. Mistakes can be SHOCKING!!
However, the way we do it here is to switch the preamp to some other source than phono, turn the volume all the way down, turn the preamp off, go ahead and swap the tubes, turn it back on, wait for the output relay to "click" and then switch back to phono with the volume turned all the way down. Cautiously advance the volume (a new defective tube or one not seated properly could make one hellofa racket) and then if all is well go ahead and listen.
Note that the gain characteristics of individual tubes vary considerably from sample to sample. We select the original tubes for matched gain (both from tube to tube and from section to section within the tube). Note that the tubes used are dual triodes, really two separate tubes in one glass envelope and each separate section can be different from the other.
We also select for low thermal noise, low microphonics, and balanced heater current draw.
Only about half of any batch of tubes we order makes the grade for phono use.
In a feedback circuit, in general, one of the major sonic characteristics of the tube is accounted for by its overall open loop gain.
The tube itself sets the open loop gain of the circuit. The feedback loop paramaters set the closed loop gain. The difference between the open loop gain and the closed loop gain is the amount of feedback now in the circuit.
Thus a high open loop gain tube provides more feedback, resulting sonically as more clarity, lower harmonic distortion, more transparency. A low open loop gain tube results in lower feedback, higher harmonic distorion, less clarity, and more "tube" sound. However, higher feedback can result in less stability, higher transient distortion --- a hardness and glare in the sound, especially if the overall circuit design does not have good headroom to handle the feedback.
Thus in many circuits "tube rolling" is a way of trying to find a way to balance the amount of feedback used by ear. Too high, glare and excessive listing fatigue, too low, too muddy and uncontrolled.
Of course, what is right to your ears may not be right to somebody else.
And of course the tube that you find is right may not be right to somebody else, and remember that their samples of that same tube brand may be very much different electrically than yours.
We try and provide consistancy in our hybrid and tube products by very carefully selecting tubes, using brands that give us the best repeatable results. You likely won't have the equipment or large number of tube samples to do this repeatably.
And finally, there are some characteristics of tubes we simply have not got figured out yet, samples from two brands with very closely matched values that we can measure still do not so necessarily sound the same in use.
Thats why "tube rolling" can still be an interesting and worthwhile proceedure for the involved audiophile.
Hope this has been of some help.
Frank Van Alstine
