I think about winding a prototype bobbin and use the other dedicated P48 Tx. as preliminary.
Reason being, I'm very confident I get the windings right, I just don't want to commit the one and only "original" bobbin yet, that came with it, until the windings have worked out for 6,3V and +250V on the prototype.
At least I think so.
I usually go for 48VAC or 50VAC secondary fro phantom. This will give you roughly 70V dc at the smoothing capacitor which gives you the 20V required for best operation of the TL783 regulator I normally use. I know this may seem a large voltage drop across the regulator but this device gives its best noise and ripple reduction in this region and since the current draw is very small the heat dissipation is usually not an issue.
Recently I read that someone says there are better sounding tubes than the 6922,
Ian, someone offers 7 or so E88CC NOS, should I avoid stocking more of those and look for a project with "better sounding tubes" at the output that is?
I'll try to come up with those alternative tubes in a second.
I made a screenshot but it's black and the tube info is lost.
The E88C is a special quality version of the ECC88. In this case special quality refers to extended service life rather than a special ruggedised construction. It was designed to withstand long periods at cut off in computers.
Electrically, the 6922 is identical to the E88CC. They are the same tube.
The question of "better sounding" is a difficult one because it tends to ignore the topology in which the tube is used. All tubes have an intrinsic distortion below which it impossible to go without using techniques like negative feedback. When I first started designing tube mic pres 20 years ago I was seduced by the audiophoolery mantra that negative feedback is bad. So I spent a lot of time searching for tubes with low intrinsic distortion and for topologies that allowed the tube to get close to it without NFB. Turns out the tubes with the lowest intrinsic distortion are the 6SN7 and its B9A equivalent the 6CG7 and the simplest topology that could approach the intrinsic distortion value was the mu follower. So the mu follower topology formed the basis of all my early mixer designs and it still occurs in project like the Classic Solo mic pre.
If instead of a mu follower you use a regular common cathode circuit you distortion will be much higher and will vary a lot with the operating point. I think this is what gives many designs their "tube colour" and at that point "better" becomes little more than a personal choice.
The trouble with the mu follower is it has poor drive capability because it is a single ended topology. If you want to build a tube mixer with several buses and a direct out then the mu follower is not going to have the required drive capability. That's when I started using the SRPP output stage. This is a true push pull topology and for the same quiescent current has many times the drive capability of the mu follower. The price you pay is that the distortion is about 10 times above the intrinsic value and the solution to that is to apply about 20dB of NFB. Long story short, the twin line amp design incorporates the necessary NFB and the 12AX7 front end provides the required additional gain to be able to produce the same kind of overall gain as the original mu follower design.
Bottom line, once you have the topology with NFB included, the performance of the amplifier is almost independent of any variations in tube properties so tube rolling is pointless.
I hope that all makes sense.
Cheers
Ian
