Folderol wrote:I would add a small caveat to Dave's excellent post.
There are two things that might be improved in some designs. The first is improved support structure to reduce microphony, and the other is the routing/shielding of the heaters to reduce hum pickup (although these days world+dog feeds them with DC).
Yes, and example of that is the TAD 7025 High Grade. That is ELECTRICALLY identical to the ECC83/12AX7 but of more rigid construction and, I assume, selected for better noise.
It should also be remembered by the cork sniffing guitar amp tweaks that many of these valves were developed for NON audio, or a least not critical quality audio purposes? The 12AT7 is a good example. Originally used in TVs (Google "sync separator") and some hum, distortion and microphonics did not matter a jot!
The 12BAH7 used in a current (superb!) 5 watt amp was also developed for TV and 'scopes and has an anode voltage rating of 2kV pulse!
DC heating? Yes Will, very easy to do now with modern components, regulators and such and really the only way to go for printed circuit design. The latter is frowned upon by the 'Old School' but PCB build is JUST as good a P2P or turret construction, is far more consistent sample to sample and is VASTLY cheaper and that puts excellent designs into the hands of those who could otherwise not afford them.
BTW the 7025 HG is an excellent replacement 12AX7 and, last I looked, not silly money.
Dave.
