Tube Rolling | Luxman CL-38uc

Hi!

Yes, those are the right tubes :) ... only 3 would be needed if you are not using phono input or tone controls.

The only caveat is that they are placed a bit differently inside on the actual unit, so a "best guess" would be needed unfortunately on picking which ones to replace. The phono input ones are clearly distinguishable, but the others not so much :(

Haven't taken the time to do that quite yet, but will try to report back when I do.

Hope that helps!

my 2 cents

i don’t have this particular luxman pre... so this is from general knowledge and a lifetime of tube rolling

12au7 and 12ax7 are usually NOT interchangeable... vastly different gain characteristics (that is why those three tube types were designed and made in the past, despite the common 12v heater dual triode small envelope designs -- 12au7 vs 12at7 vs 12ax7 - very different gain profiles from one to the other -- 1x 3x 5x gain, thus applicable for different circuits)...you can bet that putting in a 12ax7 with 5x the gain of a 12au7 in your nice luxman will create problems

you would need to contact luxman, or a knowledgeable dealer or tech to confirm specifically which three 12au7 slots are for main amplification, not tone control buffering circuitry (what is drawn on the circuit diagram is idealized, the actual circuit board may not be laid out in exactly the same fashion)... you CAN safely swap in different 12au7 tubes in those 5 positions, but without knowing for sure, you won’t be able to assess what you are hearing in the attempt to tube roll

here is a fairly complete discussion and listing of 12au7 equivalents (these are different names for the same tube, from different parts of the world, or very very similar variants made over time i.e. improved ruggedness etc etc - with identical or very similar gain profiles) - you would be well served to read it carefully and completely

http://www.audiotubes.com/12au7.htm

Hi @jfandr147 and all - I have the CL-38u and CL-38u-SE models, the tubes of which are laid out the same as in the 38uc. 

I don't know how to post an image, but I've spent some time under the hood of the 38u, traced out tube signal flows, and can describe for you the location of the tubes in question.

With the faceplate facing you, there are two rows of four tubes; a front row and a back row. For purposes of this description we'll call the back row tubes 1 through 4, from left to right. The front row holds tubes 5 through 8, from left to right. For reference, the phono tubes (with cages) are positions 3, 4, and 8. Tube positions 1 and 5 are for the EQ section. The tubes not involved in phono or EQ sections are in positions 2, 6, and 7.

I haven't done much in the way of tube rolling but can assure you with confidence the above is accurate.

I hope this helps - I'd love to hear back on any experiences with the tube rolling.

well done @acefactory -- here is a pic to accompany your excellent description

so the tubes op wants to roll are three of the four middle ones (leaving aside the one single middle one with the white spring loaded tube damper/cage)

Thank you so much @acefactory ! you just saved me ton of time trying to figure that out! :) ... I'll report back on how the tube rolling goes, will try a set of Telefunken and Mazda tubes.

@jjss49 thanks for the picture, comments and your initial remarks regarding tube differences. You are absolutely right, better to stick with 12AU tubes in this case as intended (that's what I ended up doing).