In general, in an audio amplifying device (amp, preamp, etc.) in which feedback is used (most everything out there) changing the gain of the active device (in this case a vacuum tube) will affect the overall sonic quality of the unit.
When overall loop feedback is used, the open loop gain of the circuit (before feedback is applied) will normally be very high. The feedback loop reduces the closed loop gain to that desired, and at the same time reduces harmonic distortion. Unfortunately, if too much feedback is used, this can cause the feedback loop to overload the front end of the circuit. Too little feedback and harmonic distortion rises. Too much feedback and transient distortion rises. Note that in AVA circuits we design to prevent hard feedback overload from occurring.
For example, if the open loop gain was 60dB and your wanted 20dB of closed loop gain, then the feedback loop would be designed to provide 40dB of feedback. Of course to do this it would be necessary to select active devices in the overall gain stage that did provide at least 60dB of gain. If the tube had lower gain, there would not be as much feedback possible. If the tube had higher gain, there would be more feedback than the design wanted.
Different tube types and versions within types have wildly differing amounts of gain. So changing tubes from what the designer intended without understanding how the gain of the new tube relates to that the designer wanted will change the sound of the unit in a random fashion, and not necessarily for the better.
A low gain tube will increase harmonic distortion. A bit of this makes the unit sound warm and mellow. Too low a gain tube and the sound will just get muddy and veiled. A "hot" tube with higher gain than normal make make the unit sound clearer and more "detailed" but the trade-off can be too bright and maybe even a grainy sounding high end as transient related distortion rears its head.
A good example of where a vacuum tube just will not work properly is attempting to use a single dual triode 12AX7 tube for a feedback RIAA phono circuit. To follow the specified RIAA equalization curve, the circuit must provide a range of 40dB of closed loop gain between treble and bass output. Bass is boosted 40dB in relation to treble to provide a flat output response and you need another 20dB of overall output to provide a sufficient signal level out. Unfortunately, a 12AX7 tube (the highest gain tube available) has at best 60dB of open loop gain. Thus, no matter what the parts quality of the circuit design, the phono stage will run out of open loop gain at low frequencies, and the circuit will not be able to follow the closed loop feedback demands accurately at all. Distortion at low frequencies will be high and there is nothing the designer can do about this except try again with a different kind of circuit design.
So the story is that in tube rolling, it would be useful to determine the gain of the new tube you are trying in relation to the gain of the existing tube. If the gain is different than the original, your joy at finding a "better sounding tube" may just be your impression of hearing an increase in distortion. If the gains are matched, then well, audio voodoo still abounds, we certainly don't know it all.
Regards,
Frank Van Alstine
