Tube rolling question

[anonymouse]

I have a Jadis LS9D tube preamp packaged in a Li Audio box. It uses a complement of 6 12AT7's. I currently have what appear to be Mullard CV4024's with lettering saying KQDD/K on them.

The circuit diagrams are attached.

I am considering buying the pair of Amperex and pair of Telefunken 12AT7's being sold by ND 13 here: http://www.polkaudio.com/forums/showthread.php?t=76029

Questions for the tube experts:
Would these be a step up over the Mullard's?
Given these will replace only 4 of the 6, which positions would you put them in?
Any other commentary?

[ND13]

I personally prefer Bugle Boys in the output stage, some prefer the Teles. The Mullards are normally a little lush for my taste. Just make it an experiment in audio.:p:p

[fredv]

Hmm, the schemo shows 12AX7 tube complements, and you got 12AT7. There are many many clone Jadis preamp from Asia, but none seems to be a true clone. I have the same impression as ND, BB are very pleasing to ears, kind well around tubes in general. To be, the tele's is kind of a little less energy than the BB and Mullards, but another term of this could be darker, so it is really personal preference. Regarding to how to place the 4 tubes, just try all possible position and keep the one that you prefer :-) If I have only one pick, I will use them for the first 2 stages.

[ND13]

I believe you can run 12AT7s in 12AX7 slots, but not the other way around. Same with a U7 in a T7 but not the other way. Voltage issues. IIRC, that's the way it is, I'm sure if I'm wrong, there will be plenty to tell me I'm wrong...it's kinda a famous past-time of CP.

[fredv]

These 3 types of tube have very different load lines. They are pins compatible and have same filament voltages/current draw but have very different characteristics. 12AX7 is a high gain tube, amplification factor 100, 12at7 has 60, and 12AU7 has only 17. Some people replace the 12AX7 by 12AT7 because of too much gain, but the distortion may go up if the circuit is optimized for 12AX7. 2 areas that can certainly improve the SQ more than rolling the tubes,
1. the decoupling caps (C22, C23??) they are those 2 between the first stage and second stage, values 0.47uF/400.
2. the output caps (C13, C17?) they are 3 in parallel at the output stage, looks like 3 1.0uF's.
However, it will require a little soldering to replace the caps. Trust me, it will be well worth it!!
Have fun!!

[anonymouse]

Hmm... yeah they do have 3 1uFs in parallel. Why do you think they did that? Why not just put in a single 3uF Sonicap?

[fredv]

It is indeed a good question. My gut feeling is that it might be the spacing of the board, but then all the Jadis schematic that I have seen used multiple parallel caps instead one big ones. So, it might be their practice for no good reason. You can definitely use one 3uF if the space allows. One thing to make sure, the caps must be rated 400VDC or above. If it costs too much, you can also consider buying a good small one to bypass those caps, something like 0.1uF should be good enough.

[F1nut]

The more accepted and perhaps better alternative than the 12AT7 for the 12AX7 is the 5751 at an amplification factor of 70.

I believe the reasoning behind the use of smaller value caps in parallel is that they are faster than one larger valued cap.

[george daniel]

The more accepted and perhaps better alternative than the 12AT7 for the 12AX7 is the 5751 at an amplification factor of 70.

I believe the reasoning behind the use of smaller value caps in parallel is that they are faster than one larger valued cap.

I put a 5751 in place of the ax7 in my anthem amp,,IIRC, it allows a wider gain(for lack of a better word) due to the lower factor,, hmmm,, think I'll do some reading,, one thing for sure,, you guys certainly know your tubes, thanks for the education.

If you have gain to spare(as I did) the 5751 will supposedly lower the noise floor as well.